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Design rules of thumb for naturally ventilated office buildings in Canada Edwards, Craig 2001

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DESIGN RULES OF T H U M B FOR N A T U R A L L Y V E N T I L A T E D OFFICE BUILDINGS IN C A N A D A by CRAIG EDWARDS B.Sc. in Mechanical Engineering, University of Waterloo, 1988 A THESIS SUBMITTED IN PARTIAL F U L F I L M E N T OF THE REQUIREMENTS FOR THE D E G R E E OF M A S T E R OF A D V A N C E D STUDIES IN ARCHITECTURE in THE F A C U L T Y OF G R A D U A T E STUDIES School of Architecture We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH C O L U M B I A November, 2000 © Craig Edwards, 2000 In presenting t h i s thesis i n p a r t i a l f u l f i l m e n t of the requirements for an advanced degree at the U n i v e r s i t y of B r i t i s h Columbia, I agree that the Library s h a l l make i t f r e e l y a v a i l a b l e for reference and study. I further agree that permission for extensive copying of t h i s thesis for s c h o l a r l y purposes may be granted by the head of my department or by his or her representatives. It i s understood that copying or p u b l i c a t i o n of t h i s thesis for f i n a n c i a l gain s h a l l not be allowed without my written permission. Department of /^)r o-^\ .' /t-C- r^*j -g-The U n i v e r s i t y of B r i t i s h Columbia Vancouver, Canada Date Pn_ c / V Too Q ABSTRACT Using natural ventilation to provide ventilation and/or cooling in commercial buildings has a number of direct benefits to the environment, building occupants, building owners, and architects. Despite advances now occurring in design methods, major obstacles still need to be overcome before a wide spread adoption of natural ventilation technologies will be seen. Most importantly, simple tools that can be used by architects in the initial stages of design of naturally ventilated buildings are required. Existing natural ventilation design rules of thumb were identified from published literature and building codes and standards. A computer model, capable of simulating both naturally induced airflow rates and building thermal performance, was used to evaluate natural ventilation performance in terms of ability to avoid overheating and provide ventilation for indoor air quality. First the effect of changes to building design parameters on the natural ventilation performance of a base case office building were investigated. Secondly, the validity and limitations of existing rules of thumb were evaluated. The base case building was a three story cross ventilated office building surrounded by large local wind and solar obstructions, simulated with climate data for the cities of Vancouver and Toronto. It was found that the development of most of the existing rules of thumb has been based on incomplete research, and the conditions under which they can be applied are poorly defined. When the limitations of these rules of thumb were investigated, it was found that the original rules of thumb are generally not accurate for either the climates of Vancouver or Toronto. More accurate ranges of applicability were developed for each rule of thumb for each of the two climates. The relative influence of design parameters on reducing overheating and increasing ventilation rates for indoor air quality were also established, and can be used to provide guidance into how changes made to the building form and fabric can effect overheating and indoor air quality. T A B L E OF C O N T E N T S A B S T R A C T i i LIST OF T A B L E S v LIST OF FIGURES vi 1 Introduction 1 1.1 Background 1 1.2 Need for Simple Design Tools for Architects 7 13 Thesis Objective 8 1.4 Scope 8 1.5 Methodology 9 2 Fundamental Principals of Natural Ventilation 10 2.1 Air Density Differences 10 2.2 Wind 11 2.3 Combined Stack and Wind Effects 12 3 Existing Natural Ventilation Design Rules of Thumb, Regulations, and Standards. 14 3.1 Rule 1 - Cross Ventilation 15 3.2 Rule 2 - Stack Ventilation 17 3.3 Rule 3 - Night Cooling Ventilation 18 3.4 Rule 4 - Ventilation for Acceptable Indoor Air Quality - United Kingdom 19 3.5 Rules 5a,b,c - Ventilation for Acceptable Indoor Air Quality - North America 19 4 Natural Ventilation Performance Criteria 22 4.1 Indoor Air Quality (IAQ) Performance Criteria 24 4.2 Summer Overheating Performance Criteria 29 5 Natural Ventilation Performance Modelling 33 5.1 Multizone Airflow and Thermal Modelling Tools 33 5.2 NatVent Model 34 6 Natural Ventilation Modelling Design Parameters 36 6.1 Base Case Building 36 6.2 Natural Ventilation Design Parameters Investigated 39 7 Base Case Building Performance 44 7.1 Vancouver and Toronto Climates 44 7.2 Base Case Building Performance 46 8 Parametric Analysis of Variables Effecting Thermal Comfort and Indoor Air Quality 48 8.1 Thermal Comfort Parametric Analysis 48 8.2 IAQ Ventilation Rate Parametric Analysis 57 9 Analysis of Rule of Thumb Limitations 62 9.1 Cross Ventilation Depth Limit 63 9.2 Stack Ventilation Depth Limit 73 i i i 9.3 Night Cooling Ventilation 83 9.4 Ventilation for Acceptable Indoor Air Quality 86 10 Conclusions 91 11 Recommendations For Further Work 102 i v LIST OF TABLES Table 6-1 Modelled Building Design Parameters 41 Table 6-2 Active Thermal Capacity of Structures per m 2 Gross Floor Area 42 Table 7-1 Vancouver and Toronto Climate Data 44 Table 7-2 Base Case Building Overheating Performance 46 Table 7-3 Base Case Building Ventilation for IAQ Performance 47 Table 9-1 Thermal Depth Limits of Cross Ventilated Office Buildings 65 Table 9-2 IAQ Depth Limits of Cross Ventilated Office Buildings 69 Table 9-3 Thermal Depth Limits of Stack Ventilated Office Buildings 75 Table 9-4 IAQ Depth Limits of Stack Ventilated Office Buildings 79 Table 9-5 Night Cooling Vent Size Requirments for Cross Ventilated Office Buildings 83 Table 9-6 Trickle Vent Size Required to Achieve 10 L/s per person Airflow for 99% of Occupied Hours 86 Table 9-7 Ventilation Performance with Trickle Vent Size of 4 cm 2 per m 2 of Floor Area 87 v LIST OF FIGURES Figure 2-1 Stack Ventilation and the Neutral Pressure Plane 10 Figure 3-1 Cross Ventilation Rule of Thumb 15 Figure 3-2 Stack Ventilation Rule of Thumb 17 Figure 3-3 Minimum Airflow Opening Area Required in Internal Partitions 21 Figure 4-1 A S H R A E Summer and Winter Comfort Zones 29 Figure 6-1 Base Case Building Layout 36 Figure 6-2 Base Case Cross Ventilation and Stack Ventilation Configurations Used For Parametric Analysis 37 Figure 6-3 Base Case Cross Ventilation and Stack Ventilation Configurations Used for Rule of Thumb Analysis 38 Figure 8-1 Thermal Comfort Parametric Analysis Results 49 Figure 8-2 IAQ Ventilation Rate Parametric Analysis Results 58 Figure 9-1 Base Case Cross Ventilation Configuration Used for Rule of Thumb Analysis 63 Figure 9-2 Cross Ventilation Thermal Depth Limits 64 Figure 9-3 Range of Applicability of Cross Ventilation Rule of Thumb from a Thermal Comfort Perspective 66 Figure 9-4 Cross Ventilation IAQ Depth Limits 68 Figure 9-5 Range of Applicability of Cross Ventilation Rule of Thumb from a Thermal Comfort Perspective 69 Figure 9-6 Base Case Stack Ventilation Configuration Used for Rule of Thumb Analysis 73 Figure 9-7 Stack Ventilation Thermal Depth Limits 74 Figure 9-8 Stack Ventilation Rule of Thumb - Range of Applicability of from a Thermal Comfort Perspective 75 Figure 9-9 Modified Stack Ventilation Rule of Thumb - Range of Applicability from a Thermal Comfort Perspective 77 Figure 9-10 Stack Ventilation IAQ Depth Limits 78 Figure 9-11 Stack Ventilation Rule of Thumb - Range of Applicability of from an IAQ Perspective 79 Figure 9-12 Modified Stack Ventilation Rule of Thumb - Range of Applicability from an IAQ Perspective 81 Figure 9-13 Night Cooling Rule of Thumb Range of Applicability 84 Figure 9-14 Trickle Vent Rule of Thumb Range of Applicability 88 vi 1 Introduction 1.1 Background Non domestic buildings in North America today are primarily designed with the indoor environment isolated from the outdoor environment. The indoor environment is typically controlled by artificial lighting, mechanical ventilation, and mechanical heating and cooling systems. Recently however, building occupants and designers are recognising the need to reduce the impact of buildings on local and global environments, create higher quality indoor working environments, and reduce capital and operating costs. Natural ventilation can assist the achievement of these goals by utilising the outdoor environment to create an acceptable indoor environment whenever it is beneficial. Natural ventilation can be used to provide ventilation for indoor air quality, or summertime cooling, or both. It can provide ventilation or cooling on its own, or as an integral part of hybrid or mix mode systems that use mechanical ventilation or cooling systems as a backup to natural ventilation. Natural ventilation on its own or as part of hybrid systems can have a number of direct benefits to the environment, building occupants, building owners, and architects. These benefits can include: > Reduced energy consumption > Reduced or eliminated use of ozone depleting substances > Improved quality of working environments > Improved indoor air quality and reduced causes of sick building syndrome > Reduced capital and operating costs > Increased level of control for architects over the design of the quality of indoor environments 1.1.1 Reduced Environmental Impacts of Energy Consumption Greenhouse gas emissions from the combustion of fossil fuels are leading to global warming with potentially major negative environmental and economic impacts. The energy used to cool, light, and heat buildings is a large contributor to this problem. As part of the Kyoto Agreement of 1997, Canada has committed to reducing greenhouse gas emissions to 6% below 1990 levels between 2008 and 2012. But by 1997, the latest year for which energy consumption data is available, greenhouse gas emissions from the commercial building sector were approximately 9% higher than in 19901. Energy consumed by buildings has many other negative environmental impacts such as regional air pollution from the combustion of fossil fuels, damage to wildlife habitat from the construction of large hydroelectric dams, and the dangers associated with nuclear power plants and their waste disposal. 1 Energy Efficiency Trends in Canada, Natural Resources Canada, January, 2000. 1 Office buildings in Canada consume on average approximately 35% of their total annual energy for lighting, cooling, and ventilation fan motors. This energy use could be cut by 50% or more in many buildings by the use of integrated natural ventilation, daylighting, and passive cooling designs. Naturally ventilated buildings can reduce energy consumption and the associated negative environmental impacts by reducing or eliminating the energy consumed to drive ventilation fans and operate cooling equipment. Even with hybrid natural ventilation systems, mechanical cooling can be significantly reduced or completely eliminated, and ventilation fan operation can be reduced to only those times when augmentation of natural ventilation airflow rates is required. Energy consumed for lighting, which makes up a large proportion of total energy consumption in office buildings, can also often be significantly reduced in naturally ventilated buildings. While lighting and natural ventilation are not directly connected, when natural ventilation is used to provide cooling in office buildings it is essential that electric lighting energy use be minimised to reduce cooling loads. Lighting energy can also often be reduced due to the inherent narrow plan width of most naturally ventilated buildings, which allows for a greater use of daylighting. Care must be used in designing naturally ventilated buildings to ensure that these energy reduction benefits are not offset by increased energy losses due to excessive airflow rates during the heating season. It is difficult to precisely control natural ventilation flow rates because of fluctuations in wind and outside air temperatures. However it is possible to control these airflow rates in naturally ventilated buildings as long as vents are carefully designed or mechanical ventilation is used during the coldest weather. 1.1.2 Reduced Use of Ozone Depleting Substances CFC and HCFC based refrigerants used in air conditioning systems are damaging the earth's protective ozone layer, leading to negative human health impacts such as increased rates of cancer due to ultraviolet radiation. Naturally ventilated buildings can, in some cases, provide summer cooling without the need for mechanical air conditioning and its associated negative environmental impacts. 1.1.3 Improved Quality of Working Environments Naturally ventilated buildings can produce higher quality working environments that lead to greater worker satisfaction and potentially higher productivity of staff. Since the total operating cost of a building is usually dominated by the salary costs of the building occupants, the economic benefit of improving the quality of the environment for workers is becoming well recognised. A British study2 surveyed 480 office occupiers covering all business sectors and found that 89% preferred buildings which were not air conditioned. 2 7. Ellis, R. "The British Office Market - The Workplace of Tomorrow; the Consumers View", The Harris Research Center, 1994. 2 The most important factors in the design of the building were reported as natural ventilation via opening windows and good daylighting. The provision of user control has also been shown to increase the satisfaction of occupants with the quality of their environment, and natural ventilation typically requires user involvement. A Canadian study3 suggests that there is a link between increased productivity and the occupants' perception of having control over their environment. 1.1.4 Improved Occupant Health Naturally ventilated buildings can improve indoor air quality and eliminate a number of potential causes of sick building syndrome4. They can eliminate indoor air quality problems and noxious smells associated with mold and toxins contained within poorly maintained air conditioners, humidifiers, filters, and other mechanical systems. Properly designed naturally ventilated buildings can also provide higher air change rates than poorly designed naturally or mechanically ventilated buildings. Sound from ventilation units of poorly designed or maintained mechanically ventilated buildings has also been linked to sick building syndrome, and can potentially be eliminated in naturally ventilated buildings. 1.1.5 Reduced Capital and Operating Costs Building capital costs can be significantly reduced through the downsizing or elimination of mechanical ventilation and cooling equipment. Ventilation equipment can be eliminated in all natural ventilation systems, and in hybrid systems can often be replaced with simple extract fans that are used to augment airflow rates. Cooling equipment can often be eliminated, or at least reduced in size, reducing capital costs. Capital costs of lighting equipment are also often reduced in naturally ventilated buildings, either as a result of taking advantage of increased daylighting opportunities with narrow buildings, or as a consequence of improved designs necessitated by the need to reduce cooling loads. The mechanical systems of naturally ventilated buildings can also be simpler and more robust, resulting in lower maintenance costs. In well designed naturally ventilated buildings that eliminate air conditioning, eliminate ventilation systems, or use simpler ventilation systems, there can be fewer failure modes due to the reduction in the number of components that are susceptible to failure. However natural ventilation systems can also be very complex depending on the design of control systems, and have the potential for increasing maintenance costs i f not well designed. 3 Raw, G.J., Roys, M.S., and Leaman, A.J., "Further findings from the office environment survey", Proc. 5th International Conf. Air Quality and Climate, Canada Housing and Mortgage Corporation, Toronto, 1990. 4 Daniels, Klaus, "The Technology of Ecological Building", Birkhauser Verlag, Berlin, 1997. 3 1.1.6 Greater Role for Architects in Designing Quality Indoor Environments Naturally ventilated buildings rely on the design of the building form and building envelope components to control indoor environmental conditions. As a result, they have the potential to return to architects the responsibility that they once had for design of interior environmental control. Until about the mid 19 th century, the practices of building operation and environmental control were an integral part of architectural design. While comfort expectations were much lower than they are today, the form and envelope of buildings were designed by architects to take advantage of local outside environmental conditions for interior climate control, to the extent possible. For example, in temperate climates of Europe, citadels and castles built before the mid 19 th century were designed with small windows and massive structure. The small windows increased comfort in the winter by reducing direct heat loss and air leakage through the openings. While heating systems were poor, the design of the building envelope lowered the heating requirements. In the summer the small size of the windows reduced solar gains, and the massive building structure offered high heat storage capacity. Most thermal energy coming from the outside or produced in the building was absorbed by the building masses. As a result, these buildings had minimal cooling loads and remained cool in the summer. While no additional summer cooling was necessary in these buildings, and draughty building construction provided enough ventilation to meet occupants needs, natural ventilation techniques were eventually developed in Europe to distribute heat throughout buildings. One of the first installations of such a system in a public building was in the Derbyshire Infirmary, completed in 1810, which used a 70 yard underground passageway to preheat air in the winter and cool it in the summer before passing through a furnace and into the rest of the building. To distribute the heat throughout the building and provide sufficient buoyancy usually required massive air ducts, and ventilation towers or stacks. These towers and stacks became common design features in public and private buildings constructed throughout the 19 t h century in Europe. They were generally successfully integrated into the architectural aesthetic considerations of the buildings. Some architects used exhaust towers for expressive purposes. Architect Sir Charles Barry, when designing the new Houses of Parliament in London (built in 1850's) was quite willing to accommodate the great central tower exhaust stack because he admitted that it improved his design. Another architect of the time, J.C. Louden, delighted in the imagery of using intake and exhaust towers for expression. In one instance he designed a group of cottages, all heated by a single fire at the base of a great stack located at the exact centre of the quadrangle complex. In hot arid climates such as Northern Africa the development of building forms in response to local climate conditions was quite different. The tendency in these areas was initially to locate living areas underground to utilise the coolness of the earth, and to further improve thermal comfort by using natural ventilation through the use of buoyancy. By the middle ages buildings grew taller and as solid and massive as they 4 were in Central Europe. Heat gains were minimised by the use of small windows and small roof areas. Natural ventilation techniques became more sophisticated in the use of ventilation shafts to expel warm air at the top of the structure and create a suction to draw cool fresh air into the building from cooler covered entrance areas. Another technique which was commonly used was the use of wind-catchers to capture cooling breezes. In hot humid climates the vernacular architecture which developed relied mainly on the use of natural ventilation for improving comfort levels. Houses were designed to maximise the movement of air through the building, thus providing relief from the heat and humidity by comfort ventilation. Houses were elevated to maximise exposure to prevailing breezes, and were built with large light-mass roofs to provide shading and rain protection, and light mass walls with large openings to maximise cross flow ventilation. With the start of the industrial revolution however, the practice of architecture became increasingly divorced from the practice of environmental control. Industrialisation in the 19th century in Europe brought people together at higher concentrations, produced new industrial wastes that resulted in air pollution inside buildings, and increased demands for better comfort. Raynard Banham, in his book "The Architecture of the Well Tempered Environment"5 argues that at this time the architectural profession failed to take responsibility for the maintenance of decent environmental conditions, and the responsibility fell to everybody from plumbers to consulting engineers. Indoor air quality declined due to increased levels of air pollution inside buildings and outdoors, increased crowding, and reduced air exchange rates. The rising availability of piped coal gas after the middle of the 19 t h century began a great increase in the use of coal gas lanterns. The length of the workday increased and required the unprecedented use of inefficient, polluting artificial light in above ground structures. Other factors degrading indoor air quality were the increased crowding of buildings and the grit and dusts of the urban atmosphere. At the same time heating systems improved substantially and central boilers with heating by steam or hot water became common. This resulted in the disappearance of chimneys in heated spaces, leading to a significant reduction in air exchange rates. Windows and building envelope components also became better sealed to take advantage of the improved heating systems, further reducing sources of accidental ventilation. Medical practitioners were the first to take action in response to health concerns over poor indoor air quality. They were influential in the construction of ventilation systems for public buildings such as hospitals, churches, meeting halls, and theatres, as well as houses. Their writings often reveal an intimate knowledge of the environmental performance of buildings, an expressed contempt for the architectural profession's apparent indifference to such matters, proposals for the improvement of building design, and even the construction of better designed buildings by doctors themselves. 5 Banham, Raynard, "The Architecture of the Well Tempered Environment" University of Chicago Press, 1984, pg9. 5 At the same time ventilation techniques were being developed by industry to provide air supplies in mines to extend extraction limits, evacuate noxious gasses from industry, and to improve industrial processes such as greenhouses and English and Scottish maltings. Fan driven ventilation systems using steam engines were developed in the mid 1800's and were used mainly for ventilating mines, ships, and for industrial processes. Then electric motor driven fans were invented in the 1880's. By the end of the century, mechanical ventilation was seen as a much more reliable method of providing the required ventilation rates, and was becoming common in large buildings. The availability of mechanical ventilation systems (and other technologies) ushered in a new era where technology could be used to help solve design problems that were created by the architectural design. New concepts of space were developing which were the beginnings of the concept of architectural modernity. One of the first such examples is Charles Rennie Mackintosh's Glascow School of Art, built in 1904. The provision of a mechanical ventilation system was necessary to offset the thermal discomfort from the effects of the huge north facing windows in winter. As air conditioning systems became readily available, mechanical ventilation and cooling systems were almost universally adopted as the preferred method of improving comfort, and led to dramatic changes in the built form of buildings. A futurist-inspired belief in a better environment through the exploitation of machine technology became widespread. This led to the interior climates of most commercial buildings being designed by mechanical engineers to exclude the outside environment as much as possible, and relying completely on energy intensive mechanical ventilation, heating, and air conditioning systems. Recent demands for buildings that reduce their environmental impacts, improve the quality of working environments, and allow greater user control (especially openable windows) have led to a re-evaluation of the opportunities for natural ventilation. Architects are currently responding to the push towards meeting these challenges through a renewed interest in the use of naturally ventilated buildings. However, the successful use of natural ventilation requires the integration of building form and building envelope components into the natural ventilation design strategy. Because architects are typically responsible for design of the building form and envelope components, by taking responsibility for design of these components in a naturally ventilated building, they automatically regain control over the design of the interior environment. Instead of handing over the responsibility of adding environmental control systems onto the building after the architectural design is completed, they become responsible for design of both the aesthetic and environmental control qualities of the building. The practices of building operation and environmental control once again become an integral part of architectural design, much as they were prior to the mid 19 t h century. 6 1.2 Need for Simple Design Tools for Architects Recent research into natural ventilation design methods has led to substantial improvements in building design and the ability to predict performance. Advances in computer technology have made possible the simulation of internal air movement and interior temperatures. Computer simulation and building control advances allow large buildings to be designed using natural ventilation to provide effective ventilation and summer cooling while eliminating the traditional drawbacks - such as overheating - that originally led to the decline of natural ventilation. Despite the advances now occurring in design methods, major obstacles still need to be overcome before a wide spread adoption of these technologies wil l be seen: > Design methods are still too complex, time consuming, and expensive to use, and > Architectural practices generally do not have qualified and experienced staff that can carry out complex natural ventilation computer modelling and design work. Simple tools that can be used by architects in the initial stages of design of naturally ventilated commercial buildings are required. Many of the decisions about building form are made very early in the design process and therefore rules of thumb are critical at that stage. Simple natural ventilation design rules of thumb would increase the confidence of architects in proposing natural ventilation strategies for their projects and provide guidance in development of initial building form concepts. This thesis attempts to compile and critique natural ventilation design rules of thumb to make them useful to Canadian architects. 7 1.3 Thesis Objective The objective of this thesis is to identify and critique existing design rules of thumb that can be used in the initial stages of design of naturally ventilated commercial buildings, and use current modelling tools to investigate their validity and limitations when applied to two distinct Canadian climate conditions. Specific objectives are: 1. To identify and critique existing rules of thumb, regulations, and guidelines that may be useful as rules of thumb for the design of naturally ventilated buildings. 2. To develop criteria for evaluating natural ventilation building performance to clarify the minimum acceptable level of performance of naturally ventilated buildings. 3. To use current natural ventilation software modelling tools to investigate the relative importance of key design parameters in their ability to effect performance of naturally ventilated buildings under conditions of two distinct Canadian climates. 4. To use results of the parametric analysis to investigate the applicability and limitations of natural ventilation design rules of thumb for the same two distinct Canadian climates. 1.4 Scope The following defines the scope of the study: > Type of Buildings - Commercial and Institutional Office Buildings (although the results may also apply to other types of buildings with similar form and layout, internal gains, and hours of operation). > Size - Low to mid-rise (1 to 5 stories) > Geographic Region - Two Canadian climates as represented by climates of Vancouver, B C , and Toronto, Ontario. The results of this thesis are primarily directed towards architects and H V A C designers. 8 1.5 Methodology The research process for this study involved the following nine steps: 1. Existing rules of thumb for the design of naturally ventilated office buildings were identified from published international research. Building code regulations, standards, and guidelines that could be presented as rules of thumb were also identified. 2. The primary intent of each rule of thumb and the assumptions and performance criteria used in its development were investigated. 3. Minimal criteria for acceptable performance of naturally ventilated buildings were developed to clarify the minimum acceptable level of performance of naturally ventilated buildings, and develop a performance baseline for investigating limitations of existing rules of thumb. 4. State of the art software modelling tools that could be used to simulate the performance of naturally ventilated buildings according to the performance criteria developed above were identified. The most appropriate modelling tool for meeting the objectives of this thesis was selected. 5. Design parameters were developed for a base case building based on a typical low rise office building in Canada that would be a prime candidate for using natural ventilation. 6. Practical limits of key building design parameters were developed. These limits were used to set boundaries on the degree of modification made to parameters when measuring their impact on natural ventilation performance. 7. A software model of the base case building was created and its performance modelled using reference year climate data for the Canadian cities of Vancouver and Toronto. 8. A parametric analysis was performed to investigate the impact of modification to key building design parameters on natural ventilation performance. Using the software model, building design parameter modification were each applied to the base case building in isolation from one another, and their effect on natural ventilation performance indicators identified. 9. Existing rules of thumb were critiqued by modelling the base case building to find the limitations to combined changes to building design while maintaining minimum acceptable natural ventilation performance levels. The validity of existing rules of thumb for each climate were investigated, and limitations and mitigating factors effecting natural ventilation design identified. 9 2 Fundamental Principals of Natural Ventilation Natural ventilation can be defined as airflow induced by pressure differences between the inside and outside of the building. The driving forces are wind and air density differences. The magnitude and pattern of natural air movement though a building depends of the strength and direction of these natural driving forces and the resistance of the flow path. Natural ventilation is a whole building design concept, centred on the appropriate application of fundamental principals of using wind and temperature induced pressure difference to supply outside airflow to building interiors, even when windows are closed. 2.1 A i r Density Differences Warm air is less dense than cold air, and air density in a column of air decreases with height. If two columns of air at different temperatures are separated by a boundary, there will be a difference in pressure across that boundary due to the different pressure gradients on either side. If it is warmer inside the building than outside, the pressure difference acts inwards at the bottom of the building and outward at the top. When openings are placed in the boundary separating the two air columns, an upward airflow will be created through the building, exhausting warm air at high level and replacing it by cooler air at low level. This is known as stack effect. The pressure difference between each side of the airflow opening is a function of the temperature difference and the height between the opening and the neutral pressure plane. Figure 2-1 Stack Ventilation and the Neutral Pressure Plane The neutral pressure plane is the elevation within the height of the building where inward airflow changes to an outward airflow. In Figure 2-1 the neutral pressure plane is between the second and third levels of the example building. The height of the neutral pressure plane is a function of the density difference between the two air columns and the vertical distribution of openings. Therefore, the reduced driving force at upper floors 10 needs to be counteracted by increasing the area of ventilation openings. In the example shown, the ratio of facade openings between upper and lower floors is too small, resulting in stale air from the first and second levels passing through the occupied space within the third level. This could be avoided by raising the neutral pressure plane above the third level either by increasing the size of the roof vent opening or by reducing the size of the openings on the first and second levels. Stack effects do not only occur over the height of the building. Stack pressures will be exerted over any vertically spaced openings which are inter-connected. For example, in a large window opening, air will tend to flow in at the bottom of the window and out at the top. This creates an air exchange mechanism for the room, even in single sided ventilation configurations. In a building designed to promote stack induced flows, these local stack effects wil l be superimposed on the overall pattern of air movement. As inside and outside temperatures become equal the stack pressure approaches zero and there is no driving force for ventilation. The worst case scenario for a stack driven ventilation strategy is therefore during times when the outside air temperature is equal to the inside temperature. 2.2 Wind Wind driven ventilation is caused by differences in wind pressures acting across the external surfaces of the building. Air will flow through a building from areas of high surface pressure to areas of low pressure. The magnitude of the wind pressure at any location on the surface of the building depends on the wind velocity at that location and increases as the square of the wind velocity. The wind velocity at each location and the resulting distribution of wind pressures acting on the building depend on: > The shape of the building > Height above ground of the location of interest > Roughness of the terrain surrounding the building > Local wind obstructions adjacent to the building A wind pressure coefficient is a number used to represent these effects for each location on the surface of the building for a single wind direction. The shape of the building and the direction of the wind in relation to the building effect the magnitude and direction of the wind pressure at each location on the building surface. Generally, building surfaces facing into the wind will experience positive pressures, and leeward surfaces and those parallel to the wind direction will experience negative pressures. However the magnitude and distribution of wind pressures vary greatly depending on the overall building shape and localised variations in building form . This provides an opportunity for the architectural form and detailing to enhance the potential for wind driven ventilation. 11 Wind velocity increases with height above ground due to the slowing of wind speeds near the earth's surface (the boundary layer effect). The type of terrain surrounding the building (eg. open country versus city centre) effects the boundary layer profile, and therefore wind speeds at the building site. The presence of local wind obstructions (other buildings, trees, etc) also effect local wind speeds. This provides an opportunity for site layout and landscaping to enhance wind driven ventilation. Careful orientation of a building in relation to the topography of the site can maximise the potential for wind driven ventilation. The two main strategies for applying wind driven ventilation to building design are: > Single sided ventilation, and > Cross ventilation. Single sided ventilation relies on airflow through openings on only one side of an enclosed space. With a single ventilation opening in the room the main driving force for natural ventilation is wind turbulence. Compared to other strategies, lower ventilation rates are generated and the depth of penetration of the airflow into the space is limited. If more than one ventilation opening is provided and openings are located at different heights, the ventilation rate can be enhanced by stack effect. Stack induced flows increase with the vertical separation of openings. If more than one ventilation opening is provided and openings are separated by a horizontal distance, the ventilation rate is enhanced by flow from one window to the other due to the difference in wind pressures coefficients at the location of each window. Cross ventilation relies on airflow between ventilation openings on opposite sides of a space. As the air moves across a cross ventilated space it picks up heat and pollutants. Therefore there is a limit to the depth that can be cross ventilated, which generally leads to buildings with a narrow plan depth. The worst case scenario for wind driven ventilation strategies is obviously during calm wind conditions. 2.3 Combined Stack and Wind Effects The most common situation is that both wind and stack pressures act on a building simultaneously due to the existence of non zero wind speeds, and outside air temperatures that are not equal to indoor air temperatures. Rates of airflow may be alternatively dominated by one effect or the other depending on the magnitude of outside air temperature and wind speed. Cross ventilation is a wind driven ventilation strategy, but under most climate conditions wind driven airflow rates are enhanced or reduced by stack effect ventilation acting over the floor to ceiling height of individual levels, or over the entire height of the building. 12 The air change rate of cross ventilated buildings is effected by stack ventilation acting over the floor to ceiling height of individual floors. Rates of airflow into the building are increased at openings near the bottom of the room height and rates of airflow out of the building are increased at openings near the top of the room height whenever the outside temperature is less than the inside temperature. The overall effect is a floor to ceiling height dependent increase in air change rates above that expected for wind driven cross ventilation alone. The stack effect acting over the entire height of the building also effects cross ventilation airflow. It will increase or decrease rates of airflow into the building depending on the location of the airflow opening. On lower levels the stack effect due to colder outside air temperatures will increase rates of airflow into the building on the windward side of the building, and reduce airflow out of the building on the leeward side. On upper levels above the neutral pressure plane the stack effect wil l reduce rates of airflow into the building on the windward side of the building, and increase airflow out of the building on the leeward side. Stack effect acting over the entire height of the building occurs in all buildings, but the magnitude of the effect is reduced significantly in buildings that are well sealed between floors. It is not possible to reduce the building height stack effect completely because even in well sealed buildings there is some airflow into stairway and elevator shafts through imperfect seals, and opening and closing of doors. The building height stack effect is greatest in buildings with large airflow openings between floor levels such as in buildings with each floor connected to a central atrium. In buildings with combined stack and wind effects, stack pressure differences are reduced with increasing height, moving towards the height of the neutral pressure plane. However, this effect may be partially compensated for by the increasing wind pressure at upper levels due to increasing wind speed with height. 13 3 Existing Natural Ventilation Design Rules of Thumb, Regulations, and Standards Published research papers and design guidelines were reviewed to identify existing natural ventilation design rules of thumb. Building regulations and standards were also reviewed to identify accepted criteria or guidelines that could be presented as rules of thumb. The following rules of thumb have been selected for further analysis. They are all simple to use formulas based on common building component dimensions, and can be used at the initial stages of the building design process without requiring in-depth performance modelling or analysis. For each rule selected, the assumptions and performance criteria used in its development were first investigated to discover how the rule of thumb was intended to be used and what, i f any, are the intended limitations on its applicability. The selected rules of thumb cover the following types of natural ventilation design strategies: > Cross ventilation > Stack ventilation > Night cooling ventilation, and > Ventilation for indoor air quality The majority of selected rules of thumb originate from recent publications and design guides produced by building research organisations located in Northern European countries. Very little research into design of naturally ventilated buildings has been carried out in North America, resulting in limited North American based design guidance. The one exception to this is indoor air quality requirements for naturally ventilated buildings provided by existing or developing North American building standards and codes. In general, although many of these rules of thumb exist in either building codes and design guidelines, the assumptions and performance criteria used to develop them are not explicitly defined. As a result, the conditions under which the rules may or may be applicable are also not explicitly defined. Only one rule of thumb selected for evaluation in this study explicitly states the assumptions used in its development and conditions limiting its validity. 14 3.1 Rule 1 - Cross Ventilation The Chartered Institution of Building Services Engineers (CIBSE) in the U K recommends the following rule of thumb for the design of cross ventilated buildings6: Cross ventilation is effective up to five times the floor to ceiling height. Building Depth = 5x1-1 • I Figure 3-1 Cross Ventilation Rule of Thumb The conditions under which this rule was intended to apply are not explicitly defined. The CIBSE Applications Manual references development of this rule to research presented in B R E Digest 3997. However, the only condition for this rule that is stated explicitly in the B R E Digest is that it applies to buildings with moderate to high heat gains - i.e., heat gains from people, lighting, and equipment falling into the range of 20 to 50 W/m 2 . If the internal heat gains are lower then deeper spaces can be ventilated naturally. Because the main condition associated with the rule is that it applies to buildings with moderate to high heat gains, the intention of the rule must be to address the issue of appropriate building design to avoid summertime overheating. B R E Digest 399 implies that adherence to this rule of thumb will satisfy ventilation requirements and avoid overheating in U K climates, but does not explicitly state what ventilation airflow rate and internal temperature limits are used for acceptable performance criteria. It also does not state the method used to develop the rule of thumb but implies that it was developed using simple equations contained in CIBSE Guide A 8 and BS 59259 to estimate cross ventilation airflow rates. The CIBSE Applications Manual states that research in B R E Digest 399 is based on wind speed and temperature conditions from south east England, where the external summer design temperature is 27°C. 6 "Natural Ventilation in Non Domestic Buildings, Applications Manual AM10", CIBSE, London, 1997. 7 "Natural Ventilation in Non Domestic Buildings", BRE Digest 399, Building Research Establishment, GarstonUK, 1994. 8 "Environmental Criteria for Design, CIBSE Guide Al" , Chartered Institution of Building Services Engineers, London, 1978. (Reprinted 1986) 9 "Code of Practice for Ventilation Principals and Designing for Natural Ventilation, BS 5925", British Standards Institution, 1991. 15 In another B R E research paper, Walker and White 1 0 state that this rule of thumb is based on guidance from the CIBSE Guide 1 1. The CIBSE guide advises that air distribution will be 'reasonable' in naturally ventilated buildings with room depths of up to 6 m on either side of a central corridor, resulting in an effective limit for the depth of a cross ventilated building of about 15m. Walker and White investigate the air distribution in deeper plan offices and show that airspeeds and unmixed air currents generally only penetrate to a depth of about 6 m into a space from exterior windows. But they also find that the air is well distributed and local ventilation rates are evenly spread in much deeper spaces due to mixing. They conclude that the rule of thumb depth limit can be significantly extended beyond 6m i f air distribution within the space alone is considered, but that the overall ventilation rate and overheating must also be taken into account. An important aspect of this rule of thumb is that the depth limit is based on the floor to ceiling height of the naturally ventilated space. The CIBSE applications manual that presents this rule does not provide an explanation for why the rule is based on floor to ceiling height. It states that "increased floor to ceiling heights wil l increase stratification in the space which can then lift the pollutants above the occupied zone". It is easy to understand how increased floor to ceiling height will increase temperature stratification and therefore increase the maximum overheating depth limit due to the concentration of. hot air above the occupied region of the space. But it is difficult to understand how raising the floor to ceiling height will lift pollutants above the occupied zone. Increased floor to ceiling height stack effects will result in higher air exchange rates that wil l extend the building depth limit by reducing interior temperatures and increase ventilation for indoor air quality, but they make no mention of these effects. 1 0 Walker, R.R., and White, M.K., "The Efficiency of Single-Sided and Cross Ventilation in Office Spaces", Building Research Establishment, Garston, England, 1996. 1 1 "Chartered Institution of Building Services Engineers Guide: Part B2 - Ventilation and Air Conditioning (Requirements)", CIBSE, London, 1988. 16 3.2 Rule 2 - Stack Ventilation The Chartered Institution of Building Services Engineers (CIBSE) , also recommends the following rule of thumb for the design of buildings that make use of stack ventilation: Stack ventilation can be effective across a width of 5 times the floor to ceiling height from the inlet to where the air is exhausted to the stack. 5 x H < • 5 x H <———w Figure 3-2 Stack Ventilation Rule of Thumb This rule is based on a building with a large stack or atrium running down the centre of the building and spaces on either side of the stack having a maximum depth limit of 5 times the floor to ceiling height of individual levels. For example, a building with a floor to ceiling height of 3m on each level would be limited in depth to 15m between the perimeter of the building and the stack. This results in a maximum depth building of 30m plus the depth of the central stack or atrium. The CIBSE Applications Manual states that the depth limit on either side of the stack is based on the effective limit for the width of a cross ventilated building as discussed in Section 3.1. Thus the conditions under which this rule applies are the same as for the cross ventilation rule - buildings with moderate to high heat gains (20 to 50 W/m 2 heat gains from people, lighting, and equipment), and wind speed and temperature conditions from south east England, where the external design temperature is 27°C. "Natural Ventilation in Non Domestic Buildings, Applications Manual AM10", CIBSE, London, 1997. 17 3.3 Rule 3 - Night Cooling Ventilation The European NatVent Project is a seven nation consortium of organisations that studied ways of overcoming technical barriers to low-energy natural ventilation in office-type buildings in moderate and cold climates. They developed the following rule of thumb for the design of naturally ventilated buildings that use night cooling ventilation to maintain adequate summertime thermal comfort levels": A vent opening of 2% of the floor area (200 cm per m floor area) should generally be adequate for night cooling when there is cross ventilation. As a key activity of the NatVent Project, Van Paassen, Liem, and Groninger14 developed a computer model - SIMULINK - to simulate the ventilation processes and thermal behaviour of office buildings that use night cooling ventilation for summer comfort. They used the model to investigate two aspects of night cooling ventilation - required night cooling vent opening areas, and night cooling vent control strategies - to maintain acceptable thermal comfort in one climate of the Netherlands. Based on the outputs of a large series of simulation runs, they developed two "user friendly" design tools: a stand-alone graphical chart, and a spreadsheet that uses a set of simplified equations. Both tools quickly enable the determination of the required effective (i.e. unobstructed) ventilation opening areas for night cooling based on key building design parameters. Two criteria were used to determine minimal acceptable thermal performance: > that there be no more than 100 hours in a year with an internal temperature above 25.5°C, and > no more than 25 hours in a year with an internal temperature above 28°C. Results of the study show that minimum night cooling vent opening areas required to avoid overheating in the Netherlands vary between approximately 1% and 3% of the floor area depending on internal heat gains, glazing areas, shading strategies, and control strategies. They concluded that the optimal night cooling strategy was to use a night cooling vent area of 2% of the floor area combined with the following limitations on key design variables: > Limited internal heat gains from people, lighting, and equipment - a maximum of 33 W/m 2 for high exposed interior thermal mass buildings and 26 W/m 2 for medium exposed interior thermal mass buildings. > Maximum glazing area of 40% of facade area > External shading of windows by louvers or other shading devices > Automatic control of night cooling vents 1 3 "Natural Ventilation for Offices", NatVent Project Report, BRE and the NatVent Consortium, Garston UK, 1999. 1 4 Van Paassen, AHC, Liem, SH, Groninger, BP, "Control of Night Cooling with Natural Ventilation -Sensitivity Analysis of Control Strategies and Vent Openings", Laboratory of Refrigeration Engineering and Indoor Climate Technology, Faculty of Design, Engineering and Production, Delft University of Technology, Holland, 1998. 18 3.4 Rule 4 - Ventilation for Acceptable Indoor Air Quality - United Kingdom Both the CIBSE Applications Manual "Natural Ventilation in Non Domestic Buildings" 7 5 and the European NatVent Project Report7* recommend the following rule of thumb for the control of winter ventilation rates for indoor air quality: 2 2 Trickle ventilators with an openable area of 4 cm per m of floor area, with a minimum provision of 40 cm in each room, should adequately provide the necessary background ventilation to meet occupants needs. This rule has been adopted from "The Approved Document for Part F of the U K Building Regulations"17, and is required in many European building codes. It is based on the provision of approximately 5 L/s per person of 'background ventilation' during winter months under climate conditions found in the U K . It is not intended that the trickle ventilators supply rates of fresh air that meet all occupant and pollutant loads at all times, but that they are used in conjunction with other types of ventilation opening such as operable windows. The 24 hr use of trickle ventilation is intended to provide a reservoir of fresh air which may be sufficient to maintain air quality throughout the day. However during periods of high pollutant loads ,'rapid ventilation' by opening windows or with mechanical ventilation may be required. 3.5 Rules 5a,b,c - Ventilation for Acceptable Indoor Air Quality - North America Three rules of thumb for achieving acceptable ventilation rates for indoor air quality can be derived from the prescriptive requirements for naturally ventilated buildings that are currently being reviewed as a possible addendum to A S H R A E Standard 62-1999, "Ventilation for Acceptable Indoor Air Quality."7* The draft addendum, entitled "Addendum j " replaces the current performance requirement for natural ventilation systems with a prescriptive requirement that is similar to many model building codes. For naturally ventilated buildings to achieve acceptable indoor air quality the draft addendum suggests adherence to the following requirements for size and location of natural ventilation openings: Naturally ventilated spaces shall be permanently open to and within 8 m (25 ft) of operable wall or roof openings to the outdoors, the openable area of which is a minimum of 4% of the net occupiable floor area. Where openings are covered with louvers or otherwise obstructed, openable area shall be based on the free unobstructed area through the opening. Where interior spaces without direct openings to the 1 5 "Natural Ventilation in Non Domestic Buildings, Applications Manual AM10", CIBSE, London, 1997. 1 6 "Natural Ventilation for Offices", NatVent Project Report, BRE and the NatVent Consortium, Garston UK, 1999. 1 7 "The Building Regulations, Approved Document F: Ventilation", London: HMSO, 1995. 1 8 "ASHRAE Standard 62 - 1999", American Society of Heating, Refrigerating and Air-Conditioning Engineers Inc., Atlanta, Georgia, 1999. 19 outdoors are ventUated through adjoining rooms, the opening between rooms shall be permanently unobstructed and have a free area of not less than 8% of the area of the interior room nor less than 2.3 m2 (25 ft2). An exception to this rule is provided that removes requirements for the size and location of openings for 'engineered' natural ventilation systems that have been approved by the building permit authority having jurisdiction. This exception is added to "allow specifically engineered systems such as those that use wind power, stack effect, and other natural forces to move air through conduits other than windows". The exception also may be used to show compliance for a room that exceeds 8m in depth but which can be shown to be "sufficiently ventilated" by perimeter windows. The requirements for "sufficient ventilation" are not defined. Three separate rules of thumb can be derived from the above prescriptive requirements: Rule 5a - Naturally ventilated building spaces should be limited to a depth of 8m from exterior wall or roof openings. The first part of the prescriptive requirements states that naturally ventilated building spaces should be limited to a depth of 8m from exterior wall or roof openings. This would imply that the depth of a typical office building with offices on either side of a central corridor should be limited to 16m (or possibly as much as 18m with the addition of 2m for a corridor). This requirement is similar to Rule 1 presented earlier, which limits the depth of a cross ventilated building to approximately 15m, assuming a floor to ceiling height of 3m. However, the intention of this requirement is to address the issue of ventilation for acceptable indoor air quality while the intention of Rule 1 is to address the issue of avoidance of overheating (and although not explicitly stated, possibly acceptable indoor air quality as well). The proposed "Addendum j " allows for extension beyond the 8m depth limit i f an 'engineered' natural ventilation design solution is utilised, such as those that use "wind pressure, stack effect, and other natural forces to move air through conduits other than windows." This means that the rule of thumb would apply to buildings that do not contain trickle ventilators or are specifically designed to take advantage of wind or stack effects, other than meeting the minimum requirements for opening areas in the exterior envelope and interior partitions. Therefore the rule implies that buildings that use engineered natural ventilation design solutions such as trickle ventilators could achieve ventilation for adequate indoor air quality at greater building depths. Rule 5b - Openable areas in exterior walls and roofs should be at least 4% of the net occupiable floor area The second rule of thumb requires an openable area in exterior walls and roofs of at least 4% of the net occupiable floor area. This openable area requirement is similar to 20 ventilation requirements for naturally ventilated residential and commercial buildings that are found in building codes of many North American and European countries. For example, building code regulations in England and Wales require that conventional cellular offices should have an openable window area of l/20th (5%) the floor area. The Canadian 1995 National Building Code has a similar requirement for residential buildings but not commercial buildings (See Section 4.1 for a more thorough discussion of Canadian building code requirements for naturally ventilated buildings). While the requirement for a minimum openable window area is the most commonly found rule of thumb for naturally ventilated buildings, and is often the only requirement found in building codes, its usefulness for designers of naturally ventilated buildings is limited. It addresses the amount of 'rapid ventilation' that can be achieved by opening windows but does not address the supply rates of fresh air required to meet occupant and pollutant loads at times when windows are closed. Rule 5c - Interior partitions or walls between naturally ventilated spaces and outside ventilation openings should have permanent openings of at least 8% of the floor area of the interior portion of the space, with a minimum opening area of 2.3 square meters. The third rule of thumb that can be derived from the proposed addendum is that interior partitions or walls between naturally ventilated spaces and ventilation openings to the exterior should have permanent openings of at least 8% of the floor area of the interior portion of the space, with a minimum opening area of 2.3 square meters. As a result, an opening equivalent in size to that of a large single doorway (2m x 1.15m) would meet the minimum required opening area. The minimal 2.3 m 2 opening would meet the requirements for office spaces that have up to approximately 29 m 2 of floor area contained on the interior side of the interior partition (See Figure 3-3). Larger spaces on the interior side of the partition require larger openings in interior partitions (equal to 8% of the floor area). Up To 29rrf Floor Area Behind Partition Minimum of 2.3 rrf Opening in Internal Partition Exterior Wall Airflow Openings Figure 3-3 Minimum Airflow Opening Area Required in Internal Partitions 21 4 Natural Ventilation Performance Criteria For any natural ventilation design rule of thumb to be useful, the criteria used to define a minimum acceptable level of natural ventilation performance must be established, in addition to the design conditions under which the rule can be applied. Existing natural ventilation rules of thumb presented in the previous chapter state that "ventilation is effective", "cooling is adequate", or "background ventilation is adequate" under a defined set of building design conditions. Only one rule, the night cooling ventilation rule of thumb, explicitly states the criteria used to define what is meant by an adequate, effective, or sufficient ventilation rate or level of natural ventilation performance. It is critical to define natural ventilation performance criteria when computer modelling tools are used to investigate the importance of design parameters on natural ventilation performance, or to identify limitations to rules of thumb. It is also valuable to investigate what is considered to be an accepted minimum level of performance for naturally ventilated buildings, and the variance in what is considered acceptable between North America and Europe, since the rules of thumb investigated in this study are mainly based on European design guidance. 4.0.1 Functions of Natural Ventilation First it is essential to clearly identify what is meant by the term 'naturally ventilated building', and to make a distinction between types of naturally ventilated buildings and their intended functions. There are two main functions of naturally ventilated buildings: 1) To provide "adequate" indoor air quality by supplying air to occupied spaces and removing and/or diluting pollutants. This concept is called 'Natural Ventilation for Indoor Air Quality (IAQ) Control'. Air quality is controlled through the use of ventilation supply and exhaust openings (e.g., trickle ventilators), using wind and stack effect as the primary driving forces. No fan energy is required to provide ventilation (unless mechanical extraction is applied in hybrid systems) but during the heating season energy is needed for heating the supplied outside air. Supplied airflow rates depend on wind and temperature conditions and can vary over time. Design optimisation is essential to combine "good" indoor air quality with low energy demand. 2) To provide a mechanism to remove excess heat gain from inside the building and provide adequate summer thermal comfort to occupants. This concept is called 'Natural Ventilation for Control of Summer Overheating'. Cooling can be achieved by one of three methods: a) Using natural ventilation to maximize ventilation rates of outside air when the outside air temperature is below the indoor air temperature. b) Using natural ventilation to increase indoor air speeds. This method is called "comfort ventilation". Increasing the flowrate of the air increases the upper 22 limit of the comfort zone for still air conditions, and can provide a direct physiological cooling effect even when the air is rather warm, up to a limit of about 32° C. Comfort ventilation is also effective when humidity levels are high since the higher air speeds increase evaporative cooling of sweat on the skin thus minimising the discomfort of wet skin. c) Cooling a high thermal mass building at night by ventilating the building with cool night air. This method is called "nocturnal cooling" or "night cooling." The cooled thermal mass serves as a heat sink, so that the building heats up much more slowly during the daytime when exposed to solar gains, internal ' gains, and high outside air temperatures. The interior air temperature is lowered through convection with the cool surfaces and the cooled building thermal mass also provides a radiant cooling benefit. With the radiant cooling benefit the same level of thermal comfort is achieved at a higher air temperature level than a building with a moderate air temperature and surfaces at the same temperature. Naturally ventilated buildings may be designed to serve one or both of these two functions. In summer cooling strategies, the required air flow rates are of a higher order of magnitude than those required for IAQ control. As a result, the required openings are also of a higher order of magnitude in size. Generally, summer cooling airflow rates are designed to be as high as possible, within the limits of problems caused by drafts from high airspeeds or under-cooling in the early morning hours. Precise control of the air flow rate for cooling is much less important than required for IAQ control, because energy for heating of outside air is not required. If the building is designed to perform both functions, it is generally easier to achieve greater control over performance i f separate airflow openings are provided for each of the two functions (e.g. openable windows and large vents for summertime control of overheating and trickle vents for IAQ ventilation). 4.0.2 En ergy Efficien cy It could be argued that a third main function required of natural ventilation systems is to reduce building energy consumption below that of mechanically cooled and ventilated buildings. However, while improved energy efficiency is certainly a desirable benefit, naturally ventilated buildings can provide other benefits without increasing the level of energy efficiency over mechanically cooled and/or ventilated buildings. But naturally ventilated buildings that do not provide ventilation for adequate indoor air quality or avoid overheating are not meeting basic requirements expected for all buildings. It is also difficult to set a minimum performance requirement for increased energy efficiency because the potential to reduce energy consumption depends greatly on the characteristics and performance of a reference building against which a naturally ventilated building is being compared. For example, a naturally ventilated building that provides summer cooling without air conditioning will significantly reduce energy consumption over an air conditioned reference building, but much less over a non air conditioned reference building. 23 4.1 Indoor Air Quality (IAQ) Performance Criteria When the intended function of a naturally ventilated building is to provide adequate indoor air quality, the criteria used to measure natural ventilation performance is the rate of supply of outdoor air, measured in litres per second (L/s). However, due to inevitable fluctuations in wind speed and outdoor air temperature, natural ventilation systems cannot provide a constant flow rate of ventilation air at all times. Therefore criteria for determining acceptable indoor air quality performance are more difficult to define than for mechanically ventilated buildings. 4.1.1 ASHRAE Standard 62 - 1999, Ventilation for Acceptable Indoor Air Quality In North America, the most widely accepted standard for designing ventilation systems to achieve acceptable indoor air quality in buildings is ' A S H R A E Standard 62-1999, Ventilation for Acceptable Indoor Air Quality'. A S H R A E Standard 62-1999 sets out prescriptive requirements for minimum ventilation rates in mechanically ventilated buildings. For mechanically ventilated buildings, it requires that one of the following two procedures be used to demonstrate that "adequate" ventilation rates have been provided to minimise the adverse health affects of indoor air contaminants: a) Ventilation Rate Procedure - Acceptable air quality is achieved by providing ventilation air of a specified quality and quantity to the space. Table 2 in the Standard specifies minimum flowrates of outside air for various types of building use. For office spaces, a minimum outdoor air flowrate of 10 L/s per person is required. b) Indoor Air Quality Procedure - Acceptable air quality is achieved by providing ventilation air that will control known and specifiable contaminants to acceptable concentration levels. The Standard does not specify a minimum fixed ventilation rate, but requires the designer to calculate a minimum ventilation rate that will maintain the contaminants at acceptable levels. A number of contaminants are listed in Table 3 of the Standard with recommended maximum concentration levels. Other contaminants are listed for which precise limits have not been set, and adequacy of control must rest on subjective evaluation. The indoor air quality procedure could result in a ventilation rate lower than would result from the ventilation rate procedure, but the presence of a particular source of contamination may result in increased ventilation requirements. The only requirement in the current Standard for naturally ventilated buildings is that "when natural ventilation and infiltration are relied upon, sufficient ventilation shall be demonstrable". "Sufficient ventilation" for naturally ventilated buildings is not explicitly defined in the 1999 Standard. 24 However, in 1993 A S H R A E published an official interpretation19 in response to the explicit question of what is meant by "sufficient ventilation" for naturally ventilated buildings. Unfortunately, the wording of the interpretation is confusing, and does little to clear up what is meant by "sufficient ventilation" for naturally ventilated buildings. The interpretation says that for a naturally ventilated building to have sufficient ventilation, it does not have to meet the ventilation rates specified in Table 2 (which requires 10 L/s per person for office spaces), but does have to maintain the concentration level of contaminants within limits provided in Table 3. That is, it does not have to meet the requirements of the Ventilation Rate Procedure (requiring a minimum constant ventilation rate) but does have to meet the requirements of the Indoor Air Quality Procedure (setting out maximum allowable concentrations of contaminants). The Indoor Air Quality Procedure does not help in setting indoor air quality performance criteria in naturally ventilated buildings. To use the indoor air quality procedure, the rate of release of a fixed number of contaminants, which will vary from building to building, must be specified. Then a fixed ventilation rate must be calculated that wil l dilute contaminants to acceptable concentration levels. But currently there exists no standard rate of contaminant release for buildings, and therefore it is not possible to specify minimum acceptable ventilation rates that could be used as criteria for adequate indoor air quality in naturally ventilated buildings. The interpretation further states that demonstration of ventilation rates specified in Table 2 of the Standard is another acceptable method of demonstrating sufficient ventilation. That is, the Ventilation Rate Procedure can be used i f so desired. But the ventilation rate procedure specifies a fixed ventilation rate, and changes in wind speed and outdoor temperature cause the ventilation rate in naturally buildings to fluctuate greatly. In an attempt to clarify how the designer could show that the ventilation rates of Table 2 could be met in a naturally ventilated building, the interpretation says that "acceptable means of demonstrating natural ventilation include the infiltration methods described in Chapter 23 - Infiltration and Ventilation - of the 1993 A S H R A E Handbook -Fundamentals". The methods in the A S H R A E Handbook do describe how to calculate naturally induced airflow rates in buildings based on wind, stack, and other driving forces. But they do not describe how to turn a fluctuating natural ventilation flow rate into a single representative ventilation rate that could be compared to the fixed ventilation rates of Table 2. The ventilation rate could be compared to a fixed ventilation rate i f a procedure was specified for calculating a representative "design day" natural ventilation rate, or for physically measuring ventilation rates over a period of time in constructed buildings, or for calculating a representative ventilation rate in some other way. Neither the A S H R A E Standard nor its interpretation clarify such a procedure. 1 9 "Interpretation IC 62-1989-8 of ASHRAE Standard 62-1989 Ventilation for Acceptable Indoor Air Quality", American Society of Heating, Refrigerating and Air-Conditioning Engineers Inc., Atlanta, Georgia, September 22, 1993. 25 The interpretation next goes on to say that "Acceptable means of demonstrating openable areas to the outdoors for natural ventilation are given in the model building codes." This statement also does not clarify what is meant by "sufficient ventilation". Prescriptive requirements for openable ventilation areas do not help define performance based criteria for adequate natural ventilation rates. Finally, the interpretation states that "Documentation of a background of successful natural ventilation experience in similar buildings and building uses could also be considered suitable demonstration." This means that sufficient ventilation is considered to be provided i f a building's design and use is similar to that of a building with documented "successful natural ventilation experience". But once again "successful ventilation experience" is not defined. And because the Standard does not clearly show how to meet the requirements for sufficient ventilation with the Ventilation Rate Procedure, the Indoor Air Quality Procedure must be used to provide evidence that the documented building maintains contaminant levels below maximum allowable concentration levels. The current performance requirement for natural ventilation systems specified by the Standard - i.e. demonstration of sufficient ventilation - is recognised by A S H R A E as being difficult for designers to understand and use, and difficult to enforce. As a result they are considering replacing it with a prescriptive requirement, as was discussed previously in Section 3.5. However, the proposed prescriptive requirement also does not aid in setting criteria for minimum IAQ performance in naturally ventilated buildings because it is prescriptive rather than performance based. 4.1.2 Canadian Building Codes Building codes are the other main source of guidance for ensuring adequate indoor air quality performance in naturally ventilated buildings. Most current provincial and local building codes in Canada are based on the 1995 National Building Code of Canada (NBC). For commercial buildings, Part 6 of the N B C specifies that outdoor air supplied by ventilation systems must not be less than the rates required by A S H R A E Standard 62. Then, specifically addressing naturally ventilated buildings, the N B C states that this ventilation can be provided by natural ventilation for commercial buildings with an occupant load of not more than one person per 40 m 2 during normal use. Therefore, natural ventilation is only allowed for low occupancy office buildings, and the rate of natural ventilation has to be at least that required by A S H R A E Standard 62. For large multi unit residential buildings. Part 6 of the N B C states that it does not allow natural ventilation as the method of supplying ventilation air. It says that they must use some form of mechanical ventilation, however it does not explicitly require mechanical ventilation supply systems, and bathroom and kitchen exhaust fans supplemented with natural ventilation are commonly used and generally considered to meet this requirement. For single family buildings. Part 9 of the N B C allows natural ventilation in the non heating season only, and requires mechanical ventilation in the heating season. For the 26 non heating season it specifies minimum unobstructed openable areas to the outdoors, based either on the floor area of the space to be ventilated, or the design number of occupants per space, depending on the type of space. For example, it requires 0.28 m 2 of unobstructed openable ventilation area per room for residential bedrooms, kitchens, dining rooms, and living rooms. This approach addresses the amount of 'rapid ventilation' that can be achieved by opening windows but does not explicitly address the supply rates of fresh air required to meet occupant and pollutant loads at times when windows are closed. Therefore it provides little direction in setting performance based criteria that can be used to evaluate acceptable indoor air quality in naturally ventilated buildings. 4.1.3 European Building Codes and Design Guidelines Many European building codes contain requirements for naturally ventilated commercial buildings that set minimum openable areas in exterior walls. These requirements are valid for both heating and non heating seasons and are similar to Canadian requirements for single family residential occupancies in non heating seasons. For example, regulations for England and Wales specify openable window areas of l/20 t f iofthe floor area for conventional cellular offices . European design guidance, such as that contained in the CIBSE applications manual for natural ventilation in non-domestic buildings, recommends that an (unspecified) medium term time averaged ventilation rate of fresh air be provided. This means that the fresh air rate can vary without a significant change in indoor air quality due to the reservoir effect provided by the space. For example, trickle ventilators with a minimum opening size of 4 cm 2 per m 2 of floor area are commonly required by European building codes27. This requirement is based on the provision of approximately 5 L/s per person of 'background ventilation' during winter months in the U K 2 2 . A 24 hour period of background ventilation is intended to fully purge the air in a space overnight when an office is unoccupied and provide a reservoir of clean air that, with the continuing background flow of incoming fresh air, may be sufficient to maintain acceptable indoor air quality throughout the day. In spaces with high levels of internal air contaminant generation, it may be necessary to provide rapid ventilation by opening windows for short periods to replenish the reservoir. 4.1.4 IAQ Minimum Performance Criteria Adopted In this study, a software model was used to calculate the rate of outside air flowing though an office space based on wind and stack pressures induced by outdoor wind speeds and temperature. To address the issue of IAQ airflow rates varying over time, the 2 0 "Natural Ventilation for Offices", NatVent Project Report, BRE and the NatVent Consortium, Garston UK, 1999, p8. 2 1 "The Building Regulations, Approved Document F: Ventilation", London: HMSO, 1995. 2 2 "Natural Ventilation in Non Domestic Buildings, Applications Manual A M 10", CIBSE, London, 1997, p 37. 27 building was modelled for every hour of an entire year using weather data that is considered to be representative of a typical year for the cities of Vancouver and Toronto. Since North American minimum performance criteria for acceptable ventilation rates for IAQ in naturally ventilated buildings are unclear, a very conservative approach was adopted for this study - the minimum acceptable ventilation rate criteria for mechanically ventilated office buildings from A S H R A E Standard 62-1999 of 10 L/s per person. There will of course be extreme instances where winds are calm and there is no indoor to outdoor temperature difference, and thus no driving forces for natural ventilation. Some allowance should also be made for the ability of the reservoir effect to dampen the impact of reduced ventilation rates during these times. Therefore, a conservative cut-off of 1% of occupied office hours was chosen as a maximum allowable number of hours for which the naturally induced ventilation rate could fall below 10 L/s per person. 28 4.2 Summer Overheating Performance Criteria When a building is in a cooling mode, the criteria used to measure natural ventilation performance is acceptable thermal comfort. 4.2.1 ASHRAE Standard 55 - Thermal Environmental Conditions for Human Occupancy In North America, the most widely adopted standard for establishing thermally acceptable indoor environments is A S H R A E Standard 55, "Thermal Environmental Conditions for Human Occupancy." This standard specifies conditions or comfort zones where 80% of sedentary or slightly active persons find the environment thermally acceptable, as shown in Figure 4-1 below2 . The extreme maximum acceptable indoor temperature that falls within the comfort zone is an operative temperature of between 26°C and 27°C, depending on relative humidity levels. Operative temperature is defined as the average of the mean radiant and ambient air temperatures, weighted by their respective heat transfer coefficients. It provides a more accurate measure of comfort than air temperature alone, because it also takes into account the effect of surface temperature and humidity. The comfort diagram also shows the upper acceptable comfort limit in terms of an effective temperature of 26°C. The effective temperature (ET*) combines temperature and humidity into a single index. Figure 4-1 ASHRAE Summer and Winter Comfort Zones 2 3 ASHRAE, "1997 ASHRAE Handbook - Fundamentals", American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Inc. Atlanta, Georgia, 1997. 29 Two environments with the same ET* and same level of air movement should evoke the same thermal response, even though they have different air temperatures and humidity levels. Because thermal comfort limits are defined in terms of operative temperature, air temperatures of greater than 27°C remain within the comfort zone i f the radiant temperature of the space is lowered by cooler interior floor, ceiling, and wall surfaces. The upper temperature limit of the A S H R A E comfort zone can also be extended with changes in clothing and indoor airspeeds. The comfort zone diagram specifies summer comfort zones appropriate for clothing insulation levels of 0.5 clo (e.g. trousers and short sleeve shirt). Comfort zones for other clothing levels can be approximated by increasing the temperature borders of the zone by 0.6°C for each 0.1 clo decrease in clothing insulation. A change to walking shorts and short sleeve shirt (0.36 clo) would extend the upper operative temperature limit of the comfort chart to approximately 28°C under low humidity conditions. Increasing the indoor airspeed increases the upper limit of the comfort zone by increasing convective and evaporative cooling of the body, and can provide a direct physiological cooling effect. The summer zone of the A S H R A E Standard 55 comfort diagram is based on air movements of less than 0.25 m/s. The Standard allows the upper limit of the comfort zone to be extended by 1°C for each increase of 0.275 m/s in air speed above the base case of 0.25 m/s, up to a maximum of a 2°C temperature increase at 0.8 m/s airspeeds. The net result is that the Standard allows for a maximum operative temperature of approximately 30°C under low humidity, low clothing insulation, and high air speed conditions. It sets upper extreme temperature limits under specific climate conditions, but these limits are difficult to use for evaluating performance in naturally ventilated buildings because of the inevitable fluctuations in interior temperatures due to fluctuating climate conditions. 4.2.2 Canadian Building Codes Canadian building codes do not specify indoor thermal comfort requirements and therefore they do not aid in setting thermal comfort criteria. The only reference that they make to the issue is that H V A C systems should be designed to conform to "good engineering practice" such as described in the A S H R A E Handbooks and Standards, pointing the designer back to the A S H R A E Standard 55 comfort diagram. 4.2.3 European Building Codes and Design Guidelines European design methods, standards, and codes generally allow for a greater level of thermal comfort flexibility than the North American A S H R A E Standard 55. Their acceptable upper temperature limits are similar to those suggested by the A S H R A E comfort diagram, however they often provide allowance for the fact that temperatures in a naturally ventilated building will vary through the day. They therefore encourage the use of naturally ventilated buildings. 30 For air conditioned buildings, the B R E Environmental Design Manual^4 proposes a mean summer dry resultant temperature of 23°C for a formal office (requiring jackets to be worn at all times) and 25°C for an informal office. A temperature range of +/- 2 degrees Celsius around this mean is thought to provide satisfactory conditions. The maximum acceptable indoor dry resultant temperature derived from this standard is then 27°C. Dry resultant temperature takes into account the effect of radiant heat exchange, similar to the operative temperature used by A S H R A E . At low air speeds, it can be approximated by the arithmetic mean of the air and mean radiant temperature of the space. For non air conditioned buildings, building codes of many European countries specify that the indoor dry resultant temperature may be allowed to exceed defined thresholds for specific numbers of hours based on a design weather year. For example, Holland has a requirement that the dry resultant temperature should not exceed 25°C for more than 5% of working hours, and should not exceed 28°C for more than 1 % of working hours. The design weather year is a typical year of weather data used as a common basis for calculating overheating hours in each particular climate region. It represents average weather conditions over a number of years rather than a worst case overheating scenario of extended hot weather. Therefore internal conditions in actual buildings will exceed the defined overheating hour thresholds due to inevitable periods of warmer weather than contained in the design weather year. A number of different typical weather year data formats are available, most of which are developed using a similar method in which twelve months of observed weather data are chosen from a database of 30 or more years of data. Typical Meteorological Year (TMY), Weather Year of Energy Calculations (WYEC), and Canadian Weather for Energy Calculations (CWEC) are all formats used in North America, and CIBSE Example Weather Years and CEC Test Reference Years are common formats used in the U K . CWEC is the most commonly used format in Canada. In it, each month is chosen by statistically comparing individual monthly with long term monthly means for daily total global radiation, mean, minimum and maximum dry bulb temperature, mean, minimum and maximum dew point temperature, and mean and maximum wind speed. It places additional consideration on the persistence of mean dry bulb temperature and daily total radiation in selecting months. Because each weather format differs in how it chooses the most "typical" months of weather data, they may differ significantly in their sequences of hot weather data. Unfortunately, there is not a common format that is adhered to internationally, and therefore overheating hour thresholds will vary depending on the weather data format chosen. 2 4 Petherbridge P, Milbank NO, and Harrington-Lynn J, "Environmental Design Manual", Building Research Establishment, Garston England, 1984. 31 The Zurich Canton of Switzerland allows a maximum number of degree-hours that the internal temperature can exceed a defined over-heating threshold. The number of degree-hours is defined as the temperature difference between the dry resultant temperature of the room and the threshold temperature, multiplied by the number of hours that the room temperature condition prevails. The limit is set at 30 degree-hours and is related to a variable threshold temperature, which is 28°C for outside temperatures above 20°C. This approach has been further developed in Scandinavia, where the Swedish Indoor Climate Institute recommends the use of different thermal quality classes. Three classes are defined, each with its own maximum operative temperature (25.5°C for class 1 through to 27°C for class 3). As well as temperature standards, there are corresponding specifications for air velocity, temperature gradient, radiant asymmetry, and rate of temperature change. The European NatVent Project suggests an approach similar to that adopted by Holland for naturally ventilated office type buildings in moderate and cold climates25. They suggest that the following dry resultant indoor temperatures not be exceeded for the specified number of occupied office hours: • 25.5 °C for not more than 100 hours • 28°C for not more than 25 hours 4.2.4 Summer Overheating Performance Criteria Adopted In this study, the NatVent thermal comfort criteria was adopted since it provides a limited allowance for varying indoor temperature, and is simple to use with a software model that calculates indoor temperatures for each hour of a typical year. However, indoor air temperature was used rather than dry resultant temperature, because of limitations in the NatVent software model used to simulate indoor thermal conditions. While the model calculates both interior surface and air temperatures in its thermal balance equations, it only outputs air temperature, and therefore does not allow the calculation of dry resultant temperature. This should produce an error on the conservative side, with an overestimate of overheating hours because indoor surface temperatures will most often be lower than indoor air temperatures, particularly in the case of higher thermal mass buildings and those utilising night cooling. The 25.5°C temperature threshold was also modified to 25 °C, again due to output limitations in the NatVent thermal modelling software program, also producing slightly higher estimates of the number of overheating hours. 2 5 "A Performance Specification for the Energy Efficient Office of the Future", Department of Environment, Transport and the Regions. General Information Report 30, DETR, London, 1995. 32 5 Natural Ventilation Performance Modelling The quantity of airflow entering and exiting a building depends on the size and location of airflow openings and on the magnitudes of the stack, wind, and mechanically induced pressure differences. Stack and wind induced pressure differences, which are created by external driving forces that vary with time and location on the building, cause infiltration and exfiltration of outdoor and indoor air. Pressure differences induced by operating mechanical ventilation systems can assist or counteract these airflows. Empirically determining all of the time-varying pressure differences and airflows for a multi zone office building is difficult, time consuming, and expensive. Alternatively, a detailed analysis can be carried out to simulate the multi-compartment pressure differences and airflows in a computer model of the building. 5.1 Multizone Airflow and Thermal Modelling Tools Several software models have been developed for predicting airflow, contaminant dispersal, and fire induced smoke movement in multizone buildings. Feustel and Dieris 2 6 report 50 different computer programs for multizone airflow analysis. These mathematical models typically calculate airflows based on mass balance calculations for individual zones that are connected together in a network. Two multi zone airflow models that are validated, well supported, and commonly used in North America are "CONTAM96" developed by the US National Institute of Standards and Technology (NIST) and "COMIS" developed by ten scientists in nine countries and supported by the International Energy Agency's Annex 23. While these models are excellent tools for predicting natural ventilation airflow, their usefulness is limited for design of naturally ventilated buildings because they do not also model thermal heat transfer processes. They cannot predict internal room temperatures and subsequently enable evaluation of summer overheating performance. And because they cannot predict internal temperatures, they cannot evaluate the effect of changing interior temperatures on airflow rates. A large number of thermal models have also been developed and are commonly used to evaluate building energy performance (for example DOE 2.1). However these thermal models are not capable of calculating wind and stack induced airflow rates. Very few computer models have been developed that can evaluate both airflow and thermal heat transfer, and link the effect of both processes together, due to the complexity of linking the two types of models. In the past, this lack of modelling capability has greatly restricted the ability for designers to estimate the performance of naturally ventilated buildings. 2 6 Feustel, H.E. & J. Dieris. 1992. "A survey of airflow models for multizone structures", Energy and Buildings, Vol 18, pp 79-100. 33 Several linked airflow-thermal models have now been developed, are commercially available, and are being used to aid in the design of naturally ventilated buildings. ESP-r, developed at the University of Strathclyde in the U K , and TAS, developed by EDSL in the U K are both commercially available models that allow detailed evaluation of natural ventilation building designs. Unfortunately, these programs are still fairly complex, expensive, and require extensive input from the user and specialised building airflow and thermal modelling expertise. 5.2 NatVent Model A simplified, easy to use linked airflow/thermal model called "NatVent" has been developed as part of the European NatVent Project, and has been used to estimate indoor air temperatures and airflow rates in this study. The NatVent Program is meant to serve as a pre-design tool that can be used early in the design process before building and ventilation system details are determined. Assumptions and simplifications about the building and ventilation system are made in the program. These simplifications make it valuable to use for initial design work and for this type of study - investigating the importance of design parameters on natural ventilation performance, and limitations of existing natural ventilation rules of thumb. The program is limited however, in its usefulness for evaluating detailed design options. The model is based upon a single zone approach in which the entire building or a selected part is represented by a single enclosed space of uniform temperature and indoor air pressure. A n airflow simulation algorithm is linked to a thermal heat transfer algorithm to calculate the temperature and the air flow rates into and out of the zone at each one hour time step. The airflow model calculates air pressure differences and airflow between the outside and inside of the building through windows and vents in the facade, passive stacks, supply air ducts, air fans, and cracks and imperfections in the walls and ceiling. The resulting airflow is then used in the thermal model. The thermal model calculates the internal air and surface temperatures from solar radiation on external walls and the roof, solar gains through windows and skylights, internal heat gains, heat transfer to indoor air, heat transfer between indoor air and internal surfaces, heat transfer by ventilation, and heat transfer through external walls, roof, and windows. The calculated air and interior surface temperatures are then used by the airflow model in the next time step. 5.2.1 Clint ate Data The NatVent program uses hourly weather data that can be in the form of: • summer design weather data used for estimating maximum room temperatures • winter design weather data used for estimating the size of ventilation openings needed to achieve acceptable ventilation rates of indoor air quality • reference year weather data that can be used for estimating interior air temperature and outdoor air ventilation rates for every hour in the year. 34 Weather data chosen for this study was reference year weather data for the cities of Vancouver and Toronto, in Canadian Weather for Energy Calculations (CWEC) format. CWEC format is based on the A S H R A E defined WYEC2 format and has been adopted by the National Research Institute of Canada for the creation of Canadian typical year weather files for energy calculations. 35 6 Natural Ventilation Modelling Design Parameters 6.1 Base Case Building To investigate natural ventilation design rules of thumb and the influence of different design parameters on performance, a "typical" or "base case" office building was created in which different input parameters could be modified. The base case building was designed to represent an open plan low-rise office building in Canada with a geometry and features suitable for using natural ventilation for IAQ and summer overheating control. The base case building, with its plan view as shown in Figure 6-1, is a three story office building located in an urban environment. The following is a description of its features: > The building measures 40 m long and 15 m in cross-ventilation depth, with the long walls facing east and west. > The floor to ceiling height of each level is 3m. > No internal partitions block airflow between east and west facing walls of the building. > A l l exterior walls contain windows that cover 30% of the wall area. The effective openable area of all windows is equal to 10% of the window area. > Trickle vents are evenly distributed within exterior walls. These vents could be considered to be either trickle vents or a very small area of night cooling vents. > The building has low internal heat gains, a low occupancy ratio, light thermal mass construction, clear double pane windows, and medium size overhangs over the windows. > The occupancy schedule is 8:00 a.m. to 6:00 p.m., 5 days per week. > The building is surrounded by obstructions equal to its height, such as neighbouring buildings, within a distance of one building height. > The building is constructed to a high level of airtightness (1.6 L/s per m envelope area @ 50 Pa pressure) with the trickle vents closed. A single zone within the centre of the building was selected for natural ventilation performance analysis. This zone is representative of a 10m wide office in the centre of the length of the building, open the entire 15m depth of the building, with solid partitions isolating airflow between it and other zones of the building. 15 m r i Modelled Zone Figure 6-1 Base Case Building Layout 36 6.1.1 Building Form -Parametric Study The first part of the computer simulation modelling work performed in the study was a parametric analysis of the effect of building design modifications on natural ventilation performance. The building configurations used are shown in Figure 6-2. A large atrium runs down the length of the three storey building. Parameter modifications such as changes to window size, overhang size, thermal mass, etc. were first made to a cross ventilated building with no stack on top of the building, as shown in Figure 6-2 (a). As one of the parameter modifications, a passive stack was added to the top of the atrium as shown in Figure 6-2 (b). This base case building configuration enables both wind pressure and stack effects to induce natural ventilation airflow. It represents the performance resulting from combined stack and cross ventilation driven airflow through the building since large airflow openings exist between floor levels. This configuration was chosen because it is a common configuration found in naturally ventilated low rise office building design. The C K Choi Building in Vancouver, BC, is an example of this type of configuration. The parametric analysis was used to provide a broad overview of the relative magnitude of the impact of a wide range of building design modifications on this type of building configuration. Airflow and interior temperatures used to evaluate natural ventilation performance were measured on the ground floor level only. Cross Ventilation Base Case Building 15m Building Depth (a) 3 = > Stack Ventilation Base Case Building (b) Figure 6-2 Base Case Cross Ventilation and Stack Ventilation Configurations Used For Parametric Analysis 37 6.1.2 Building Form - Investigation of Rules of Thumb The second part of the computer simulation modelling work performed in the study was an evaluation of the limitations of existing rules of thumb. Many of the rules of thumb presented in Chapter 3 are based on cross ventilation strategies, however the building configuration shown in Figure 6-2 (a) is strongly effected by stack effects over the entire building height. Therefore, to evaluate the effect of cross ventilation in greater isolation of stack effect, the base case building was modified so that individual building floors are isolated from one another, as shown in Figure 6-3 (a). Airflow and interior temperatures used to evaluate natural ventilation performance were measured on the ground floor level only. The building configuration used to evaluate stack ventilation rules of thumb remained unchanged from that used for the parametric analysis, as shown in Figure 6-3 (b). The stack ventilation rule of thumb evaluated is based on the same type of configuration -representing the performance of combined cross ventilation and stack ventilation airflow. Cross Ventilation Base Case Building Variable Building Depth Stack Ventilation Base Case Building (a) (b) Figure 6-3 Base Case Cross Ventilation and Stack Ventilation Configurations Used for Rule of Thumb Analysis 38 6.2 Natural Ventilation Design Parameters Investigated A large number of building design parameters effect natural ventilation performance: > Some building parameters directly effect the rate of ventilation that can be used to control summer overheating, such as the window opening area, or the height of a passive ventilation stack. > Other parameters, such as interior lighting loads, solar shading, or wall and roof insulation levels do not directly effect rates of natural ventilation airflow, but instead effect the thermal balance between heat gains and losses within the building. Because they play a key role in determining the building's ability to avoid overheating, they are considered indirect natural ventilation design parameters. > A third grouping of parameters effect the performance of the building in terms of its ability to provide adequate ventilation rates for indoor air quality. Tickle vent size is a key variable effecting ventilation rates when windows are closed. Many of the parameters that effect ventilation rates that control summer overheating also effect ventilation rates that provide adequate indoor air quality. For example, stack ventilation parameters and other parameters that effect cross ventilation such as local wind shielding or building orientation effect rates of airflow through trickle vents. And parameters that effect the rate of airflow through windows also effect ventilation for indoor air quality. 6.2.1 Modified Par am eters The following key natural ventilation design parameters, grouped according to the above categories, were modified to investigate their effect on the natural ventilation performance of the base case building: 1) Natural Ventilation For Control of Summertime Overheating a) Parameters that directly effect ventilation rates used to control summer overheating: Cross Ventilation • Openable window area • Local wind shielding • Building orientation • Floor to ceiling height Stack Ventilation • Passive Stack Area • Passive Stack Height • Building Height 39 Night Cooling • Night Cooling Vent Size b) Parameters that effect building thermal balance and therefore summer overheating: • Interior lighting and equipment loads • Occupant Density • Window glazing area • Window shading due glass colour or blinds or shading devices • Window overhang size • Building orientation • Window type - U Value and transmissivity • Wall and roof insulation levels • Interior active thermal mass capacity 2) Natural Ventilation for IAQ Control Cross Ventilation • Trickle vent size • Openable window area • Local wind shielding • Building orientation Stack Ventilation • Passive Stack Area • Passive Stack Height • Building Height Each of these natural ventilation design parameters were modified within their practical design limitations. In some cases the range of variation of parameters is limited by physical contraints, and in others it is limited by design practicality or aesthetic considerations. For example, the total window area within exterior walls was modified between a maximum of 100% (including window frames) of wall area and a minimum of 10% of wall area. A minimum of 10 % window area was chosen rather than 0% because it is not likely that a building would be designed with no windows. As another example, the passive stack height was varied between zero height and 10 m height above the building. While greater stack heights are possible, a 10 m high stack is already stretching the limits of practicality in terms of building aesthetics. The range of variation modelled for each design parameter and values for the base case building are shown in Table 6-1. 40 Table 6-1 Modelled Building Design Parameters Natural Ventilation Design Parameter Base Case Modelling Limits Minimum Cooling and/or IAQ Performance Maximum Cooling and/or IAQ Performance Cross Ventilation: Site Local Wind and Solar Shielding Full Building Height Obstructions Full Building Height Obstructions No Obstructions Building Form Building depth 15 m 50 m 5m Building height 3 stories 2 stories 10 stories Window Area 30% of Wall Area 10% of Wall Area 100% of Wall Area Unobstructed Openable Window Area 3% of Wall Area 1% of Wall Area 10% of Wall Area Orientation of Window and Airflow Opening Walls Facing East West Facing East - West Facing North - South Building Details Solar Shading Overhang Size (Angle from centre of window to overhang edge) Medium Overhang (40°) No Overhang (0°) Large Overhang (60°) Solar Shading Factor (Due to Solar Protective Glazing, Interior Blinds, or Exterior Blinds or Shading Devices) 0.6 (Limited solar protective glazing) 1.0 (No Protective Glazing, Blinds, or Shading) 0.2 (External Blinds or Shading) Window Type (Without varying colour) Double Clear Single Clear Double Clear + Low-e + Argon Wall and Roof Insulation Level U=0.35 W/m2K (RSI = 3) U=0.8 W/m2K (RSI =1) U=0.2 W/m2K (RSI = 5) Thermal Mass - Interior Active Thermal Capacity (Wh/K m2) - See Table 6-2 Medium (80 Wh/K m2) Very Light (40 Wh/K m2) Very Heavy (160 Wh/K m2) Internal Design Conditions Occupant density 20 m 2 per occupant 5 m 2 per person 50 m 2 per person Lighting and Equipment Heat Gain 10 W/m2 60 W/m2 5 W/m2 IAQ Ventilation: Trickle Vent Size 10 cm2/m2 floor area (750 cm2 per wall)* 0 cm2/m2 floor area 16 cm2/m2 floor area Night Cooling: Night Cooling Vent Size 0.1% Floor Area (750 cm2per wall)* No Vents 1.3% Floor Area (10,000 cm 2 orl m2 per wall) Stack Ventilation: Stack Height (Fixed area of 2.8 m2) No Stack 0 m (Hole in roof) 10 m Stack Airflow Area (Fixed hgt of 7 m) No Stack Om 2 16 m 2 *The trickle vents and night cooling vents are the same vents in the base case building. 41 The variance in interior thermal mass modelled is shown in Table 6-2, with an example of the type of interior surface materials that could be used in each thermal mass category. Interior air and surface temperatures increase over the course of the day due to increased solar and internal heat gains from occupants, lighting, and equipment. Interior thermal mass dampens the amplitude of this increase, and delays the time of day when peak temperatures occur, preferably to a time beyond occupied hours. The effect of thermal mass increases with the density and thermal heat capacity of interior wall, roof, and floor materials. The active thermal capacity used by the NatVent model is a product of the material thermal heat capacity (Wh/kg K) , density (kg/m3), and the active volume of material (m3), averaged over the interior of the building and presented on a square meter of floor area basis. The active volume of thermal mass depends on the depth of material that plays an active role in dampening interior temperatures. This depth depends on the type of material but is usually limited to a few inches (5 to 10 cm) in depth. Table 6-2 Active Thermal Capacity of Structures per m 2 Gross Floor Area. Description Internal Construction Active TKermal|Capacity (Wh/K m2) Very Light Light walls, floor and ceiling, e.g. steel studs and gypsum board walls, T bar ceiling, carpeted floor over underlay and plywood, no heavy structures. 40 Light Some heavy structures, but not well exposed, e.g. concrete slab with wooden floor or light-weight concrete walls clad in plaster. 80 Medium Several heavy structures, e.g. exposed concrete slab with brick or concrete walls. 120 Very Heavy Heavy walls, floor and ceiling made of exposed concrete, brick or stone. 160 6.2.2 Fixed Parameters A limited number of parameters were explored in this study to investigate their effect on natural ventilation performance. Many other parameters can also have significant impacts, several of which are described below: > Internal Partitions - The base case building is assumed to have no internal partitions that inhibit airflow across the depth of the building. Depending on their size and location, internal partitions can reduce cross ventilation or stack ventilation driven airflow, and therefore degrade natural ventilation performance. > Length of the Building - The length of the building was held constant at 40m. The length of the building effects the wind pressure profile, since the shape of the building has a direct effect of the magnitude and distribution of wind pressure coefficients (Cp values). The length of the building also effects solar heat gains through end walls. The longer the building, the lower the impact due to the averaging of fixed end wall heat gains over a larger building floor area. For the base case building modelled in 42 this study, with interior temperature evaluated in a central zone within the building, it was found that as the overall building length was incrementally increased to approximately 20m the effect of end wall solar gains was reduced. Beyond 20m, increasing building length showed insignificant impact. > Occupancy Schedule - The building was assumed to have a fixed occupancy schedule of 8:00 a.m. to 6:00 p.m., 5 days per week. This is a relatively long schedule for an office building, leading to a conservative estimate of natural ventilation performance, because overheating performance is based on the number of overheating hours per year that occur while the building is occupied. > Factors that Effect Wind Pressure - A number of building and site specific factors that effect wind pressure coefficients and wind velocities around the building were also not investigated. These include: • Building Shape - Wind pressure coefficients for each location on the building can vary significantly depending on building shape. • Roof Design - Roof design has a particularly significant impact on roof level wind pressure coefficients that effect airflow performance of passive stacks. ••• Terrain Roughness - Surrounding terrain roughness (open flat land versus city centre) effects local wind speeds, which then effect wind driven ventilation rates. Only city centre type terrain roughness was investigated. • Local Wind Obstructions - The height and location of local surrounding buildings and landscape features can significantly effect local wind speeds. While the effect of changing the height of local wind obstructions was investigated, the effects of the exact placement of obstructions, which can also have significant impacts due to wind funnelling, were not modelled. A l l of these factors can have either positive or negative effects on natural ventilation performance. > Occupant Control of Windows - The operation of windows was modelled as being opened by occupants to provide cooling only during occupied hours, and only when the indoor temperature is greater than 20°C. Different scenarios of occupant control could dramatically effect the rates of ventilation airflow, increasing or decreasing indoor temperature conditions and ventilation rates for IAQ. 43 7 Base Case Building Performance The base case cross-ventilated building was modelled with the NatVent simulation model using hourly reference year weather data for the cities of Vancouver and Toronto. 7.1 Vancouver and Toronto Climates The cities of Vancouver and Toronto were chosen as two example climates because they represent extremes in summer temperature conditions within Canada. Vancouver has mild summer temperature conditions and therefore has great potential for naturally ventilated office buildings. Toronto was chosen to represent a very difficult climate for using natural ventilation for cooling because it has very hot and humid summer weather. An overview of climate characteristics for Vancouver and Toronto, as they relate to natural ventilation performance, are shown in Table 7-1. Shown in the table are relevant A S H R A E design climate conditions, and statistics taken from the CWEC reference year weather data used for natural ventilation performance modelling. Table 7-1 Vancouver and Toronto Climate Data . Climate Vancouver Toronto ASHRAE Design Conditions 1% Summer Design Dry Bulb Temperature (July) 26 32 2.5% Summer Design Dry Bulb Temperature (July) 25 31 Mean Daily Temperature Range (July) 9 11 Prevailing Wind Direction at 2.5% Summer Design Dry Bulb Temperature WNW SW Prevailing Wind Direction at 97.5 % Winter Design Dry Bulb Temperature East North CWEC Reference Year Weather Files Number of hours per year of dry bulb temperature at or above 25°C 6 372 Number of hours per year of dry bulb temperature at or above 28°C 0 121 Mean Annual Wind Speed 12km/hr 15 km/hr Mean July Wind Speed 11 km/hr 11 km/hr Percentage of hours per year with zero wind speed 9 7 Percentage of occupied hours (8:00am-6:00pm) per year with zero wind speed 5 4 7.1.1 Vancouver Climate For control of summer overheating, Vancouver has a mild climate that is ideal for natural ventilation. As shown in Table 7-1 the CWEC reference year used to model building performance shows only six hours per year with outside air dry bulb temperatures equalling or exceeding 25°C and zero hours per year at or exceeding 28°C. These numbers are slightly low when compared to historical weather data for Vancouver. Environment Canada Principal Station Data" complied from all hourly observations 2 7 Atmospheric Environment Service, "Principal Station Data" Vancouver International Airport, Environment Canada, Downsview, Ontario, 1983. 44 between 1951 and 1980 show the percentage frequency of the dry bulb temperature equalling or exceeding 25°C at approximately 0.1% in the month of May, 0.5% in June, 2% in July, 1% in August, and 0.1% in September. During all other months the occurrence of temperatures exceeding 25°C is negligible. This works out to an average of approximately 27 hours per year with the temperature at or exceeding 25°C. A similar calculation for the number of hours at or exceeding 28°C reveals an historical average of approximately 3 hours per year. This discrepancy is relatively insignificant because both sources show very few hours per year - out of 8760 total hours per year - with outside air temperatures exceeding 25°C. The rest of the time outside air is cool enough to provide direct cooling to indoor spaces. Table 7-1 also reveals information about diurnal temperature differences - effecting the potential for night cooling - as well as wind speed and direction information useful for looking at the potential for summer and winter ventilation rates and for orienting the building. As shown, on average the Vancouver night time outside air temperature in July decreases by approximately 9°C from the daily maximum temperature, making night cooling an effective cooling strategy. During the month of July, when wind is required as a driving force for summer cooling, the average wind speed is 11 km/hr. The mean prevailing wind direction during the hottest 2.5% of hours per year is West Northwest. Therefore to take maximum advantage of wind direction during the hottest weather conditions (although not necessarily also the best direction from a solar gain point of view), the building should ideally be oriented with its longest walls facing West-Northwest and East Southeast rather than directly East and West as in the base case building. During winter months, when ventilation for IAQ is required but cooling is not, the prevailing wind direction in Vancouver is from the East. Therefore for provision of ventilation for indoor air quality, the ideal building orientation is with long building walls facing East and West as in the base case building. Calm wind conditions in Vancouver exist for approximately 5% of all occupied hours (8:00 am and 6:00 p.m) in the typical year weather data used in this study. For the remainder of the time some level of wind is available to provide a wind pressure driving force that can be used to provide ventilation. The percentage of hours with no driving force to provide air movement for ventilation will actually be less than the 5% of hours with calm wind conditions, because during part of this time there will be temperature differences between the inside and outside of the building, providing a stack ventilation driving force for ventilation airflow. 7.1.2 Toronto Climate Toronto has hot summer weather that is less than ideal for control of summer overheating using natural ventilation. The CWEC reference year used to model building performance shows 372 hours and 121 hours per year with outside air dry bulb temperatures at or exceeding 25°C and 28°C respectively. 45 However, on average the night time outside temperature in July decreases by approximately 11°C from the daily maximum temperature, making night cooling an effective cooling strategy part of the time. During the month of July, when wind is required as a driving force for summer cooling, the average wind speed is 11 km/hr, the same as Vancouver. The mean prevailing wind direction during the hottest 2.5% of hours per year is Southwest. Therefore to take maximum advantage of wind direction during the hottest weather conditions, the building should ideally be oriented with its longest walls facing Southwest and Northeast rather than directly East and West as in the base case building. Toronto has weather conditions that are similar to those of Vancouver in terms of providing driving forces for ventilation for indoor air quality. Calm wind conditions in Toronto exist for approximately 4% of the time between occupied building hours of 8:00 am and 6:00 p.m. used in this study. During winter months, the prevailing wind direction in Toronto is from the North. Therefore for provision of ventilation for indoor air quality, the ideal building orientation is with long building walls facing North and South, rather than East and West as in the base case building. 7.2 Base Case Building Performance 7.2.1 Summer Overheating The overheating performance of the base case buildings used to study the effect of building design modification are shown in Table 7-2. As shown, neither the Vancouver nor Toronto base case buildings meet the minimum criteria for control of overheating as set out in Chapter 4. The Toronto base case building greatly exceeds the acceptable number of overheating hours for both 25°C and 28°C criteria. While the number of overheating hours in the Vancouver building is much lower than in the Toronto building, it also shows occupied overheating hours exceeded for both temperature criteria. Table 7-2 Base Case Building Overheating Performance Minimum Acceptable Performance Criteria Vancouver Base Case Building Toronto Base Case Building Occupied Hours per Year over 25°C 100 287 406 Occupied Hours per Year over 28°C 25 30 137 46 7.2.2 IAQ Ventilation Rates The IAQ performance of the base case buildings are shown in Table 7-3. As shown, Vancouver and Toronto base case buildings show identical ventilation rate performance and neither building meets the minimum acceptable IAQ criteria as set out in Chapter 4. Modelled with either Vancouver or Toronto weather data, the base case buildings show a naturally induced ventilation rate falling below the 10 L/s per person criteria for approximately 4% of all occupied hours over the year. Table 7 - 3 Base Case Building Ventilation for IAQ Performance Minimum Acceptable Performance! Criteria : Vancouver Base Case Building Toronto Base Case Building % of Occupied Hours per Year with Ventilation Rate Below 10 L/s per Person 1% 4% 4% 47 8 Parametric Analysis of Variables Effecting Thermal Comfort and Indoor Air Quality The base case office building was modelled to determine the relative effect of modifying key building design parameters on natural ventilation performance. The effect on the following two performance indicators that were used to set the natural ventilation performance criteria were investigated: 1. Overheating - measured by the number of occupied hours per year that the interior air temperature of the building exceeds 25°C. 2. Indoor Air Quality (IAQ) ventilation rates - measured by the percentage of occupied hours during which the naturally induced ventilation rate (i.e. with no mechanical assistance) falls below 10 L/s per person. Design parameter modifications were each applied to the base case building in isolation of one another. The upper and lower limits of each parameter were based on the limits presented in Table 6-1. Combined effects of parameter modifications were not investigated. The combined sum of effects will vary greatly depending on interactions specific to each parameter combination. 8.1 Thermal Comfort Parametric Analysis The results of modifying parameters that effect thermal comfort inside the building are summarised in Figure 8-1. The number of occupied hours per year when the internal temperature exceeds 25°C are shown in separate graphs for Vancouver and Toronto. The dashed line in each figure shows the number of overheating hours for each of the base case buildings. Graph bars extending to the left of the dashed line indicate a reduction in overheating hours from the base case building, and to the right indicate an increase in overheating hours. Results of the analysis show that parameters differ significantly in their abilities to reduce or increase the number of overheating hours in the base case building. Parameters with the greatest ability to reduce overheating in the Vancouver base case building, ordered from greatest to least effect, are: 1. Utilisation of night cooling vents 2. Increasing operable window area 3. Addition of a passive stack 4. Decreasing non operable window area 5. Increasing building height above floor level of interest 6. Increasing solar protective glazing, blinds, or other shading devices 7. Decreasing local wind shielding obstruction height 8. Decreasing lighting and equipment loads 48 Figure 8-1 Thermal Comfort Parametric Analysis Results Parameters Effecting Thermal Comfort Vancouver Base Case Office Building Passive Stack Area Passive Stack Height Night Cooling Vent Size Openable Window Area Non Openable Window Area Building Height Solar Protective Glazing or Blinds Local Wind Shielding Lighting and EquipmentLoads Trickle Vent Size Wall Insulation Occupant Density Increasing Interior Thermal Mass Window Overhang Size Building Orientation Window Type 1 IT i IT"" 1 • - Base Case • Positive Effect H Negative Effect 0 100 200 300 400 500 600 700 800 900 Occupied Overheating Hours per Year (Above 25C) Parameters Effecting Thermal Comfort -Toronto Base Case Office Building Passive Stack Area Passive Stack Height Openable Window Area Night Cooling Vent Size Non Openable Window Area Building Height Local Wind Shielding Solar Protective Glazing or Blinds Lighting and EquipmentLoads Occupant Density Trickle Vent Size Building Orientation Wall Insulation Window Overhang Size Increasing Interior Thermal Mass Window Type tHHIl i s (- — Base Case • Positive Effect 0 Negative Effect 0 100 200 300 400 500 600 700 800 900 Occupied Overheating Hours per Year (Above 25C) Parameters with a much lower ability to reduce overheating in the Vancouver base case building, ordered from greatest to least effect, are: 9. Increasing Wall Insulation 10. Decreasing occupant density 11. Increasing interior thermal mass 12. Increasing window overhang size 13. Changing building orientation 14. Changing window type (with no colour changes) The following is a brief explanation of how each parameter was modelled, how its modification effects thermal comfort, and any major differences between Vancouver and Toronto results. Parameters are presented in the order of greatest to least ability to reduce overheating within the Vancouver base case building. 8.1.1 Night Cooling Vents Night cooling vents were added to the east and west facing walls of the base case cross ventilated building to allow an increase in night time ventilation rates. Cool night air removes heat that was stored in the building's thermal mass over the course of the day, and cools the thermal mass so that it can provide direct radiative cooling and reduce interior air temperatures the following day. The addition of night cooling vents is the parameter with the most significant ability to decrease overheating for the Vancouver base case building, and the second most significant for the Toronto base case building. Figure 8-1 also shows that a decrease in the small night cooling vent area in the base case building leads to a significant increase in overheating hours. This decrease in vent area results from the removal of the trickle vents contained in the base case building. 8.1.2 Openable Window Area The unobstructed openable area of the windows in the base case building was next modified. Increasing the openable window area has one of the greatest abilities to decrease overheating for both Vancouver and Toronto buildings. Increasing the openable window area increases the rate of cross ventilation and stack ventilation airflow through the building, removing heat from the interior of the building when the outside air temperature is below the indoor air temperature. The operation of windows was modelled as being opened by occupants to provide cooling only during occupied hours, and only when the indoor temperature is greater than 20°C. It is easy to understand how increasing operable window area can significantly decrease overheating in Vancouver because the outdoor temperature is rarely greater than 25°C. However a very large reduction in overheating hours is also observed for the Toronto 50 building, even though there is a high number of hours that the outdoor temperature exceeds 25°C. A possible explanation for this behaviour is that increased window opening areas preferentially increase airflow of cooler air, due to stack effect. If the building was subjected to cross ventilation alone, increasing window opening areas would increase airflow into the building at all times, independent of outdoor air temperature. Whether overheating increased or decreased would depend on the relationship between heat gain due to increased flowrates of hot mid day air, versus heat losses and thermal storage effects (similar to night cooling effects) due to increased flowrates of cool early morning air. However, the base case building is also subjected to stack effect due to the fact that individual floors are not completely isolated from one another. Increasing window opening areas will then also increase the rate of airflow due to stack effect, which occurs when the outside air temperature is lower than the inside air temperature. Therefore the rate of airflow at cool temperatures may be increased more than the rate of airflow at warm temperatures, resulting in greater cooling of the building's thermal mass, leading to a lower number of overheating hours. 8.1.3 Addition of a Passive Stack A passive natural ventilation stack was added to the base case building, increasing the thermal buoyancy driven natural ventilation airflow rate. The base case building already takes advantage of wind pressures that generate cross ventilation airflow, and thermal buoyancy driven airflow through the three stories of building height. Adding a passive stack to the top of the building increases the total height of stack above the ground floor and enhances thermal buoyancy driven airflow. A stack was added to the base case building under two scenarios. In the first, a stack with a small opening area (2.8m ) was added to the roof of the three storey building and progressively increased in height. Even when a passive stack of zero height is added to the building (i.e. a simple opening is made in the roof), there is a significant reduction of overheating. Increasing the height of this small passive stack to a maximum of 10m has less of an impact than the initial addition of an opening in the roof of the building. This can be seen in the passive stack height results shown in Figure 8-1, which shows the range of impact possible from modifying stack height. In the second scenario a stack of fixed 7 meter height (and initially zero area) was added to the top of the building and progressively increased in opening area. Results show a large potential reduction in overheating in both Vancouver and Toronto climates. A large diameter stack can be thought of as representing the effect of either a traditionally shaped stack, or the airflow opening areas within an atrium that extends above the height of the building. 51 8.1.4 Non Operable Window Area The non-operable or fixed glazing area of all windows was modified by increasing or decreasing the total window area while maintaining a constant airflow opening area. Decreasing the fixed glazing area reduces the amount of solar gain entering the building, and greatly reduces overheating in both Vancouver and Toronto base case buildings. Increasing glazing area increases overheating in both climates, but the impact is much greater for the Vancouver building. This would imply that the model predicts that control of summer solar gains is more important in Vancouver than Toronto, the cause for which is not clear. 8.1.5 Building Heigh t Increasing the building height increases the stack effect and results in increased airflow into the ground floor of the building, the location where thermal performance was measured in this study. The height of the building was modified from 1 story to a maximum of 10 stories. The effects of increasing or decreasing overheating by changing the building height as shown in Figure 8-1 apply to the ground level floor only and assume that there are large airflow openings between floors offering little airflow resistance. The effect of increasing building height is much more difficult to generalise than for other parameters investigated here, because it is strongly dependent on location within the building and detailed airflow opening design over the entire exterior surface of the building. When the outside temperature is lower than the inside temperature, the ground level receives the greatest airflow and cooling benefit from increased building height related stack effect. Stack induced rates of airflow are reduced for higher floor levels, and reversed for floor levels that are located above the neutral pressure plane. However, the location of the neutral pressure plane is dependent on the complex interaction of wind, stack, and mechanical ventilation induced pressure differences, and is strongly effected by the distribution of airflow openings over the entire exterior surface of building. Raising the height of the neutral pressure plane to take maximum advantage of stack effect is a key component of the overall natural ventilation design, particularly in taller buildings. It is generally accomplished by increasing the size of airflow openings with height, and/or adding passive stacks to the roof of the building. Such a detailed design and analysis is not possible with the use of a simplified airflow modelling tool. Therefore, the analysis results presented here should be understood to be a maximum possible benefit only, with more detailed modelling, using more sophisticated techniques, required for specific locations within the building. 8.1.6 Solar Protective Glazing or Blinds Increasing window solar shading by using solar protective glazing, internal blinds, or external blinds reduces the amount of solar energy that enters the building interior. Windows in the base case building are double pane windows with a solar shading factor 52 of 0.6, which is representative of glazing with limited daylight colouring, or clear glazing with limited use of light coloured internal blinds. Solar shading was increased by adding significant daylight colouring or external shading devices to the base case windows, and decreased by removing all colouring. Increased solar shading resulted in a significant decrease in overheating. The effect was more pronounced in the Vancouver building that the Toronto building. Similar to results for the impact of non operable window area, the Vancouver building is more sensitive to changes in solar gains than the Toronto building. 8.1.7 Local Wind Shielding Three local wind shielding options were modelled: > no local wind obstructions > wind obstructions equal to 1/2 the building height, and > wind obstructions equal to the full building height. Obstructions were assumed to be located at a horizontal distance of one building height from the modelled building. Decreasing the size of wind obstructions leads to increased local wind speeds at the building site, increasing wind pressure on the building, and increased natural ventilation airflow and cooling. As expected, as the height of wind obstructions were reduced from the base case condition of full building height obstructions, the number of overheating hours was reduced in both climates. 8.1.8 Lighting and Equipment Loads Lighting and equipment loads were decreased from 10 W/m in the base case building to a minimum of 5 W/m 2 , representing an office with very low lighting and equipment loads. They were increased to a maximum of 60W/m 2, representing an office with very high lighting and equipment loads. Decreasing lighting and equipment loads has a large effect on reducing summer overheating. The significance of its impact is actually much greater than indicated by its placement as eighth in order of ability to reduce overheating because very low lighting and equipment gains were chosen for the base case building, leaving little room available for improvement. In a building with high internal heat gains a reduction in these gains could be the parameter modification with the greatest ability to reduce overheating. 8.1.9 Wall and Roof Insulation Increased levels of insulation reduce overheating by reducing the rate of conductive heat transfer through walls and the roof when the outside temperature is greater than the inside temperature. The Toronto building exhibits a greater reduction in overheating resulting 53 from increased insulation levels. This result is expected since Toronto has higher peak daytime temperatures than Vancouver. 8.1.10 Occupant Density Modifications to occupant density change the total internal heat gains. Changes to internal heat gains due to occupants have the same effect on overheating as changes to heat gains due to lighting and equipment loads. 8.1.11 Interior Thermal Mass Increasing or decreasing the interior thermal mass of the building effects its ability to dampen the amplitude of the interior temperature increase that occurs as a result of interior heat gains from lighting, equipment, occupants, and solar gains. It also effects the ability of the building to delay, ideally until after occupied hours, the time of day when peak internal temperatures occur. Modelling results show a relatively minor impact on overheating resulting from large changes to the building's thermal mass, for both Vancouver and Toronto climates. The impact is probably much greater than shown in these results however, due to limitations in the reporting format of the computer model. The NatVent model calculates the damping and storage effects of interior thermal mass, models the interaction between thermal mass and interior air temperature, and calculates interior surface temperatures at each time step. However, output from the model reports interior air temperature only. Surface temperatures are not reported, making it impossible to calculate a resultant or operative temperature that takes into account both air temperature and surface temperature. Therefore it is not possible to measure the radiative cooling effect - the true benefit of increased thermal mass - even though the temperature damping and shifting effects are likely significant. 8.1.12 Window Overhang Size The window overhang size effects the amount of solar radiation that enters the building. Larger overhangs can reduce solar gains and therefore reduce overheating, particularly on south facing windows. Overhangs are less effective at blocking solar radiation on east and west facing walls due to low sun angles at the beginning and end of the day. Overhangs can also reduce day lighting, resulting in a requirement for more electric lighting and increasing internal heat gains. Large changes in overhang size were found to have very little impact on overheating in both Vancouver and Toronto base case buildings. The impact is likely small due to the form and orientation of the base case buildings and the size of surrounding obstructions. The base case building is oriented with its long walls facing east and west, with the modelled zone in the centre of the building length, relatively isolated from solar gains in the south facing wall. Windows in the west facing walls provide the greatest potential for increased overheating because of late afternoon solar gains through these windows which 5 4 coincide with the time of peak internal temperature cycles, due to the delay caused by thermal mass. However the window overhangs are not very effective at reducing these solar gains due to the low angle of the sun. The base case building is also surrounded by local obstructions equal to the building height. As a result the solar gains are significantly reduced, and therefore the potential for overhangs to reduce overheating is also significantly reduced. The impact of overhangs could be much greater for other building designs and orientations with less solar shielding. 8.1.13 Building Orientation Changing building orientation can potentially effect overheating due to the net result of two factors: (a) changes in solar gains, and (b) changes to wind pressures against the building. In south facing windows, solar gains can be easily controlled with the use of overhangs. Orienting the building so that more windows face west will increase solar gains in the later portion of the day, and increase overheating. These solar gains are also very difficult to reduce with the use of overhangs due to low sun angles. Rotating the building so that cross ventilation airflow through the building is parallel to summer prevailing wind conditions will increase airflow rates, due to increased wind pressure when the wind blows directly towards a surface. The optimum orientation for airflow would take advantage of the direction of prevailing wind conditions that occur at night or early and late in the day when air temperatures are lower, rather than at midday when air temperatures are at their highest. Changing building orientation was found to have a small impact on overheating in the base case building, for both Vancouver and Toronto climates. However, solar gains are significantly reduced in the base case building because it is surrounded by local obstructions equal to the building height. As a result the potential for changes in building orientation to reduce overheating is also significantly reduced. For building sites with less shading from surrounding obstructions, the impact of rotating the building so that west facing windows face south and solar gains are blocked with overhangs is much greater. As a comparison, the building was remodelled with local wind and solar obstructions removed. Using Vancouver weather data it was found that changes to building orientation resulted in a 5 times greater change in overheating hours in the non shaded building compared to the building surrounded by local obstructions equal to the building height. Another factor that could explain the relatively small impact of orientation on overheating is the potential for benefits of reduced solar gains to be cancelled out by reduced airflow rates due to changes in orientation with respect to the prevailing wind direction. For example, to reduce solar gains the Vancouver building should be oriented with windows facing south and north instead of east and west. But an east-west 55 orientation of window openings is preferred to increase wind driven airflow rates due to the prevailing east-west wind direction. 8.1.14 Changing window type (with no colour changes) The type of window was modified in terms of the number of panes of glazing and overall insulation rating, without changing the glass colour from clear. Increasing the number of glazing panes has the effect of reducing solar transmittance, leading to reduced solar gain. Increasing the number of glazing panes, or other changes that increase the insulation value of the window, reduces conductive heat transfer through the window and reduces overheating, similar to the effect of increasing wall insulation levels. Modelling results found little reduction in overheating due to replacement of double pane windows with energy efficient argon filled low-e double or triple pane windows. A small increase in overheating was observed however, from the replacement of double pane windows with single pane windows. Single pane windows increase solar gains due to their higher rates of transmittance, and allow greater heat transfer into the building due to their reduced insulation levels. 56 8.2 I A Q Ventilation Rate Parametric Analysis A parametric analysis was carried out to investigate the effect of building design modifications on naturally induced ventilation rates for providing adequate indoor air quality. Building design modifications included only those that directly effect natural ventilation airflow rates. Parameters that have little direct effect on airflow rates- such as internal loads, solar shading, etc were not included in the airflow parametric analysis because of their low potential impact. Figure 8-2 shows modelling results for both Vancouver and Toronto climates in terms of the percentage of occupied hours per year when the naturally induced ventilation rate falls below 10 L/s per person. The dashed line in each figure shows the performance of the base case building (4% of occupied hours in both Vancouver and Toronto). Graph bars extending to the left hand side of the dashed line show the potential for increased ventilation rates (a reduction in percentage of hours with low ventilation rates), and to the right indicate the potential for decreased ventilation rates. Results of the analysis show that parameters differ significantly in their abilities to increase or reduce ventilation rates in the base case building. Parameters ordered from greatest to least range of effect on the ventilation rate required to maintain indoor air quality, for both the Vancouver and Toronto base case buildings, are: 1. Addition of a passive stack 2. Occupant density. 3. Trickle vent size 4. Building height 5. Openable window area 6. Local wind shielding 7. Building orientation Results for Vancouver and Toronto were found to be very similar, with the same order of greatest to least impact. The main difference between results for the modelled buildings in the two climates is a greater negative effect for two parameters - trickle vents and occupant density - using Toronto weather data. 57 Figure 8-2 IAQ Ventilation Rate Parametric Analysis Results Parameters Effecting IAQ Ventilation Rate Vancouver Base Case Office Building Passive Stack Area Passive Stack Height Trickle Vent Size Building Height Local Wind Shielding Occupant Density Openable Window Area Building Orientation - Base Case • Positive Effect • Negative Effect •f-itj 0 5 10 15 20 25 30 35 % Occupied Hours With Ventilation Less Than 10 L/s.person Parameters Effecting IAQ Ventilation Rate Toronto Base Case Office Building Passive Stack Area Passive Stack Height Trickle Vent Size Building Height Local Wind Shielding Occupant Density Openable Window Area Building Orientation - f -.<ILII&<-I Base Case • Positive Effect B Negative Effect •a.. ' ' J S £ . .WW •- *)•. 4TT1 1 0 10 20 30 40 50 % Occupied Hours With Ventilation Lower Than 10 L/s.person 58 The following is an explanation of how the modification of each parameter effects IAQ ventilation rates, and any major differences between Vancouver and Toronto results. 8.2.1 Addition of a Passive Stack The effect of adding a passive natural ventilation stack to each of the base case buildings is shown at the top of each graph shown in Figure 8-2. A stack was added to the base case building under two scenarios. In the first, a stack with a small opening area (2.8m2) was added to the roof of the three storey building and progressively increased in height. In the second scenario a stack of fixed 7 meter height (and initially zero area) was added to the top of the building and progressively increased in opening area. The addition of almost any stack - even of zero height or very small diameter - has the effect of dramatically increasing IAQ ventilation rates, meeting the IAQ ventilation criteria of a naturally induced ventilation rate of at least 10 L/s per person for a minimum of 99% of occupied office hours. This result does not strictly present theresults of adding stack ventilation in isolation to the building. It shows the effect of adding a passive stack to a building that already utilises cross ventilation through windows, and stack ventilation resulting from buoyancy driven airflow through the three stories of building height. This is a much more common situation than adding a passive stack to a building in isolation of other natural ventilation mechanisms. 8.2.2 Occupant Density Occupant density strongly effects indoor air quality when the criteria for ventilation performance is set out on a flow rate per person basis, such as 10 L/s per person. Using this criteria, a building with fewer occupants will obviously require a lower rate of total airflow to maintain 10 L/s per person. The occupant density for the base case building was set at 20m 2 per person. As occupant densities were gradually decreased, in both Vancouver and Toronto base case buildings the IAQ ventilation criteria could be met at occupant densities of 25 m 2 per person, (i.e. naturally induced ventilation rate of at least 10 L/s per person for a minimum of 99% of occupied office hours). 8.2.3 Trickle Vents The modelling results show that the use of trickle vents that are open at all times, independently of whether or not windows are open, can dramatically increase indoor air quality ventilation rates. As shown in Figure 8-2, without any trickle vents the Vancouver and Toronto base case buildings have naturally induced ventilation rates that are less than 10 L/s per person for approximately 27% and 40% of occupied hours per year respectively. Because the modelling assumed that windows are only opened when the indoor air temperature is greater than 20°C, the only mechanism for winter ventilation is air leakage through the building envelope. However a level of airtightness that would be considered high for new office buildings was assumed for the exterior envelope of the 59 base case building. Less air-tight buildings with no trickle vents would show higher ventilation rates. Increased trickle vent size dramatically increases airflow rates. The base case building 2 2 contains trickle vents with an equivalent opening area of 10 cm perm of floor area. In both Vancouver and Toronto, trickle vents have to be increased to approximately 13 cm 2 per m 2 of floor area to meet the IAQ ventilation criteria of naturally induced ventilation rate of at least 10 L/s per person for a minimum of 99% of occupied office hours. 8.2.4 Building Height Increased building height increases the stack effect when outdoor temperatures are lower than indoor air temperatures, resulting in increased airflow into the ground floor of the building, the location where ventilation performance was measured in this study. Increased stack effects then increase airflow rates through the trickle ventilators, leakage areas of the building envelope, and windows (when open). The height of the building was modified from 1 story to a maximum of 10 stories. It was found that a height of 4 stories increases the airflow rate on the lower floor to the point that the IAQ ventilation criteria was met in both Vancouver and Toronto buildings. 8.2.5 Openable Window Area Increasing the openable window area increases indoor air quality ventilation rates due to decreased resistance to airflow through window openings. In the modelled building, windows are assumed to be opened by occupants only to provide cooling when the indoor air temperature is greater than 20°C. Therefore, increased window opening area does not effect indoor air quality ventilation rates during a large portion of the year when cooling is not required. As a result, modifying the openable window area has a relatively small impact on IAQ ventilation rates compared to other parameters. 8.2.6 Local Wind Shielding Decreasing local wind shielding increases the wind velocity and wind pressure on the building. This results in higher airflow rates through trickle ventilators, windows, and other leakage areas of the building envelope. It was found that airflow rates were increased, to the point that the IAQ ventilation criteria were met in both Vancouver and Toronto buildings, when local wind shielding was decreased by lowering the height of local wind obstructions to Vi building height from the base case condition of full building height. The level of local wind shielding is usually determined by existing buildings and natural features on the building site and within adjacent properties. As a result, changing the level of local wind shielding is often not a design strategy available to the building designer. 60 8.2.7 Building Orientation Rotating the building changes the orientation of open windows and trickle vents in relation to the prevailing wind direction. In the base case building, walls containing windows and vents face directly east and west. Rotating the Vancouver building in any other direction reduces IAQ ventilation rates slightly since the prevailing wind direction in winter months is from the east. In Toronto the prevailing wind direction in winter months is from the north. Therefore changing the orientation of the east and west facing walls to any other direction shows an increase in ventilation rates. 61 9 Analysis of Rule of Thumb Limitations The base case office building was modelled with the NatVent program to determine the validity and limitations of existing rules of thumb for use in Vancouver and Toronto climates. Design parameter modifications were each applied to the base case building using their upper and lower limits as presented in Table 6-1. Instead of looking at the effect of parameter modifications individually as was performed in Chapter 8, the effects of combined parameter modifications were investigated to examine the full extent of rule of thumb limitations. The criteria used to measure minimum acceptable natural ventilation performance were those developed in Chapter 4, and are summarised below: 1. Thermal comfort limitations were evaluated according to the criteria that the following indoor air temperature limits for the specified number of working hours per year should not be exceeded: 0 25°C for not more than 100 occupied hours 0 28°C for not more than 25 occupied hours 2. Ventilation for indoor air quality limitations were evaluated according to the criteria that naturally induced ventilation rates should provide 10 L/s of outdoor air per person for a minimum of 99% of occupied office hours. 62 9.1 Cross Ventilation Depth Limit The rule of thumb that "cross ventilation is effective up to five times the floor to ceiling height" was investigated by modelling the natural ventilation performance of the base case cross ventilated building using both Vancouver and Toronto weather data. The base case building was modified from the configuration used to carry out the parametric analysis presented in the previous chapter, to enable more accurate modelling of rules of thumb that are based on a cross ventilation strategy alone. This was accomplished by isolating floors from one another to reduce the stack effect due to building height. The configuration for the cross ventilated base case building is shown in Figure 9-1. Cross Ventilation Base Case Building 3 = = ! > = > = > = * ? - = > => => ^1 • _ a—»»»> ^ • | ^ Variable Building Depth ^ | Figure 9-1 Base Case Cross Ventilation Configuration Used for Rule of Thumb Analysis Natural ventilation performance was based on temperature and airflow measurements observed on the lower floor of the three story cross ventilated building. To investigate the effect of floor to ceiling height, two scenarios were modelled - one with a typical office building floor to ceiling height of three meters and another with an exaggerated floor to ceiling height of nine meters. 9.1.1 Cross Ventilation Overheating Depth Limit Maximum depth limits achievable in the base case cross ventilated building without causing overheating are shown in Figure 9-2. The figure shows how the depth limit of an office building, under the two different floor to ceiling height scenarios, can be extended through the progressive cumulative addition of eight cross ventilation parameter modifications. While a cut off of 50m building depth is presented in the figures, in certain cases building depths of greater than 50 m were achieved while maintaining acceptable thermal comfort conditions. These are represented with an extended dashed line. 63 Cross Ventilation Thermal Depth Limit Vancouver Base Case Building Increase Openable Window Area 1. Increase Wall and Roof Insulation 2. Significant Solar Protective Glazing 3. Increase Window Overhang Size 4. Increase Thermal Mass 5. Improve Orientation 6. Add External Window Shading 7. Decrease Local Wind Shielding 8. • 3 m Floor to| Ceiling Height • 9 m Floor to| Ceiling Height 10 20 30 40 Overheating Depth Limit (m) 50 c 2 E C Cross Venti lat ion Thermal Depth Limit Toronto Base Case Building Increase Openable Window Area 1. Increase Wall and Roof Insulation 2. Significant Solar Protective Glazing 3. Increase Window Overhang Size 4. Increase Thermal M ass 5. Improve Orientation 6. Add External Window Shading 7. Decrease Local Wind Shielding 8. • 3 m Floor | to Ceiling Height • 9 m Floor | to Ceiling Height 0 10 20 30 40 50 Overheating Depth Limit (m) Figure 9-2 Cross Ventilation Thermal Depth Limits 64 Before the addition of any parameter modifications, the 3 m floor to ceiling height building was found to have a maximum building depth achievable, without exceeding the overheating performance criteria, of 5m and 3m for Vancouver and Toronto buildings respectively. For the 9 m building the same depth limits were found to be 1 Om and 6m for Vancouver and Toronto buildings respectively. Each parameter was modified to its practical maximum cooling limit as presented in Table 6-1. The order of addition of parameter modifications was based on an assumed level of difficulty of implementation in new building design, from easiest (1) to hardest (8), based on practicality and cost. Several parameters that have little impact on extending thermal depth limits were not included. For example, even though they can have a significant impact on overheating, the effects of decreasing occupant density and lighting and equipment loads were not included because in the base case building these parameters are already close to their limits of lowest impact on overheating. The resulting cross ventilation thermal depth limits were converted into a ratio of building depth to floor to ceiling height. These results are presented in Table 9-1. The effect of local wind shielding was presented separately because, unlike other parameter modifications carried out in the analysis, it is a site dependent variable rather than a building design variable and has such a large impact on natural ventilation performance. Decreasing local wind shielding is not usually possible in urban environments. Parameters 1 though 7 however could be fairly easily implemented in most cross ventilation building design projects. Overheating Depth Limit j With Local Wind Shielding Reduction Without Local Wind Shielding Reduction, Depth (m) Depth/Floor to Ceiling Height Depth : (in) Depth/Floor to Ceiling Height Vancouver 3m floor to ceiling height 49 16 27 9 Vancouver 9 m floor to ceiling height 100 11 56 6 Toronto 3m floor to ceiling height 22 7 12 4 Toronto 9 m floor to ceiling height 32 4 20 2 Table 9-1 Thermal Depth Limits of Cross Ventilated Office Buildings 65 A graphical summary of the resulting ranges of overheating depth limits for Vancouver and Toronto climates are shown in Figure 9-3. Cross Ventilation Overheating Depth Limit Range • Bat ^Original Rul£ of Thumb 1 1 0 ' 5 10 15 20 Overheating Depth Limit (Ratio of Building Depth / Floor to Ceiling Height) Figure 9-3 Range of Applicability of Cross Ventilation Rule of Thumb from a Thermal Comfort Perspective 9.1.1.1 Vancouver What these results show is that from an overheating perspective, the Vancouver building was able to easily achieve, and go far beyond, the depth limits suggested by the cross ventilation rule of thumb. Using cross ventilation alone, the Vancouver building is able to maintain adequate thermal comfort conditions for building depths far greater than 5 times the floor to ceiling height, for both floor to ceiling height scenarios modelled, and whether or not local wind shielding was reduced from that of the base case building. In the best case scenario, the 3m floor to ceiling height Vancouver building is able triple the building depths suggested by the rule of thumb. 9.1.1.2 Toronto Modelling results using Toronto weather data show that in this climate it is also possible to achieve an overheating depth limit of five times the floor to ceiling height using cross ventilation alone. However, it is difficult to extend this limit significantly. The rule of thumb depth limit was only achieved using the combined effect of all cross ventilation parameters modelled, including the reduction in local wind shielding, and only for the lower floor to ceiling height scenario. The rule of thumb depth limit was not achieved for the larger floor to ceiling height scenario, or for any scenario where the effect of reduced local wind shielding was not included. 9.1.1.3 Floor to Ceiling Height Effects The modelling results show that cross ventilation depth limits are not dependent on cross ventilation airflow alone, but are highly influenced by stack effects as well. As floor to ceiling height increases, the thermal buoyancy driven stack effect increases over the height of space. The rate of airflow into the building through windows and openings near 66 floor level is increased, as is the rate of airflow out of the building at ceiling level. The result is a greater rate of airflow and increased cooling, leading to a greater overheating depth limit. However, modelling results shown in Figure 9-2 show that increased floor to ceiling height is not directly proportional to increased thermal depth limits. Results for both Vancouver and Toronto show that an increase in floor to ceiling height of three times leads to less than a doubling in corresponding thermal depth limits. Therefore, this non direct proportionality should be taken into account when using the cross ventilation rule of thumb based on floor to ceiling height. 9.1.2 Cross Ventilation IAQ Depth Limit The base case office building was modelled with the NatVent program to determine how the depth of the building could be increased by modifying key natural ventilation building design variables, while maintaining adequate ventilation rates for acceptable indoor air quality. The occupancy rate in office buildings is typically specified in terms of a ratio of floor area per occupant. Therefore as the building depth is increased, the number of occupants increases, resulting in a greater required volume of ventilation airflow into the building. Modelling results of maximum depth limits achievable in the base case building while maintaining ventilation rates of 10 L/s per person for 99% of time are shown in Figure 9-4. Before the addition of parameter modifications to the 3m floor-ceiling height base case building, it was found that the criteria of maintaining ventilation rates of 10 L/s per person for 99% of time was not achievable, no matter how much the building depth was reduced, in both Toronto and Vancouver. In the 9 m floor-ceiling height base case building, the building depth could be increased to a maximum of approximately 12m in Toronto and 13 m in Vancouver, while maintaining the IAQ ventilation rate criteria. These results show that the cross ventilation airflow rates are significantly augmented with increasing floor to ceiling height. Figure 9-4 also shows how the depth limit of an office building, under the two different floor to ceiling height scenarios, can be extended through the progressive cumulative addition of cross ventilation parameter modifications that improve ventilation rates. Each parameter was modified to its practical maximum limit of benefit, as presented in Table 6-1. The order of addition of parameter modifications is based on an assumed level of difficulty of implementation in new building design, from easiest (1) to hardest (5), in terms of cost and practicality. The resulting cross ventilation IAQ depth limits were converted into a ratio of building depth to floor to ceiling height. These results are presented in Table 9-2. 67 Cross Venti lat ion IAQ Depth Limit Vancouver B a s e C a s e Building Increase Openable Window A r e a 1. Increase Trickle Vent S ize 2. Improve Orientation 3. Decrease Occupant Density 4. Decrease Local Wind Shielding 5. 10 20 30 40 IAQ Depth Limit (m) • 3 m Floor to Ceiling Height • 9 m Floor to Ceiling Height 50 Cross Venti lat ion IAQ Depth Limit Toronto B a s e C a s e Building Increase Openable Window A r e a 1. Increase Trickle Vent S ize 2. Improve Orientation 3. Decrease Occupant Density 4. Decrease Local Wind Shielding 5. 10 20 30 40 IAQ Depth Limit (m) | Q 3 I T I Floor to Ceiling Height 19 m Floor to Ceiling Height 50 Figure 9-4 Cross Ventilation IAQ Depth Limits . IAQ Ventilation Depth Limit Depth (m) Depth/Floor-Ceiling Height Vancouver 3m floor to ceiling height 25 8 Vancouver 9 m floor to ceiling height 50 6 Toronto 3m floor to ceiling height 25 8 Toronto 9 m floor to ceiling height 50 6 Table 9-2 IAQ Depth Limits of Cross Ventilated Office Buildings A graphical summary of the resulting ranges of overheating depth limits for Vancouver and Toronto climates is shown in Figure 9-5. C r o s s Vent i la t ion IAQ D e p t h L imi t R a n g e Toronto Vancou\er 0 IAQ Depth Limit (Ratio of Building Depth / Floor to Ceiling Height) Figure 9-5 Range of Applicability of Cross Ventilation Rule of Thumb from a Thermal Comfort Perspective 9.1.2.1 Vancouver and Toronto What these results show is that, from an indoor air quality perspective, it is possible to achieve the five times floor to ceiling height depth limits set out by the cross ventilation rule of thumb, under both floor to ceiling height scenarios, in both Vancouver and Toronto climates. The scenario with the greatest ratio of depth to floor-ceiling height is the 3m floor to ceiling height building with the combined benefit of all cross ventilation parameter modifications. In this scenario the depth limit was extended to 8 times the floor to ceiling height. The results also show that indoor air quality depth limits were found to be almost identical using typical year weather data for Vancouver and Toronto climates. 9.1.2.2 Three Meter Floor to Ceiling Height It was found that the 3m floor to ceiling height base case building had difficulty achieving the ventilation rate criteria under any depth limit, even with successive cumulative modifications to building design parameters. Using the combined benefits of , Original Rul£ of Thumb 10 15 20 69 maximised window opening and trickle vent areas, and optimised building orientation for airflow, the building still could not meet the ventilation rate criteria under any depth limit. Only by reducing the occupancy ratio to between 40 and 50 m 2 per person, or by eliminating local wind shielding was it possible for the 3m floor to ceiling height building to meet the ventilation rate criteria, and achieve building depth limits equal to or greater than those specified by the rule of thumb. This is a very interesting result because it agrees very closely with requirements for naturally ventilated commercial buildings contained in the 1995 National Building Code of Canada (NBC). The N B C specifies that outdoor air supplied by ventilation systems must not be less than the rates required by A S H R A E Standard 62, and that this ventilation can be provided by natural ventilation only for commercial buildings with an occupant load of not more than one person per 40 m 2 during normal use. The conclusion that can be made is that the modelled results from this study agree with the N B C limitations on occupancy density in commercial buildings that are naturally ventilated, for cross ventilated buildings with floor to ceiling heights of 3m, located in urban environments with local obstructions of full building height. 9.1.2.3 Nine Meter Floor to Ceiling Height The cross ventilated building with 9m floor to ceiling height was found to have much higher ventilation rates. Before the addition of any parameter modifications, it was found that this base case building was able to achieve the ventilation rate criteria up to depth limits of 13m and 12m for Vancouver and Toronto climates respectively (less than 2 times the floor to ceiling height). Increased trickle vent size, or decreased occupant density, or decreased local wind shielding were all found to extend the depth limit to and beyond the limitation of the cross ventilation rule of thumb. In the best case scenario with all of these parameter modifications combined the IAQ depth limit was extended to 6 times the floor to ceiling height. 9.1.2.4 Floor to Ceiling Height Effects Similar to modelling results for cross ventilation overheating depth limits, it was found that cross ventilation IAQ depth limits are not dependent on cross ventilation airflow alone, but are highly influenced by stack effects. As floor to ceiling height increases, the thermal buoyancy driven stack effect increases over the height of the space. Changes to floor to ceiling height were found to have a much greater influence on ventilation for IAQ depth limits than they had on overheating depth limits. The relationship between floor to ceiling height and ventilation depth limit is roughly directly proportional in the case of modifications to occupancy density. For other parameter modifications it is not possible to evaluate this relationship because no depth limit existed for the 3 m floor to ceiling height scenarios. 70 9.1.3 Cross Ventilation Rule of Thumb Mitigating Factors A number of influences other than the design parameter modifications modelled in this study can directly effect cross ventilation performance and application of the cross ventilation rule of thumb. These include: 0 Internal heat gains - The original rule of thumb is based on office buildings in the U K with moderate to high heat gains (20 to 50 W/m 2 heat gains from people, lighting, and equipment). The rule states that deeper spaces are possible i f heat gains are lowered. In this study, internal heat gains of approximately 14 W/m 2 were assumed, which represents a building with low internal loads. Higher internal heat gains would reduce the cross ventilation overheating depth limits presented above. Ideally the rule of thumb should be presented for building's with low internal heat gains because high internal heat gains should be avoided as part of the overall design strategy. 0 Surface temperatures - Indoor air temperature was used to measure overheating performance rather than dry resultant or operative temperature, because of output data reporting limitations in the NatVent software model. This should produce an overestimate of overheating hours and an underestimate of overheating depth limits, because indoor surface temperatures will most often be lower than indoor air temperatures, resulting in lower dry resultant or operative temperatures. 0 Location within the building - Overheating and ventilation performance was evaluated on the lower level of a three storey cross ventilated building. Wind speeds increase with height above ground, and the effect of local wind obstructions decrease. Therefore cross ventilation airflow rates wil l be higher on higher floor levels of the building. Cross ventilated buildings that are isolated from stack effects over the height of the building will have extended overheating and ventilation depth limits on higher floor levels of the building. 0 Non isolated floors - The building modelled was assumed to have floor levels completely isolated from one another. If floor levels are connected by atriums, open stairways, or other airflow openings, then stack ventilation wil l increase airflow rates and increase the overheating and ventilation depth limits on floor levels below the neutral pressure plane. 0 Internal partitions, the effect of which were ignored in this study, can greatly reduce cross ventilation airflow for ventilation and cooling. Overheating and ventilation depth limits will be reduced for buildings that have internal partitions that obstruct cross ventilation airflow. 0 Temperature stratification - Increased floor to ceiling height can lead to temperature stratification, isolating the hottest air on each floor level from the 71 occupied zone. Temperature stratification was not taken into account in this study. If it was, overheating depth limits could be extended. 0 Occupancy schedule - The building occupancy schedule can have significant impacts on the overheating depth limit. The base case building was considered occupied from 8 am to 6 p.m., five days per week. Reduced occupancy hours would reduce the number of overheating hours, particularly i f occupancy is reduced at the end of the day when overheating is most likely to occur due to building thermal storage delays. This would lead to extended cross ventilation overheating depth limits. 0 Local terrain roughness - The surface roughness of local terrain wil l have large effects on wind speed at the building site. The base case building modelled is located in an urban terrain setting. As a result, wind speeds at the site are significantly reduced due to boundary layer effects. Locations that are more open wil l receive higher wind speeds, resulting in increased ventilation rates and increased IAQ and overheating depth limits. 0 Drafts - Drafts associated with trickle ventilators in cold climates can be an issue. If drafts are too high, windows and vents may be closed by occupants, decreasing the maximum thermal and IAQ depth limits. However, designs that properly slow and temper the incoming air can eliminate problems associated with drafts. The main strategy to avoid problems is to temper and slow the incoming air by incorporating it into the heating system, or by providing contact with the building's thermal mass. 72 9.2 Stack Ventilation Depth Limit The rule of thumb that "stack ventilation can be effective across a width of 5 times the floor to ceiling height from the inlet to where the air is exhausted to the stack" was investigated by modelling the natural ventilation performance of the base case stack ventilated building using both Vancouver and Toronto weather data. The base case building is a three story office building with a large atrium running down its centre. The atrium forms a passive stack by extending above the roof of the building. Office spaces on either side of the atrium are directly connected to the atrium and stack. The configuration for the stack ventilated base case building is shown in Figure 9-6. Stack Ventilation Base Case Building OI •s 1 a =*=> => t 2 i a '5 X •~...| Variable Building Depth Figure 9-6 Base Case Stack Ventilation Configuration Used for Rule of Thumb Analysis The rule of thumb as stated implies that the maximum depth limit is directly proportional to the floor to ceiling height of individual floors. However the rule was originally developed based on a building configuration with office spaces open to a large atrium, such as that shown in Figure 9-6. With this type of configuration there is a continuous column of air running from the base of the building to the height of airflow openings in the stack on top of the building. As a result, the magnitude of stack effect induced pressure differences on the lower floor depend on the height of the column of air over the entire height of the building plus the height of the stack above the building. Therefore the stack effect depends on the building height and stack height, but not the floor to ceiling height of individual floors. To explore the limitations of the rule of thumb the building performance was modelled first with no stack and then with the tallest and largest size stack that was modelled in the previous parametric study - a passive stack with a height of 7m above the height of the building and an airflow opening area of 16 m 2 . Because the natural ventilation performance is also highly dependent on building height, two building height scenarios were also modelled: 73 a) A building height of 3m, representing a one story building with a typical office building floor to ceiling height of 3m, and b) A building height of 9m, representing a three story office building with a floor to ceiling height of 3m on each level. In the case of the three story stack ventilated building the natural ventilation performance was evaluated based on temperature and airflow measurements observed on the lower floor. On higher floor levels the stack induced airflow rates will be reduced, resulting in lower overheating and IAQ ventilation depth limits. 9.2.1 Stack Ventilation Overheating Depth Limit The maximum depth limits achievable without causing overheating are shown in Figure 9-7. The figure shows how the total depth limit of a stack ventilated office building can be extended through the addition of a passive stack, under the two different building height scenarios, each with a passive stack with a height of 7m above the top of the building and an airflow opening area of 16 m 2 . Stack VentilationOverheating Depth Limits Vancouver and Toronto Toronto Base Case, No Stack Toronto - Passive Stack Vancouver Base Case, No Stack Vancouver - Passive Stack • One Story • Three Story 10 20 30 40 Overheating Depth Limit (m) 50 Figure 9-7 Stack Ventilation Thermal Depth Limits Before the addition of the passive stack, in the one storey base case building it was found that the maximum building depth achievable without exceeding the overheating performance criteria was 5m in the Vancouver building and 3m in the Toronto building. For the three story base case building the depth limits were found to be 10m and 6m for Vancouver and Toronto buildings respectively. These results show that the stack induced airflow rates and resulting overheating depth limits on the lower floor of the building are significantly augmented with increasing building height, even without the use of a passive stack on top of the building. Once a passive stack is added to each building, the stack induced ventilation rates and resulting overheating depth limits are significantly increased for both the one and three 74 story buildings. These modelled stack ventilation thermal depth limits were converted into a ratio of building depth to (a) floor to ceiling height, (b) building height, and to (c) total building plus stack height, and are presented in Table 9-3. The ratio of building depth to floor to ceiling height was calculated using an assumed floor to ceiling height of 3m in both one story and three story buildings. 5 (a) (b) ir (c) •: Overheating Depth/ Depth/ • Depth/ Depth Limit > Floor-Geiling Building Building Plus • (in) T Height Height * Stack Height Vancouver One Story 42 14 14 4.2 Vancouver Three Story 100 33 11 6.3 Toronto One Story 12 4 4 1.2 Toronto Three Story 18 6 2 1.1 Table 9-3 Thermal Depth Limits of Stack Ventilated Office Buildings A graphical summary of the resulting ranges of overheating depth limit to floor to ceiling height ratios for Vancouver and Toronto climates are shown in Figure 9-8. The original rule of thumb is shown as a ratio of overheating depth limit to floor to ceiling height of 10. This ratio results from using the rule of thumb of 5 times the floor to ceiling height on either side of the stack, without taking into account the width of the stack. Stack Ventilation Overheating Depth Limit Range Toronto Vancouver Original Rul^ of Thumb 0 5 10 15 20 25 30 35 Overheating Depth Limit (Ratio of Building Depth / Floor-Ceiling Height) Figure 9-8 Stack Ventilation Rule of Thumb - Range of Applicability of from a Thermal Comfort Perspective 9.2.1.1 Vancouver What these results show is that from an overheating perspective, the Vancouver base case building with an added passive stack was able to achieve and exceed the depth limits suggested by the stack ventilation rule of thumb, but the Toronto building was not. 75 The Vancouver base case building is able to maintain adequate thermal comfort conditions for building depths equal to and much greater than 10 times the floor to ceiling height, for both building height scenarios modelled. A maximum depth limit of approximately 33 times the floor to ceiling height was achieved in the taller 3 story building scenario, using an assumed floor to ceiling height of 3 m. 9.2.1.2 Toronto Modelling results using Toronto weather data show that in this climate it is not possible to achieve an overheating depth limit of 10 times the floor to ceiling height using stack ventilation alone added to the base case building. Maximum depth limits of approximately 6 times the floor to ceiling height were achieved with the taller 3 story building scenario, representing building depths of slightly greater than one half of that suggested by the rule of thumb. Stack ventilation is much more effective at providing cooling in Vancouver than Toronto because Vancouver has lower summertime outdoor air temperatures. Stack ventilation is only effective when the outdoor air temperature is lower than the indoor air temperature. Vancouver summer daytime temperatures rarely exceed 25 °C, resulting in effective stack ventilation flow rates of cool air for all but very rare occasions. 9.2.1.3 Building Height and Stack Height Effects The ratios (a), (b), and (c) calculated in Table 9-3 show that while the rule of thumb is based on a ratio of building depth to floor to ceiling height, a much better relationship is the ratio of building depth to the height of the building plus the height of the stack above the building. The ratio of building depth to floor to ceiling height as shown in column (a) of Table 9-3 was calculated using an assumed floor to ceiling height of 3m in both one story and three story buildings. The resulting depth limit ratios vary greatly between the one story and three story buildings. Because the ratio varies greatly, this shows that there is a poor correlation between building depth limits and floor to ceiling height. This result is expected because each floor level is open to the atrium, and therefore stack effects are dominated by the height of the column of air in the atrium rather than the height of the column of air on each floor level. The overheating depth limit is highly dependent on the total height of the column of air that acts as the stack in the building, as shown in column (c). This total height is made up of the height of the building above the location of interest plus the height of the stack above the roof of the building. The relationship between overheating depth limit and total stack height is roughly directly proportional, at a ratio of approximately 5:1 in Vancouver and 1:1 in Toronto. Because of this good relationship, the rule of thumb should be based on this total stack height rather than on floor to ceiling height. The ratio of depth limit to building height, as shown in column (b), shows greater variance between one story and three story buildings than the ratio of depth limit to 76 building plus stack height. This is due to the fact that the depth limits are based on using the same size stack on top of both the one story and three story buildings. Therefore the one story building has a stack that is proportionally larger compared to building height than the three storey building. A maximum stack height of 7m above the roof was used for both buildings based on the assumption that the limits to stack height are based on aesthetics alone, and 7m was assumed to be a reasonable limit for either building height. The stack ventilation rule of thumb for overheating could be improved by basing it on the relationship between thermal depth limits and building plus stack height. A graphical summary of modelling results presented using this ratio are shown in Figure 9-9. Stack Ventilation Overheating Depth Limit Range Toronto Vancouver 0 1 2 3 4 5 6 7 Overheating Depth Limit (Building Depth / Building Plus Stack Height) Figure 9-9 Modified Stack Ventilation Rule of Thumb - Range of Applicability from a Thermal Comfort Perspective As shown, the overheating depth limit of the base case building could be extended to 1.2 times the building plus stack height in Toronto, and to 6.3 times the building plus stack height in Vancouver. Using these results a modified stack ventilation rule of thumb was developed as: > Stack ventilation can be effective for overheating control across a building width of 6 times the building plus stack height in Vancouver and 1 times the building plus stack height in Toronto. This rule is valid for buildings up to three stories in height with parameters similar to the base case building. The modelling results are based on airflow and temperature conditions found on the ground floor of the building. Therefore in buildings with more than one floor level, the stack induced airflow rates, and hence overheating depth limits presented, are reduced with increasing height within the building. 77 9.2.2 Stack Ventilation IAQ Depth Limit A passive stack was added to the base case office building to determine how much the depth of the building could be increased while maintaining adequate ventilation rates for acceptable indoor air quality. Figure 9-10 shows modelling results of how the IAQ depth limit of office buildings, of one and three story heights, can be extended through the addition of a passive stack, while maintaining ventilation rates of 10 L/s per person for 99% of time. The passive stack on top of the building measures 7m in height above the roof of the building and has an airflow opening area of 16 m 2 . Stack Ventilation IAQ Depth Limits Vancouver and Toronto Toronto Base Case, No Stack Toronto - Passive Stack Vancouver Base Case, No Stack Vancouver - Passive Stack -• One Story • Three Story -10 20 30 40 IAQ Depth Limit (m) 50 Figure 9-10 Stack Ventilation IAQ Depth Limits Before the addition of the passive stack to the one storey base case building, it was found that the criteria of maintaining ventilation rates of 10 L/s per person for 99% of time was not achievable, no matter how much the building depth was reduced, in both Toronto and Vancouver. In the three story base case building, the building depth could be increased to a maximum of approximately 12m in Toronto and 13 m in Vancouver. These results show that the stack induced airflow rates on the lower floor of the building are significantly augmented with increasing building height, even without the use of a passive stack on top of the building. Once a passive stack is added to each building, the stack induced ventilation rates are significantly increased for both the one and three story buildings. These modelled stack ventilation thermal depth limits were converted into a ratio of building depth to (a) floor to ceiling height, and (b) total building plus stack height, and are presented in Table 9-4. 78 The ratio of building depth to floor to ceiling height was calculated using an assumed floor to ceiling height of 3m in both one story and three story buildings. • •••fi' • r -(aF-"-'"; fV (b) IAQ Depth Depth/ Depth/ Limit (hi) Floor-Geiling % Building Plus Height Stack Height Vancouver One Story 50 17 5 Vancouver Three Story 100 33 6 Toronto One Story 50 17 5 Toronto Three Story t 80 27 5 Table 9-4 IAQ Depth Limits of Stack Ventilated Office Buildings A graphical summary of the resulting ranges of IAQ depth limit to floor to ceiling height ratios for Vancouver and Toronto climates are shown in Figure 9-11. The original rule of thumb is shown as a ratio of IAQ depth limit to floor to ceiling height of 10. This ratio results from using the rule of thumb of 5 times the floor to ceiling height on either side of the stack, without taking into account the width of the stack. Stack Ventilation IAQ Depth Limit Range Toronto Vancouver 1 ; 141- HI Original Rule of Thumb ; 1 — i 1 1 0 5 1 0 1 5 2 0 2 5 3 0 3 5 IAQ Depth Limit (Ratio of Building Depth / Floor to Ceiling Height) Figure 9-11 Stack Ventilation Rule of Thumb - Range of Applicability of from an IAQ Perspective 9.2.2.1 Vancouver and Toronto What these results show is that from an indoor air quality perspective, the Vancouver and Toronto base case buildings with added passive stacks were both able to achieve and exceed the depth limits suggested by the stack ventilation rule of thumb. 79 The Vancouver base case building was able to maintain adequate IAQ ventilation rates for building depths up to a maximum of approximately 33 times the floor to ceiling height - in the case of the three storey building with an assumed floor to ceiling height of 3m. The Toronto base case building was able to maintain adequate IAQ ventilation rates for building depths of up to a maximum of 27 times the floor to ceiling height under the same scenario. The stack effect has a greater influence on extending IAQ depth limit in Vancouver than Toronto. This is due to the fact that summertime outside air temperatures are cooler in Vancouver, leading to a stronger stack effect. 9.2.2.2 Building Height and Stack Height Effects Similar to stack ventilation rule of thumb overheating depth limits, it was found that while the rule of thumb is based on a ratio of building depth to floor to ceiling height, a much better relationship is the ratio of building depth to the height of the building plus the height of the stack above the building. The ratio of building depth to floor to ceiling height as shown in column (a) of Table 9-4 was calculated using an assumed floor to ceiling height of 3m in both one story and three story buildings. The resulting depth limit ratios vary greatly between the one story and three story buildings (range of 17 to 33). The variance shows a poor correlation between building IAQ depth limits and floor to ceiling height. As expected due to the interconnection between individual floors and the atrium, the IAQ depth limit is highly dependent on the total height of the column of air that acts as the stack in the building, as shown in column (b) of Table 9-4. This total height is made up of the height of the building above the location of interest plus the height of the stack above the roof of the building. The relationship between overheating depth limit and total stack height is roughly directly proportional, at ratios varying between 5:1 and 6:1. The stack ventilation rule of thumb for IAQ ventilation could be improved by basing it on the relationship between IAQ depth limits and building plus stack height. A graphical summary of modelling results presented using this ratio are shown in Figure 9-12. As shown, the IAQ depth limit of the base case building could be extended to approximately 5 times the building plus stack height in Toronto, and to approximately 6 times the building plus stack height in Vancouver. Using these results a modified stack ventilation rule of thumb for IAQ ventilation was developed as: > Stack ventilation can be effective for IAQ ventilation across a building width of 6 times the building plus stack height in Vancouver and 5 times the building plus stack height in Toronto. 80 Stack Ventilation IAQ Depth Limit Range Toronto Vancouver 0 1 2 3 4 5 6 7 IAQ Depth Limit (Building Depth / Building Plus Stack Height) Figure 9-12 Modified Stack Ventilation Rule of Thumb - Range of Applicability from an I A Q Perspective This rule is valid for buildings up to three stories in height with parameters similar to the base case building. The modelling results are based on airflow and temperature conditions found on the ground floor of the building. Therefore in buildings with more than one floor level, the stack induced airflow rates, and hence overheating depth limits presented, are reduced at increasing height locations within the building. 9.2.3 Stack Ventilation Rule of Thumb Mitigating Factors A number of other influences that can directly effect stack ventilation performance include: 0 Cross Ventilation Parameters - Stack ventilation strategies also rely on cross ventilation between air inlets on the exterior walls of the building and the stack. Limits of the stack ventilation rule of thumb were investigated based on the base case building with no cross ventilation parameter modifications. As a result, all factors that effect cross ventilation overheating and IAQ performance will effect stack ventilation overheating and IAQ performance. Cross ventilation parameter modifications were found to greatly extend overheating and IAQ depth limits of the cross ventilated base case buildings, and would have similar effects on extending depth limits of the stack ventilated base case buildings. 0 Stack outlet design - Good design of the stack outlet can improve stack ventilation performance. The form of the stack outlet can increase wind 81 speeds, creating a negative pressure at the top of the stack (venturi effect) and aiding stack ventilation. Negative wind pressures can also be created with multiple stack vents controlled to take maximum advantage of leeward negative pressure effects. 0 Building form - The roof profile and overall building form wil l also effect the wind pressure coefficient at the stack outlet. Stack performance is enhanced with designs that use the building form to increase wind speeds and create negative pressures at the top of the building. Aerodynamic design of the entire building form and roof profile can be done to maximise this effect. 0 Stack temperature (solar chimneys) - The temperature of air in the stack or atrium effects stack ventilation performance, with higher air temperatures creating higher stack pressures. The stack temperature is effected by heat losses, which are determined mainly by the quantity of exposed surface area and the level of insulation. The stack temperature is also effected by its exposure to heat gains, and its ability to absorb those heat gains. Exposure to solar gains depends on stack orientation, with exposed south facing stacks being best for absorbing solar radiation. Solar chimneys are designed to maximise solar radiation gains, and increase the effectiveness of absorbing solar energy, particularly in spaces above the occupied zones. 82 9.3 Night Cooling Ventilation Cross Ventilation Base Case Building The rule of thumb that "a vent opening of 1/5 Oth or 2% of the floor area (200 cm 2 per m 2 floor area) should generally be adequate for night cooling when there is cross ventilation" was investigated by modelling the natural ventilation performance of the base case cross ventilated building using both Vancouver and Toronto weather data. No passive stack was included in the building design because the rule is based on cross ventilation only. However, two scenarios were modelled to investigate the effect of floor to ceiling height on night cooling performance - one a cross ventilated building with a typical office building floor to ceiling height of 3m . and another with a much higher floor to ceiling height of 9m. Two scenarios were also modelled to investigate the effect of building depth on night cooling performance - a 15m building depth and a 50 rri building depth. 15 Variable Building Depth 9.3.1 Minimum Vent Size Requirements The minimum night cooling vent size required to avoid overheating for each scenario is shown in Table 9-5. Night Cooling Vent Size (cm /rri floor area) 15 in Depth 50 m Depth Vancouver 3m floor to ceiling height 253 140 Vancouver 9 m floor to ceiling height 35 27 Toronto 3m floor to ceiling height 507 500 Toronto 9 m floor to ceiling height 192 190 Table 9-5 Night Cooling Vent Size Requirments for Cross Ventilated Office Buildings A graphical summary of the resulting ranges of overheating depth limits for Vancouver and Toronto climates is shown in Figure 9-13. 83 Required Night Cooling Vent Area Toronto Vancouver • Original Rule of Thumf) 1 I 1 2 4 6 8 Night Cooling Vent Size (% of Floor Area) 10 Figure 9-13 Night Cooling Rule of Thumb Range of Applicability These results show that in Vancouver, a vent opening of approximately 2.5% of the floor area is adequate for night cooling in cross ventilated buildings similar to the base case building. In Toronto, a larger night cooling vent size of approximately 5% of the floor area is required. Increased floor to ceiling height was found to dramatically reduce the night cooling vent size required to avoid overheating. This indicates that increased floor to ceiling height increases stack pressure differences between the inside and outside of the building, increasing airflow rates though night cooling vents. The difference in size requirements for night cooling vents between the two floor to ceiling height scenarios is much greater in the Vancouver building. This indicates that stack pressure differences have a much greater effect in Vancouver than Toronto over the course of the cooling season due to lower night time outside air temperatures. The size of night cooling vents required to avoid overheating was also found to decrease with increasing building depth. The base case building represents a zone within the centre of a long building. As the building depth is increased, solar gains through windows do not increase in this zone because the building zone is not exposed to the exterior along its sides. Therefore as building depth increases, the ratio of internal gains to floor area decreases, resulting in a lower average cooling load, requiring a lower ratio of cooling vent area to floor area. 9.3.2 Night Cooling Vent Rule of Thumb Mitigating Factors Night cooling strategies generally rely on cross ventilation between night cooling vents • on opposite or adjacent exterior walls of the building. As a result, the previously discussed factors that effect cross ventilation overheating performance wil l also effect night cooling effectiveness. A number of other mitigating factors can directly effect night cooling performance including: 84 0 Interior thermal mass storage capacity - The thermal mass storage capacity of inside surfaces will directly effect night cooling. Night cooling relies on the cooling of internal surfaces with cool night air. The ability of these surfaces to provide radiant cooling the next day is strongly dependent on their total thermal mass capacity. 0 Thermal mass rate of heat transfer - The heat transfer rate into and out of the thermal mass effects its ability to be cooled and to cool occupants the following day. The characteristics of the material in terms of its thermal conductivity, thickness, location, emissivity, and surface area in relation to the night cooling ventilation air path all effect its rate of heat transfer. 85 9.4 Ventilation for Acceptable Indoor Air Quality 9.4.1 Ventilation for Acceptable Indoor Air Quality - The United Kingdom The rule of thumb that "trickle ventilators with an openable area of 4 cm per m of floor area, with a minimum provision of 40 cm 2 in each Cross Ventilation Base Case Building 3 3 3 3 3 room, should adequately provide the necessary background ventilation to meet occupants needs" was investigated by modelling the natural ventilation performance of the base case cross ventilated building using both Vancouver and Toronto weather data. Similar to the analysis performed for other rules of thumb, two scenarios were modelled to investigate the effect of floor to ceiling height on ventilation performance - one a cross ventilated building with a typical office building floor to ceiling height of 3m and another with a much higher floor to ceiling height of 9m. => Variable Building Depth In each scenario the trickle vent size was increased until the ventilation criteria of 10 L/s of outdoor air per person for a minimum of 99% of occupied hours was met. The trickle vent size results are shown in Table 9-6. Trickle Vent Size 2 2 (cm /m floor area) Vancouver 3m floor to ceiling height Not achieved Vancouver 9 m floor to ceiling height 12 Toronto 3m floor to ceiling height Not achieved Toronto 9 m floor to ceiling height 14 Table 9-6 Trickle Vent Size Required to Achieve 10 L/s per person Airflow for 99% of Occupied Hours The base case building with a floor to ceiling height of 3m was not able to achieve the IAQ ventilation criteria - of 10 L/s of outdoor air per person for a minimum of 99% of occupied hours - no matter how much the trickle vent size was increased, in either Vancouver or Toronto. As was discussed previously in Section 9.1.2, the only way that the 3m floor to ceiling height base case cross ventilated building could meet the ventilation criteria was by decreasing the occupancy rate to approximately 40 m 2 per person, by decreasing local wind shielding, or using additional stack ventilation to increase airflow rates. The ventilation rate criteria was met in 9m floor to ceiling height base case buildings with 2 2 2 2 * trickle vent opening areas of 12 cm per m of floor area in Vancouver and 14 cm /m in 86 Toronto. Therefore, based on this initial examination, it could be concluded that to meet the ventilation criteria adopted in this study trickle vent sizes in the range of 12 to 14 cm 2 per m 2 of floor area are required. Additionally, the building must be designed to use some level of stack ventilation (such as increased floor to ceiling height) to increase airflow rates above those achieved with cross ventilation alone in a standard floor to ceiling height building. However, the trickle vent rule of thumb of 4 cm per m of floor area originates from British building code requirements that are based on the provision of approximately 5 L/s per person of 'background ventilation' during winter months in the U K . This rate of ventilation is intended to fully purge the air in a space overnight, providing a "reservoir" of clean air. A reservoir of clean air, combined with the continuing background flow of incoming fresh air, and rapid ventilation by opening windows for short periods to replenish the reservoir, is intended to maintain acceptable indoor air quality throughout the day. The performance of the base case building was re-evaluated using this 5 L/s per person ventilation rate criteria and the rule of thumb vent size of 4 cm 2 per m 2 of floor area. The percentage of occupied hours that the ventilation rate does not achieve the criteria of 10 L/s per person and 5 L/s per person were measured, and are shown in Table 9-7. % of Occupied Hours Not Meeting the Ventilation Rate Criteria 10 L/s per person 5 L/s per person Vancouver 3m floor to ceiling height 32 27 Vancouver 9 m floor to ceiling height 27 3 Toronto 3m floor to ceiling height 39 20 Toronto 9 m floor to ceiling height 33 3 Table 9-7 Ventilation Performance with Trickle Vent Size of 4 cm2 per m 2 of Floor Area These results show that when the criteria of 10 L/s per person is used to evaluate ventilation performance, the base case building with a trickle vent size of 4 cm 2 per m 2 of floor area provides an inadequate ventilation rate for a large percentage of occupied hours (27 to 39%) in both floor to ceiling height scenarios, in both Vancouver and Toronto climates. When the ventilation rate criteria is reduced to 5 L/s per person, ventilation in the 9m floor to ceiling height building meets the criteria most of the time (97% of the time). However, the 3m floor to ceiling height building falls short of meeting the criteria for a large portion of time (20 to 27%). Therefore it can be concluded that i f the ventilation rate criteria of 10 L/s per person is adhered to, the rule of thumb of 4 cm of trickle vents per m of floor area is not valid under any of the scenarios modelled in either Vancouver nor Toronto. Greater trickle 87 vent sizes of 12 to 14 cm 2 per m 2 of floor area can achieve the ventilation rate criteria, but only with assistance from stack ventilation. If the ventilation rate criteria of 5 L/s per person is used instead, then the rule of thumb of 4 cm 2 of trickle vents per m 2 of floor area is not valid for standard floor to ceiling height buildings with no stack ventilation assistance. However it is roughly valid (as long as not meeting the criteria for 3% of occupied hours is considered acceptable) in buildings where some level of stack ventilation, such as increased floor to ceiling height, is used to assist cross ventilation airflow. A graphical summary of the resulting ranges of applicability of the rule of thumb are shown in Figure 9-14. These ranges take into account both the 5 and 10 L/s per person ventilation rate criteria. R e q u i r e d T r i c k l e V e n t A r e a . Original Rule of Thumb . Li i i I 5 10 15 20 Trickle Vent Size (cm2/m2 Floor Area) Figure 9-14 Trickle Vent Rule of Thumb Range of Applicability The rule of thumb would be valid under the scenarios modelled in this study i f its wording was modified to reflect the identified limitations. A new wording that is more appropriate is: Trickle vents of 4 cm2 per m2 of floor area provide a ventilation rate of 5 L/s per person for the majority of the time in buildings that use the combined effect of cross ventilation and stack ventilation. Toronto Vancouver 0 88 9.4.2 Ventilation for Acceptable Indoor Air Quality - North America Three rules of thumb for achieving acceptable ventilation rates for indoor air quality were derived from North American prescriptive building code requirements for naturally ventilated buildings. Natural ventilation performance was not modelled for any of these rules of thumb - either because the rule of thumb is similar to those previously analysed, or because performance modelling was beyond the scope of this project. The potential validity and limitations of each rule are discussed to the extent possible below. 9.4.2.1 Rule 5a - Naturally ventilated building spaces should be limited to a depth of 8m from exterior wall or roof openings. This rule suggests that to achieve adequate indoor air quality, cross ventilated building spaces should be limited to a depth of approximately 16m. This requirement is similar to the cross ventilation rule of thumb presented previously, which limits the depth of a cross ventilated building to approximately 15m, assuming a floor to ceiling height of 3m. The analysis and discussion for the cross ventilation rule of thumb, as it applies to indoor air quality ventilation rates, also applies here. This analysis concluded that for buildings with 3m floor to ceiling heights as typically found in office buildings, it is difficult to meet this depth limit while maintaining the ventilation rate criteria adopted in this study. The only viable options for meeting the criteria in the 3 m floor to ceiling height base case building studied were found to be decreasing occupancy rates to below approximately 40 m per person, by decreasing local wind shielding, or by using additional stack ventilation to assist ventilation rates. However it was found to be relatively easy to meet and vastly exceed this depth limit in buildings with greater floor to ceiling heights, or buildings that make use of additional passive stacks. 9.4.2.2 Rule 5b - Openable areas in exterior walls and roofs should be at least 4% of the net occupiable floor area Modelling performed in this study showed no effect of openable window area on the number of hours with ventilation rates of less than 10 L/s per person, unless the openable window area was reduced to the size of vents used to provide trickle ventilation. In any event, the performance of openable windows cannot be described or measured in these terms. The size of openable window area addresses the amount of 'rapid ventilation' that can be achieved by opening windows. It does not address the supply rates of fresh air required to meet occupant and pollutant loads throughout the year, because openable windows are closed most of the time during cold weather. The specification of a minimum trickle vent 89 size would be much more effective at achieving the goal of adequate IAQ ventilation rates. Examination of the validity of this rule requires a study of the flushing potential of openable windows, which beyond the capability of the predictive tool used in this study. This analysis could be done with a multizone airflow modelling tool and the development of minimum performance criteria. 9.4.2.3 Rule 5c - Interior partitions or walls between naturally ventilated spaces and outside ventilation openings should have permanent openings of at least 8% of the floor area of the interior portion of the space, with a minimum opening area of 2.3 square meters. The effect of internal partitions was not addressed in this study, and therefore it is not possible to comment on the validity or limitations of this rule. 9.4.3 Ventilation for Acceptable Indoor Air Quality Mitigating Factors The provision of outside air ventilation for IAQ relies on cross ventilation or stack ventilation to provide the airflow rates through trickle vents, windows, or other openings in the exterior of the building. As a result, all of the parameter modifications and mitigating factors that effect cross and stack ventilation airflow rates for IAQ, as discussed in previous sections, will also effect the applicability of the IAQ ventilation rules of thumb discussed in this section. 90 10 Conclusions Simple natural ventilation design rules of thumb were compiled and critiqued in an attempt to make them useful to Canadian architects, and provide much needed design aids for the initial stages of natural ventilation building design. The following conclusions can be made from the findings of this study: a) Limitations of rules of thumb as applied to Canadian climates were found to differ greatly from original rules of thumb. Simple natural ventilation design rules of thumb were compiled from published literature and building codes and standards. The development of most of these existing rules of thumb were found to be based on incomplete research, and the conditions under which they are meant to apply are either not stated or are poorly defined. The limitations of applicability of these rules of thumb were investigated in this study using computer modelling analysis. Natural ventilation parameter modifications were applied to an example three storey office building located in an urban setting using weather data for the cities of Vancouver and Toronto. The computer model used to simulate building performance is a linked airflow/thermal model called NatVent. It is a comprehensive model in its procedure for calculating interior temperatures and airflow rates. At one hour time steps it calculates interior air temperatures, taking into account the effects of thermal mass and surface temperatures. It then uses interior air and surface temperatures in its calculations of airflow rates. It was chosen to perform the natural ventilation analysis because of its ability to link interior temperature and airflow rate calculations, and because it is a pre design tool that is easy to use because it defaults many of the detailed design parameters. While it should provide much more accurate estimates of interior temperatures and airflow rates than other natural ventilation design models and procedures that have been available in the past, its results have not been validated against field test measurements, and therefore should be used with caution. Modelling results showed that the limitations stated by many of the original rules of thumb are not accurate when applied to the two Canadian climates investigated. Results of computer modelling found limitations that differ greatly from those presented in the original rules of thumb. In some cases original rules of thumb were found to be conservative and in other cases they were overly optimistic in representing ventilation or cooling performance. The limitations of each rule of thumb were also found to vary greatly depending on assumptions used for the base case building, and on complex interactions between specific combinations of parameters. As a result, it is more appropriate to define rules of thumb in terms of ranges of applicability, with an understanding that the presented limits of these ranges could also be extended under specific combinations of building design parameters. Therefore the performance of natural ventilation systems in any unique 91 building requires confirmation by additional modelling with conditions that are specific to that building. A comparison between original rules of thumb and the ranges of applicability found through modelling the performance of the base case building are shown below: Original Rule # i - Cross ventilation is effective up to five times the floor to ceiling height. C r o s s Vent i la t ion O v e r h e a t i n g D e p t h L imi t R a n g e Toronto Vancouver 19 : ^Original Rul£ of Thumb i 1 0 5 10 15 20 Overheating Depth Limit (Ratio of Building Depth / Floor to Ceiling Height) C r o s s Ven t i l a t ion IAQ D e p t h L imi t R a n g e Toronto Vancouver Original Rul£ of Thumb I 0 5 10 15 20 IAQ Depth Limit (Ratio of Building Depth / Floor to Ceiling Height) The cross ventilation overheating depth limits were extended to the upper limits shown (farthest extension to the right) under the following conditions: > Base case building with a floor to ceiling height of 3m. > The following parameters cumulatively modified to their practical limits: • Increased openable window area. • Increased wall and roof insulation. • Significant solar protective glazing. • Increased window overhang size. • Increased thermal mass. • Optimised orientation. • External window shading. • Decreased local wind shielding. The cross ventilation IAQ depth limits were extended to the upper limits shown under the following conditions: > Base case building with a floor to ceiling height of 3m. > The following parameters cumulatively modified to their practical limits: • Increased openable window area. • Increased trickle vent size. • Optimised orientation. • Decreased occupant density. • Decreased local wind shielding. 92 Original Rule #2 - Stack ventilation can be effective across a width of 5 times the floor to ceiling height from the inlet to where the air is exhausted to the stack. S t a c k Vent i la t ion O v e r h e a t i n g D e p t h Limit R a n g e Toronto Vancouver • Orijginal Rute of Thumb 0 5 10 15 20 25 30 35 Overheating Depth Limit (Ratio of Building Depth / Floor-Ceiling Height) S t a c k Vent i la t ion IAQ D e p t h Limit R a n g e Toronto Vancouver „ Original Rule of Thumb . 0 5 10 15 20 25 30 35 IAQ Depth Limit (Ratio of Building Depth / Floor to Ceiling Height) The stack ventilation overheating and IAQ depth limits were extended to the upper limits shown under the following conditions: > Base case three story building. > Each floor connected to a central open atrium. > Floor to ceiling height of individual floors of 3m. > A passive stack is added to the building as an extension of the central atrium and has a height of 7 m above the top of the building and an airflow opening area of 16m . > Performance is based on conditions on the lower level of the three storey building. While the original rule of thumb is based on a ratio of building depth to floor to ceiling height, a much more representative relationship was found to be the ratio of building depth to the height of the building plus the height of the stack above the building. The range of limitations of the rule of thumb using this modified relationship are shown below. The stack ventilation overheating and IAQ depth limits were extended to the upper limits shown under the same conditions used to extended limits of the original rule of thumb, as presented above. S t a c k Vent i la t ion O v e r h e a t i n g D e p t h Limit R a n g e Toronto Vancouver 0 1 2 3 4 5 6 7 Overheating Depth Limit (Building Depth / Building Plus Stack Height)! S t a c k Vent i la t ion IAQ D e p t h Limit R a n g e Toronto Vancouver 0 1 2 3 4 5 6 7 IAQ Depth Limit (Building Depth / Building Plus Stack Height) 93 Original Rule #3 -A vent opening of 2% of the floor area (200 cm2 per m2 floor area) should generally be adequate for night cooling when there is cross ventilation. R e q u i r e d N i g h t C o o l i n g V e n t A r e a Toronto Vancouver Original pule of Thumf) 2 4 6 8 Night Cooling Vent Size (% of Floor Area) 10 The upper limits of night cooling vent size requirements are based on the following conditions: > Base case three story cross ventilated building. > Floor to ceiling height of individual floors of 3m. > Building depth of 15m. The lower limits of night cooling vent size requirements are based on the following conditions: > Base case three story cross ventilated building. > Floor to ceiling height of individual floors of 9m. > Building depth of 50m. 2 2 Original Rule #4 - Trickle ventilators with an openable area of 4 cm per m of floor area, with a minimum provision of 40 cm2 in each room, should adequately provide the necessary background ventilation to meet occupants needs. R e q u i r e d T r i c k l e V e n t A r e a Toronto Vancouver Original Rule of Thumb r 0 5 10 15 20 Trickle Vent Size (cm2/m2 Floor Area) 94 The upper limits of trickle vent size requirements shown are based on the following conditions: > Base case three story cross ventilated building. > IAQ ventilation rate criteria of 10 L/s of outdoor air per person for a minimum of 99% of occupied hours. > Stack effect assistance (Ex. - Extension of Floor to ceiling height of individual floors to 9m). The lower limits of trickle vent size requirements are based on the following conditions: > Base case three story cross ventilated building. > IAQ ventilation rate criteria reduced to 5 L/s of outdoor air per person for a minimum of 97% of occupied hours. > Stack effect assistance (Ex. - Extension of Floor to ceiling height of individual floors to 9m). b) The relative order of importance of natural ventilation parameters was identified for one example building configuration. This ranking of importance can aid designers in prioritising design changes for improving natural ventilation thermal or indoor air quality performance. Single natural ventilation design parameters were modified in the base case building to examine their ability to reduce overheating, or increase ventilation rates for IAQ. The following parameters were able to reduce overheating in the base case building, ordered from greatest to least effect: 1. Utilisation of night cooling vents. 2. Increasing operable window area. 3. Addition of a passive stack. 4. Decreasing non operable window area. 5. Increasing building height above floor level of interest. 6. Increasing solar protective glazing, blinds, or other shading devices. 7. Decreasing local wind shielding obstruction height. 8. Decreasing lighting and equipment loads. 9. Increasing wall insulation. 10. Decreasing occupant density. 11. Increasing interior thermal mass. 12. Increasing window overhang size. 13. Changing building orientation. 14. Changing window type (with no colour changes). The effect that many of these parameters can have on reducing overheating can change significantly due to interactions with other parameters. Several parameters also have 95 greater potential impacts than presented due to limitations in the ability of the NatVent model to show their impacts. Those parameters that could potentially show the greatest variance from presented results are: > Lighting and equipment loads - Very low lighting and equipment gains were assumed for the base case building, leaving little room available for improvement. In buildings with high internal heat gains a reduction in these gains could potentially be the parameter modification with the greatest ability to reduce overheating. > Interior thermal mass - Modelling results show a relatively minor impact on overheating resulting from large changes to the building's thermal mass, for both Vancouver and Toronto climates. The impact is probably much greater than presented however, due to limitations in the reporting format of the computer model. The NatVent model does not output resultant or operative temperature values so is not possible to measure the radiative cooling effect - the true benefit of increased thermal mass - even though the temperature damping and shifting effects are likely significant. > Orientation and solar shading - The base case building is assumed to be surrounded by large local obstructions which reduce solar gains within the base case building. Changes to building orientation, overhang size, and other solar shading techniques will have greater effects on buildings that are less shaded. > Building height - Temperature and airflow performance was measured on the lowest level of multi story buildings. As a result, overheating hours on higher floor levels would be higher in stack ventilated buildings due to the reduction in stack induced pressure differences and thus airflow rates with increasing height. Overheating hours in cross ventilated buildings with isolated floors would be lower on higher floor levels because of the effect of increasing outdoor wind speeds with height. The following parameters were able to increase indoor air quality ventilation rates, ordered from greatest to least effect: 1. Addition of a passive stack. 2. Decreasing occupant density. 3. Increasing trickle vent size. 4. Increasing building height. 5. Increasing openable window area. 6. Decreasing local wind shielding. 7. Optimising building orientation. The effect that many of these parameters can have on reducing overheating can change significantly due to interactions with other parameters. Several parameters also have greater potential impacts than presented due to limitations in the ability of the model to show their impacts. Those parameters that could potentially show the greatest variance from presented results are: 96 > Building orientation - The base case building is assumed to be surrounded by large local obstructions which reduce wind speeds around the base case building. Changes to building orientation that orient airflow openings towards prevailing wind directions will have greater effects on buildings that are less wind shielded. > Building height - Airflow performance was measured on the lowest level of multi story buildings. As a result, ventilation rates on higher floor levels would be lower in stack ventilated buildings due to the reduction in stack induced pressure differences and airflow rates with increasing height. Ventilation rates in cross ventilated buildings with isolated floors would be higher on higher floor levels because of the effect of increasing outdoor wind speeds with height. c) While the accuracy of modelling results is limited by the accuracy of the model and base case building parameter assumptions, the results provide a first order evaluation of limitations of existing rules of thumb and the relative importance of design parameters in Canadian climates. The NatVent model is more accurate than many other natural ventilation design methods because of three major advantages: > It is a linked airflow and thermal model that uses interior air and surface temperatures in its calculation of airflow rates. > It takes into account thermal interactions of interior thermal mass, and > It can evaluate interior air temperatures and airflow rates on an hour by hour basis over an entire year. However its accuracy is also limited by the following factors: > The building is modelled as having a single zone only. Temperature and airflow performance is always measured on the lowest level of multi story buildings. As a result, winter IAQ ventilation rates that would occur on higher floor levels are overestimated due to the stack effect enhancement of airflow rates. Overheating hours on higher floor levels would be higher in stack ventilated buildings due to the reduction in stack induced pressure differences with increased height. Overheating hours in cross ventilated buildings with isolated floors would be lower on higher floor levels because of the effect of increasing outdoor wind speeds with height. > The model only reports air temperature, rather than effective temperature, even though it calculates surface temperatures and uses them in its calculations of indoor air temperature. As a result, the number of overheating hours are overestimated since cool interior surfaces will generally lower the effective temperature below the air temperature. > The model uses a simplistic representation of building details, and doesn't allow detailed placement of airflow openings. This means that the effect of 97 features such as internal partitions are not taken into account. It also does not allow evaluation of airflow and temperature conditions at different locations within the building. > The model doesn't allow detailed modification of wind pressure coefficients. Default wind pressure coefficients taken from wind tunnel experiments are used for several generic building shapes. While these are accurate for the original building shapes, their modification for alternative building shapes is not possible. > The model's predictions have not been validated against other models or field data measurements, therefore its results have to be used with caution. Development of linked airlfow/thermal models has only begun recently, and none of the models created to date have been extensively validated due to the difficulty and expense of doing so. Parameter assumptions used for the base case building also have a strong influence on the accuracy of the results presented here. There are complex interactions between natural ventilation parameters that have significant effects on natural ventilation performance. Some parameters are strongly dependent on other parameters, with their full performance benefits achieved only with modification of another dependent variable. Therefore it is difficult to define a "typical" base case building to which modifications are made. The limitations of the software model and choice of base case building clearly effect the outcome of results. Therefore care must be used in interpreting and applying the results of this study. However, these results do provide a good first look at the validity and limitation of existing rules of thumb. They show that the rules of thumb differ greatly by climate for overheating, but not for IAQ ventilation. They also show general ranges of applicability for each rule of thumb for overheating and indoor air quality performance, for the two example cities. The results also show the relative importance that different design parameters have on overheating and indoor air quality performance for the example base case building. Used with an understanding of the base case building design to which they have been applied, these results can aid designers in focusing their efforts on the design of parameters with the greatest impact, and provide guidance into how changes made to the building form and fabric can effect overheating and indoor air quality. With this information, building designers can now use these rules of thumb as a starting point for natural ventilation building design in Canada. 98 d) Criteria for evaluating minimum acceptable performance of naturally ventilated buildings in North America are poorly defined. A number of European countries have well defined requirements for thermal comfort that allow for a level of fluctuation in interior temperature. Similar North American standards that allow for the inevitable fluctuation in interior temperature in naturally ventilated buildings do not exist. One European standard was adopted in this study and could be adopted for use in North American. However to do so would first require further investigation into levels of thermal comfort deemed acceptable by North Americans. It is often speculated that North Americans have lower levels of tolerance towards indoor temperature fluctuations and would demand cooler indoor temperature limits, so it is not clear whether or not the European overheating hour limits would be accepted or not. In North American we are also lagging Europeans in development of well defined "design weather year" weather data that has been specifically designed for simulating natural ventilation performance using computer models. A number of different typical weather year data formats are available in every country, most of which are developed using a similar method in which twelve months of observed weather data are chosen from a database of 30 or more years of data. Because each weather format differs in how it chooses the most "typical" months of weather data, they may differ significantly in their sequences of hot weather data. They generally represent average weather conditions over a number of years rather than a worst case overheating scenario of extended hot weather. Therefore internal conditions in actual buildings will definitely exceed the defined overheating hour thresholds in some years due to inevitable periods of warmer weather than contained in the design weather year. In the U K the Chartered Institution of Building Services Engineers (CIBSE) has developed "design weather year" weather data for a wide number of locations that specifically addresses this problem by ensuring that weather data files contain appropriate sequences of extended hot weather, developed using a common format. This ensures a common method of comparison to thermal comfort criteria based on a set number of hours per year exceeding a defined temperature threshold. In North America we do not have design weather year data files developed on this basis, and so our typical weather year data does not necessarily include periods of extended hot weather, and performance modelling results would not be consistent from location to location. While North American and European countries have well defined requirements for ventilation rates for adequate indoor air quality in mechanically ventilated buildings, neither have developed ventilation rate performance criteria that allow for a limited level of fluctuation in ventilation rates provided by naturally ventilated buildings. Without established ventilation rate requirements it is not possible to evaluate indoor air quality performance. As a result, a criteria for minimum acceptable ventilation rates that takes into account some level of flexibility for naturally ventilated buildings was developed in this study. The criteria developed was based mainly upon standard North America requirements for ventilation rates in mechanically ventilated buildings. Not surprisingly, upon evaluation of natural ventilation design options it was found that most natural 99 ventilation building designs have difficulty meeting this criteria. This may imply that the developed criteria is overly conservative, however further study that is beyond the scope of this study is required to make that evaluation. e) It is possible to meet the natural ventilation performance criteria set out in this study with a broad range of building designs and natural ventilation design strategies. This study looked at an example three story office building located in an urban setting and showed that before any modifications were made to it, its performance in terms of avoiding overheating and providing ventilation for IAQ fell far short of the adopted natural ventilation performance criteria in both Vancouver and Toronto climates. In Vancouver, it was shown that a broad range of design parameters could be modified, either singly or in combination with other parameters, to easily meet the minimum performance criteria for both overheating and IAQ. Toronto has much higher summertime temperatures and therefore naturally ventilated buildings have greater difficulty meeting the overheating criteria. However it was shown that with specific combinations of parameter modifications it is also possible to design an office building that meets both IAQ and overheating requirements in this climate. f) This study investigated the limitations of strictly naturally ventilated buildings. Mixed mode or hybrid ventilation systems can eliminate any shortcomings of naturally ventilated buildings and may ultimately be the preferred system of choice. This study looked at the performance of buildings that were cooled and ventilated without the assistance of any mechanical cooling or ventilation equipment. Because of the inevitable fluctuations in outside air temperatures and wind speed conditions it is not possible to guarantee natural ventilation performance at all times. Hybrid systems that use mechanical ventilation or cooling equipment to provide backup to natural ventilation systems can eliminate this performance uncertainty, while maintaining most of the potential benefits. The difficulty in using natural ventilation alone to provide ventilation for IAQ was shown in this study. This is essentially a worst case scenario because IAQ performance can be substantially improved with the use of small mechanical ventilation backup systems. The simplest systems use central exhaust fans located in passive stacks and are connected to sensors that turn on the fans at specific CO2 setpoints. The exhaust fans then provide assistance to,the natural ventilation systems only when needed, and ensure that inadequate ventilation rates never occur. Another option is to use mechanical ventilation systems exclusively during cold weather to avoid the difficulty of designing natural ventilation systems that do not over ventilate, and lead to excessive energy consumption. 100 Mechanical cooling can also be used in hybrid systems. Air conditioning can be used as a backup to natural ventilation to reduce the number of overheating hours. Used in conjunction with natural ventilation, the size and cost of this backup cooling equipment can be substantially reduced due to reduced cooling loads. Alternatively the air conditioning equipment can be used to provide cooling only to specific zones that require tighter temperature control. 101 11 Recommendations For Further Work The following are recommendations for future work that should be performed to improve the results of this study and the usefulness of simplified natural ventilation design tools: Confirmation of results - The validity and limitations of rules of thumb were investigated using a computer model that simulates natural ventilation performance. These results should ideally be confirmed with field measurements of airflow rates and indoor temperatures in built offices in Vancouver and Toronto. Development of improved ventilation rate performance criteria — While North American and European countries have well defined requirements for ventilation rates for adequate indoor air quality in mechanically ventilated buildings, neither have developed ventilation rate performance criteria that allow for a limited level of fluctuation in ventilation rates provided by naturally ventilated buildings. As a result it is not possible to evaluate indoor air quality performance in terms of established ventilation rate requirements. European design guidance recommends that a "reservoir effect" should be taken into account when designing ventilation for indoor air quality in naturally ventilated buildings, and that a medium term time averaged ventilation rate of fresh air be provided. Recommended values of medium term ventilation rates and the time period over which they are measured must be developed for this concept to be useful. Similarly, North America's A S H R A E Standard 62 should provide a limited allowance for fluctuation of ventilation rates in naturally ventilated buildings. To do this, a method must be developed to turn a fluctuating natural ventilation airflow rate into a single representative ventilation rate that could be compared to the fixed ventilation rates of Table 2 in the Standard. The ventilation rate could be compared to a fixed ventilation rate i f a procedure was specified for calculating a representative "design day" natural ventilation rate, or for specifying a fixed maximum number of hours that modelled ventilation rates can be below a set limit, or by physically measuring ventilation rates over a period of time in constructed buildings. Development of North American thermal performance criteria and design weather years specifically for design of naturally ventilated buildings - A number of European countries have well defined requirements for thermal comfort that allow for a level of fluctuation in interior temperature. Similar North American standards that allow for the inevitable fluctuation in interior temperature in naturally ventilated buildings must be developed. However these standards should take into account the levels of thermal comfort deemed acceptable by North Americans. "Design weather year" weather data, that has been specifically designed for simulating natural ventilation performance using computer models, must also be developed for North American locations. While a number of different typical weather year data formats are available, they differ in their representation of sequences of hot weather data. They 102 generally represent average weather conditions over a number of years rather than a worst case overheating scenario of extended hot weather. A new format for weather data should be developed in North America that specifically addresses this issue by ensuring that weather data files contain appropriate sequences of extended hot weather developed using a common format. Investigate the Effect of Partitions - The rule of thumb that specifies a minimum allowable airflow opening area in partitions within naturally ventilated spaces was not investigated in this study, due to limitations within the model used to measure natural ventilation performance. The validity and limitations of this rule when applied to Canadian climates could be fairly easily investigated with multizone airflow modelling analysis. If the validity of this rule of thumb were confirmed, then it would provide an extra level of information on conditions for validity of other rules of thumb. 103 

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