February 26, 2021
Madeleine Lunde, EIT, and Marc Trudeau, P.Eng., Architect AIBC, BEMP, CPHD, LEED AP
Buildings are designed for the people who live and work within them. Yet, more than 40 per cent of occupants of office facilities have reported dissatisfaction with their thermal environment (For more information, read Percentage of commercial buildings showing at least 80% occupant satisfied with their thermal comfort by Caroline Karmann, Stefano Schiavon, and Edward Arens (2018), proceedings of 10th Windsor Conference: Rethinking Comfort). This is because building design often disregards thermal comfort due to the analysis involved in quantifying a person’s subjective human response to the surrounding conditions. Thermal comfort is defined as “the condition of mind that expresses satisfaction with the thermal environment,” and is, therefore, experienced differently by each person (This definition is from the American National Standards Institute, American Society of Heating, Refrigerating and Air-conditioning Engineers (ANSI/ASHRAE) 55-2017, Thermal environmental conditions for human occupancy). Thermal comfort is a personal interaction between how one’s body generates metabolic heat (thermoregulatory response and activity level) and how that heat can be lost because of clothing insulation and environmental conditions. Ensuring people can achieve thermal equilibrium within a space is essential to their satisfaction, workforce productivity, and overall health and well-being.
When a person feels uncomfortable with the indoor environment, they will seek to alleviate that discomfort through measures such as opening a window when hot or using a personal space heater when cold. This can result in suboptimal building operation that could cause excessive energy use and additional operational costs. If thermal comfort is not considered in the design process, the building is more likely to be improperly operated because of unsatisfied occupants responding to their discomfort. If thermal comfort issues can be identified prior to occupancy, it is possible to optimize building design such that passive and energy-efficient strategies can be incorporated rather than relying solely on HVAC systems to meet (or not meet) personalized thermal comfort demands.
While the goal is to achieve 100 per cent thermal satisfaction for every building space, this is an impossible goal due to the large variations, physiologically and psychologically, from person to person (This definition is from the American National Standards Institute, American Society of Heating, Refrigerating and Air-conditioning Engineers (ANSI/ASHRAE) 55-2017, Thermal environmental conditions for human occupancy). However, with careful analysis and design, comfort can be achieved for the vast majority of occupants without having to alter the operations of the space.
Due to the complexity of designing for thermal comfort, guidelines have been developed to help building professionals achieve thermal comfort within occupied building spaces. The American Society of Heating, Refrigerating and Air-conditioning Engineers (ASHRAE) 55, Thermal Environmental Conditions for Human Occupancy, is the industry standard by which North American building codes, incentive programs, and certifications base their requirements. ASHRAE 55 provides analytical methods of evaluating the interaction between the following six conditions relating to thermal comfort:
These factors, when considered concurrently and in steady state, can depict an accurate forecast of the thermal satisfaction of future building occupants. ASHRAE 55-2017 predicts thermal comfort through the adaptive thermal comfort (ATC) and whole-body thermal-balance comfort (WBC) models.
Adaptive thermal comfort model
ATC is used in naturally ventilated buildings and suggests a range of acceptable indoor operative temperatures based on mean outdoor temperatures with the assumption occupants can adapt to their environments as necessary. To comply with this method, an 80 per cent acceptability limit must be achieved (i.e. 80 per cent of occupants in a naturally ventilated space must be satisfied with the thermal conditions). This method can be used when the mean outdoor temperature is greater than 10 C (50 F) and less than 33.5 C (92 F). The allowable indoor operative temperature can be determined using the 80 per cent acceptability limits from Figure 1, which is taken from Figure 5.4.2 of ASHRAE 55-2017 (This definition is from the American National Standards Institute, American Society of Heating, Refrigerating and Air-conditioning Engineers (ANSI/ASHRAE) 55-2017, Thermal environmental conditions for human occupancy).
The focus of this model is to ensure naturally ventilated buildings, typically with improved building envelopes, do not overheat due to lack of design strategies that limit heat gain, and promote passive cooling. This method encourages the implementation of passive design strategies. Some of the strategies are operable windows, exterior shades, interior blinds, and windows with low solar heat gain co-efficient (SHGC) and low shading co-efficient (SC) to reduce the risk of overheating in summer months.
Whole-body thermal comfort model
For buildings where mechanical conditioning is used, the WBC model is required, as it is assumed people will rely on building conditioning systems rather than natural ventilation for thermal comfort. Unlike the adaptive thermal model, the WBC one considers personal factors explicitly, requiring that building occupants have metabolic rates typical of indoor office activities (1.0 to 2.0 met) and clothing with thermal insulation of 1.5 clo or less (This definition is from the American National Standards Institute, American Society of Heating, Refrigerating and Air-conditioning Engineers (ANSI/ASHRAE) 55-2017, Thermal environmental conditions for human occupancy). The WBC model aims to predict the thermal sensation and dissatisfaction of occupants in a space employing a predicted mean vote (PMV) and predicted percentage dissatisfied (PPD). The PMV model uses heat balance principles to relate the six governing conditions of thermal comfort to the average response of occupants based on a thermal sensation scale, defined as follows: +3 hot, +2 warm, +1 slightly warm, 0 neutral, – slightly cool, –2 cool, and –3 cold. People voting hot, warm, cool, or cold are considered thermally dissatisfied. PPD is an important index. It is calculated using PMV, and is employed to determine if a building will meet acceptability limits. The thermal comfort range is defined as –0.5 < PMV < 0.5 and PPD ≤ 10 per cent, which corresponds to an 80 per cent acceptability limit (This definition is from the American National Standards Institute, American Society of Heating, Refrigerating and Air-conditioning Engineers (ANSI/ASHRAE) 55-2017, Thermal environmental conditions for human occupancy).
Codes and certifications
Thermal comfort is referenced in global building rating systems, such as the Leadership in Energy and Environmental Design (LEED), WELL, and Passive House, as well as Canadian building codes such as the B.C. Energy Step Code and the City of Vancouver Energy Modelling guidelines. To meet these codes and certifications, it is required to show plots and calculations verifying the design parameters meet the specified thermal comfort requirements. Thermal comfort is not currently a requirement of the National Energy Code for Buildings (NECB) and is therefore only considered in buildings pursuing relevant green building certification credits (e.g. LEED) or in provinces that have elected to enact regulations relating to thermal comfort.
LEED v4 provides one credit in Indoor Environmental Quality for thermal comfort. This credit requires the design of HVAC systems and building envelopes to meet the requirements of either ASHRAE 55-2010 or the International Organization for Standardization (ISO) and the European Committee for Standardization (CEN) standards. For building types, such as schools and health-care projects, group thermal comfort controls must be provided for all shared multi-occupant spaces and individual comfort controls must be provided for at least 50 per cent of individual occupant areas. Thermal comfort controls should allow occupants to adjust at least one of the following in their local environment: air temperature, radiant temperature, air speed, and humidity.
WELL v2 puts an even greater emphasis on thermal comfort compared to LEED, with one prerequisite, six credits, and two beta credits used to evaluate thermal comfort. The thermal performance prerequisite requires compliance with the 80 per cent acceptability limit for naturally ventilated spaces and –0.5 < PMV < 0.5 and PPD ≤ 10 per cent for 95 per cent of regularly occupied mechanically ventilated spaces, as per ASHRAE 55-2013. WELL provides thermal comfort points if any of the following additional credits can be achieved:
WELL continues to make advancements on what it means to achieve thermal comfort. Two credits that encourage enhanced operable windows and outdoor thermal comfort are currently in beta testing. WELL has a stringent and in-depth thermal comfort criterion, which makes it an important resource to reference when prioritizing thermal comfort design, regardless of the pursuit of a WELL certification.
As an example, thermal comfort targets were required on a multilevel student residence building. This project illustrates the analysis and strategies that are involved in designing for thermal comfort.
The project had multiple comfort targets.
B.C. Energy Step Code 2018
Interior dry bulb temperatures of occupied spaces shall not exceed the 80 per cent acceptability limit for naturally conditioned spaces, per ASHRAE 55-2010, for more than 200 hours per year for any zone.
Owner’s technical requirements
Naturally conditioned space shall be designed to satisfy the following criteria through passive design practices and shall be verified using thermal modelling simulation:
Operable windows and 15 per cent additional supply air capacity to account for extreme temperature events shall be provided.
Energy modelling overview
As with any building simulation, thermal modelling results are only as accurate as the representation of the building being constructed. Building geometry was setup in the virtual environment according to the architect’s floor plans and elevation drawings. Building zones were setup with representative internal gains and envelope constructions. HVAC systems were included corresponding to the mechanical designs. Since this building was to be naturally ventilated, it was also important to assign opening types and properties for all windows and doors in the model. Natural ventilation was simulated by having operable openings that opened when outside air was able to helpfully regulate the temperature within each space.
Once the virtual building had been constructed such that it reflected the design, thermal comfort analysis was performed to identify if each occupied space met design targets. In this summary, only results from level 4 of the building are highlighted.
The B.C. Energy Step Code requires each building space to not exceed the 80 per cent acceptability limit for more than 200 hours per year, as stipulated by ASHRAE 55. Each space was, therefore, modelled to determine how many hours each month the operative temperature exceeded the acceptability limit. Since the total yearly hours for each zone did not exceed 200 hours, it was concluded the building would not overheat by the B.C. Energy Step Code standards, and therefore met the requirements. Results are shown in Figure 2.
The following analysis performed on the building involved evaluating the building’s thermal comfort with respect to the owner’s technical requirements. It was found the proposed design, which contained no mechanical cooling and 0.007 m3/s (15 cfm) of outside air per person, did not meet the owner’s requirements. Both south-facing student residence zones exceeded 24 C for more than 150 hours a year, as seen in Figure 3.
The results indicated mechanical cooling would be required in student residences to meet specified thermal comfort requirements. As an alternative to full cooling, ‘partial’ cooling based on cooling of ventilation air was assessed. It was found providing partial cooling at 18.3 C (65 F) through the ventilation system and increasing outdoor air to 0.02 m3/s (50 cfm) per person would be sufficient to meet the owner’s thermal comfort requirements.
With these strategies, however, the building did not meet the WELL v2 Enhanced Thermal Performance credit. Following WELL v2 requirements, overheating occurred in south- and west-facing student rooms, and both north and south study lounges.
Analysis was performed to determine if passive strategies could be used to reduce the maximum operative temperature and the hours each room was above the 90 per cent acceptability target. Strategies to reduce solar gain included using windows with a lower SHGC and installing vertical shading on the west façade of the building. The results from this study can be seen in Figure 4.
Both strategies yielded significant improvements in both the peak operative temperature and number of hours above the 90 per cent acceptability limit, meeting the WELL v2 credit for Enhanced Thermal Performance. To further exemplify the effect of reducing SHGCs, Figure 5 illustrates the difference in solar gain in the level 4 lounge with an SHGC of 0.37 compared to an SHGC of 0.23.
These design changes had an immense effect on the solar gain in the level 4 lounge, yet it likely would not have been considered had there not been a thermal comfort model. Reducing the SHGC of the windows and implementing exterior fixed shading, coupled with operable windows, increased outdoor airflow, and partial cooling resulted in a building that met thermal comfort requirements and limited building energy use.
This project is an example of a building that did not meet thermal comfort requirements with the initial building design, but was able to achieve targets by identifying areas needing improvement and applying both passive and mechanical strategies to reduce heat gain where overheating was found to be an issue. Many strategies can be used to achieve thermal comfort, including but not limited to the following:
Designing for future thermal comfort
It is important to keep in mind that although buildings may meet thermal comfort demands today, they may not be thermally comfortable in the near future. Due to global warming, outdoor temperatures have been increasing, and this trend can be expected to continue. This is concerning because overheating in buildings can result in dangerous conditions for occupants. Prolonged exposure to temperatures above 26 C (79 F) can increase risk of premature mortality and emergency medical service calls (Read “Towards establishing evidence-based guidelines on maximum indoor temperatures during hot weather in temperate continental climates” by Glen P. Kenny, Andreas D. Flouris, Abderrahmane Yagouti, and Sean R. Notley (2019)).
Considering the effects of global warming, it is imperative buildings are designed with future climatic conditions in mind. However, how does one design for unknown conditions? Currently, energy modelling guidelines require the use of Canadian Weather year for Energy Calculation (CWEC) 2016 weather file, which takes 12 ‘typical’ meteorological months selected from a database of 30 years. Past weather files are not a good guide for a rapidly changing future. Fortunately, groups such as the Pacific Climate Impacts Consortium (PCIC) are developing future weather-shifted data representing potential scenarios. Nationally, future climate files are being developed for the National Building Code (NBC), and requirements may be included as early as 2025.
Although future weather files will give a better indication of what design features will be necessary to avoid overheating, there are still limitations. Future weather files are based on ‘average-year’ CWEC data, and, therefore, will not account for extreme weather events. Global warming will result in a wider range of conditions than before. Therefore, it is essential to design buildings with passive systems that have been proven to increase resilience to extreme temperatures.
With the uncertainty of future climatic conditions and the limitations of weather files, it important to design for worst-case scenarios so that if overheating occurs, systems capable of combating harsh conditions are in place. By implementing both passive and mechanical strategies, it is less likely temperatures will exceed thermal comfort acceptability limits, even in hotter and more extreme conditions.
Thermal modelling analysis is under-utilized in the building industry despite the importance of thermal comfort for the health and productivity of occupants. Without further analysis and design alterations, it is unrealistic to expect a building to deliver thermally satisfying conditions for the majority of occupants. Thermal comfort modelling and analysis is the approach to optimize the design of a building for the satisfaction of future occupants while conserving energy and minimizing expenses. After all, buildings are nothing if not for the people who live and work within them.
Marc Trudeau, P.Eng., Architect AIBC, BEMP, CPHD, LEED AP, is the principal and team lead for building performance modelling at AME Group. He enjoys participating in projects to establish strategies for energy, greenhouse gas, thermal comfort, and sustainability performance. Trudeau can be reached at email@example.com.
Madeleine Lunde, EIT, LEED GA, is a building performance engineer at AME Group. She focuses on achieving energy and sustainability targets for buildings through energy modelling. She can be reached via e-mail at firstname.lastname@example.org.
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