Thermal
Part of Comfort, A WELLography™
First Edition

Copyright

Copyright© 2017 International WELL Building Institute PBC. All rights reserved.

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Unauthorized use of the Thermal Comfort WELLography™ violates copyright, trademark, and other laws and is prohibited.

Credits

The International WELL Building Institute also acknowledges the important work of Melcher Media in bringing this document to market in its current state.

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Acknowledgments

The International WELL Building Institute (IWBI) and Delos Living LLC (Delos) acknowledge the work of the following IWBI and Delos technical staff that developed and created the WELLographies: Oriah Abera; Niklas Garrn; Trevor Granger; Soyoung Hwang; Michelle Martin; Vienna McLeod; Anja Mikic; Renu Nadkarni; Brendan O’Grady; Chris Ramos; Eric Saunders; Sara Scheineson; Nathan Stodola; Regina Vaicekonyte; Sarah Welton; Kylie Wheelock; Emily Winer.

IWBI also is grateful for the input and insight provided by the following Subject Matter Experts:

Air: Terry Gordon, PhD; Eric Liberda, PhD; Tim McAuley, PhD; Ellen Tohn, MCP

Water: Eric Liberda, PhD; Tim McAuley, PhD; Margret Whittaker, PhD, MPH, CBiol, FSB, ERB, DABT, ToxServices LLC

Nourishment: Sharon Akabas, PhD; Alice H. Lichtenstein, DSc; Barbara Moore, PhD

Light: Chad Groshart, LEED AP BD+C; Samer Hattar, PhD; Steven Lockley, PhD, Consultant, Delos Living LLC and Member, Well Living Lab Scientific Advisory Board, Neuroscientist, Brigham and Women’s Hospital and Associate Professor of Medicine, Harvard Medical School

Fitness: Dr. Karen Lee, MD, MHSc, FRCPC, President & CEO, Dr. Karen Lee Health + Built Environment + Social Determinants Consulting; Jordan Metzl, MD

Thermal Comfort: Alan Hedge, PhD, CPE, CErgHF; David Lehrer, MArch; Caroline Karmann, PhD, MArch

Acoustics: Arline L. Bronzaft, PhD, Professor Emerita of The City University of New York; Charles Salter, PE

Materials: Clayton Cowl, MD; Matteo Kausch, PhD, Cradle to Cradle Products Innovation Institute; Megan Schwarzman, MD, MPH; Margret Whittaker, PhD, MPH, CBiol, FSB, ERB, DABT, ToxServices LLC

Mind: Anjali Bhagra, MBBS; Lisa Cohen, PhD; Keith Roach, MD; John Salamone, PhD; Nelida Quintero, PhD

Disclaimer

None of the parties involved in the funding or creation of the WELL Building Standard™ and the WELLographies™, including Delos Living LLC, its affiliates, subsidiaries, members, employees, or contractors, assume any liability or responsibility to the user or any third parties for the accuracy, completeness, or use of or reliance on any information contained in the WELL Building Standard and the WELLographies, or for any injuries, losses, or damages (including, without limitation, equitable relief) arising from such use or reliance.

Although the information contained in the WELL Building Standard and the WELLographies is believed to be reliable and accurate, all materials set forth within are provided without warranties of any kind, either express or implied, including but not limited to warranties of the accuracy or completeness of information or the suitability of the information for any particular purpose.

The WELL Building Standard and the WELLographies are intended to educate and assist building and real estate professionals in their efforts to create healthier work and living spaces, and nothing in the WELL Building Standard and the WELLographies should be considered, or used as a substitute for, medical advice, diagnosis or treatment.

As a condition of use, the user covenants not to sue and agrees to waive and release Delos Living LLC, its affiliates, subsidiaries, members, employees, or contractors from any and all claims, demands, and causes of action for any injuries, losses, or damages (including, without limitation, equitable relief) that the user may now or hereafter have a right to assert against such parties as a result of the use of, or reliance on, the WELL Building Standard and the WELLographies.

Welcome to WELL

The buildings where we live, work, learn and relax have a profound effect on our well-being: how we feel, what we eat and how we sleep at night. By examining our surroundings and our habits, and making key optimizations and changes, we have the power to cultivate spaces that promote wellness, and support efforts to live healthier, active, mindful lives – a right for every human.

The WELL Building Standard™ (WELL) envisions this reality and opens this critical dialogue. It provides a roadmap and a comprehensive set of strategies for achieving building and communities that advance human health.

WELL consists of a comprehensive set of features across seven concepts (Air, Water, Light, Nourishment, Fitness, Comfort and Mind). Together, these components address the various individual needs of the people inside buildings, while also setting forth a common foundation for measuring wellness in buildings as a whole. The standard was developed by integrating scientific research and literature on environmental health, behavioral factors, health outcomes and demographic risk factors that affect health; with leading practices in building design and management. WELL also references existing standards and best practice guidelines set by governmental and professional organizations, where available, in order to clarify and harmonize existing thresholds and requirements.

The result is the premier building standard for advancing human health and wellness – and a blueprint for creating better buildings that can enhance productivity, health and happiness for people everywhere.

How to Use This WELLography™

WELLographies™ present research relevant to health and well-being in buildings and communities. The sources included span health, wellness, and scientific and professional literature specific to the seven concepts within WELL, and other core focus areas. WELLographies are meant to complement the WELL Building Standard™ (available at standard.wellcertified.com) and provide architects, building managers, engineers, and interior designers, among others, with health- and science-focused background to support and guide their efforts to advance the healthy buildings movement.

WELLographies have three primary goals:

1. Provide background information for key topics relevant to understanding human health as it relates to the built environment.
2. Synthesize and present the science that underpins the WELL Building Standard.
3. Outline specific, evidence-based strategies that building professionals can apply to create spaces that promote health and well-being in buildings and communities.

There are nine WELLographies:

1. Air
2. Water
3. Nourishment
4. Light
5. Fitness
6. Thermal
7. Acoustics
8. Materials
9. Mind

The Thermal Comfort WELLography™ has the following sections:

Thermal Comfort and the Built Environment, which broadly describes how thermal comfort relates to the human experience in buildings.

Properties of Thermal Comfort, describes important technical components, including any terms that will be discussed throughout the WELLography.

Thermal Comfort and the Human Body, which provides an explanation of the biological mechanisms relating to thermal comfort, describing how the body functions under normal, healthy conditions.

Elements of Thermal Comfort, which describes environmental conditions or behaviors that are linked to health, focus, or occupant comfort, and that are subject to interventions in building design or policy. Each element includes coverage of associated health effects as well as solutions, interventions that can be implemented to impact the element. Some solutions may address several different elements.

Explanations of Solutions, which provides a definition and/or more detail for each solution.

Introduction

In the pursuit of comfort, humans have long sought control over their environment. The intentional, controlled use of fire is an early example of an effort to improve thermal comfort; subsequently, it likely brought with it many lessons about proper ventilation, thermal adaptation, and strategies to better harness the benefits of heat.

Today, the heating and air conditioning industry follows comprehensive guidelines based on extensive studies, with the most common standard, known as Standard 55, coming from the American Society for Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE).¹

Despite great advances in knowledge and technology, many people still find themselves feeling uncomfortable during the work day due to the complexities involved in controlling the complex interaction between people and the buildings in which we live, learn, work, and play.

Thermal comfort is determined by multiple personal and environmental factors, including metabolic rate, clothing, air temperature, humidity, air movement, mean radiant temperature (MRT) of surrounding surfaces, and other contextual factors.² Achieving optimal thermal comfort requires some level of control over these parameters in any given environment, and an understanding of how to best optimize thermal comfort for the maximum number of people, not just the 80% of a building’s total occupancy as prescribed under ASHRAE Standard 55.¹

New strategies continue to evolve in pursuit of the optimal thermal conditions for the safety, health, and productivity of people in buildings.

Thermal engineers and scientists work to systematically quantify and model the conditions of our environments, going beyond the body’s internal and external heat balance and delving into the environmental, physiological, and psychological factors that determine what each person subjectively defines as thermally comfortable. ASHRAE Standard 55 and EN ISO 7730 not only provide a framework for establishing comfortable thermal conditions indoors, but also provide useful definitions for thermal comfort noting that it is “a condition of mind that expresses satisfaction with the thermal environment.”^{1 3} ASHRAE further describes thermal comfort as “a cognitive process involving many inputs influenced by physical, physiological, psychological, and other processes.”⁴ While an oversimplification of the complex biology involved, thermal comfort is reached when the body is heat balanced against the environment and when skin temperature is conducive to maintaining core body temperature (CBT) of of 37°C [98.6°F].⁵ However, due to interpersonal factors it will take varying amounts of time for each person to respond physiologically to the prevailing thermal conditions of the environment.

Variables of Thermal Comfort

The ASHRAE Standard 55 has been influenced by the work of many scientists, notably, Frederick Rohles.⁶ His work, along with the work of P.O. Fanger, has helped to identify six core parameters of thermal comfort: air speed, dry-bulb temperature (DBT), MRT, relative humidity, metabolic rate, and clothing insulation (Figure 1).²

When a person does not feel comfortable, he or she is likely to identify one or more of these parameters as the source of their discomfort. The colloquial terms used are more recognizable and personally descriptive (i.e. muggy, balmy, drafty, chilly, hot), but the discomfort is actually a result of the combination of the aforementioned parameters, not necessarily any single one.

The driving force behind any biologic response to environmental thermal conditions is to maintain CBT stability. The body is constantly working to maintain a CBT within a narrow range of 36.1°C [96.8°F] to 37.2°C [98.9°F].⁷ Within this range is an even narrower range that constitutes and individuals’ perceived thermal comfort zone. Beyond the six core drivers of thermal comfort mentioned above, factors such as temperament, preferences, social and cultural norms, and seasonal variation all play an important role in determining individual thermal comfort. Collectively, these variables underpin thermal comfort’s subjective nature and highlight why a one-size-fits-all approach to thermal comfort in buildings invariably fails for large groups of people.

The Thermal Comfort WELLography^TM offers an in-depth look at the perception of thermal comfort and how it can affect an individual’s performance and health. This WELLography^TM also provides a variety of strategies that can be implemented throughout the built environment in order to achieve maximum levels of thermal comfort.

Thermal Comfort and the Built Environment

ASHARE defines thermal comfort as a subjective state of mind that expresses satisfaction with one’s immediately surrounding environment.¹

Controlling and predicting thermal comfort in buildings is crucial in air conditioning systems design. The operation schedule of air conditioning systems often does not match the diurnal fluctuations of outside temperatures or the windows exposure to direct sunlight. Especially in commercial buildings, poorly designed and installed ventilation systems adversely affect human productivity. Studies show a clear connection between optimal thermal comfort conditions and increased productivity. Minimizing or eliminating potential adverse health outcomes is also an important aspect of thermal comfort.

It is relatively straightforward to design thermally comfortably environment from the very early design stages, particularly when it comes to individual rooms or single office spaces. However, designing the entire office building for acceptable thermal comfort is challenging, because each room and floor have quite different thermal comfort parameters: different number of occupants, different size, placing of windows, diversity of the electronic equipment or the vicinity of special areas such as server rooms, central heating systems, staircases and other service premises. In addition, since an individuals’ perception of their thermal environment will vary, it is challenging for a building to provide thermal conditions that will satisfy everyone in the space. Thus, it is not uncommon for a large proportion of an indoor environment to be considered thermally “uncomfortable” for periods of time throughout the day. In efforts to address thermal comfort in buildings, ASHRAE standards for thermal comfort work to satisfy at least 80% of all people in buildings.⁴

Methods for Evaluating Thermal Comfort

Thermal Comfort Modeling

For many years, the predictive modeling of thermal comfort in air-conditioned spaces has been based on the six parameters previously mentioned, along with estimates of those satisfied or dissatisfied at a given temperature (static model of thermal comfort). The static model of comfort, developed from laboratory experiments, provides estimates of two variables - predicted mean vote (PMV) and predicted percentage dissatisfied (PPD) — and reduces these variables to a steady-state heat balance equation. ⁸ To better address other subjective considerations and seasonal variability in naturally ventilated spaces, ASHRAE Standard 55 also includes a model that takes such variables into account. The adaptive model, developed from field study data, applies specifically to naturally ventilated environments and attempts to consider the way people’s interactions with the environment affect their expectations and thermal preferences, in order to better predict comfortable temperature ranges. ⁹

Static Model of Comfort – PMV/PPD Model.

The traditional model for thermal comfort is based on a heat balance model of the human body that predicts thermal sensation exclusively as a function four environmental factors (air speed, temperature, thermal radiation and humidity) and two personal factors (metabolic rate and clothing). This model, published by Fanger et al., in 1970, uses a PMV bipolar scale to determine how the majority of people in a given environment will describe their thermal sensation. ⁸ PMV values range from cold (-three) to hot (+three) as depicted in Figure 2 with zero indicating thermal neutrality. ⁸ This scale is derived from previous research conducted in controlled climate chamber laboratory studies where participants gauged comfort using a similar method. ¹⁰

The second element of this model uses the measure of PPD. ⁸ It is important to note that:

1. PPD is based on the assumption that thermal sensation votes of +/- two or three are associated with dissatisfaction.
2. The 5% lower bound of the PPD model represents the inability to satisfy everyone, even under conditions most consider pleasant.

The model takes into account that differences in body size and general preference will always impact perception. The laboratory data that led to the development of the PMV and PPD empirical models produced a relationship where a maximum of 95% of the target audience were satisfied at even the ideal conditions (i.e., where the average thermal sensation was neutral), and that within two deviations of the mean (±1 PMV score), more than 50% of the population is accounted for (Figure 3).

The ASHRAE thermal comfort zone is based on achieving 80% satisfaction in a given space and assumes 10% of dissatisfied occupants experience general discomfort (measured by PPD <10%, or -0.5 < PMV < +0.5) and the remaining 10% experience local discomfort.¹ To put it more simply, it is assumed that respondents who identify their thermal sensation as +two, +three, -two, or -three would likely complain of discomfort because of warm, hot, cool, or cold conditions, respectively.

Despite drawbacks, the PMV/PPD model has been the dominant paradigm in thermal comfort engineering.

The model was originally intended to be used in all environments, under all conditions, without any modifications.⁹ Its inclusion in ISO 7730 (1984) as well as ASHRAE 55 (1992) have made it a pillar in thermal comfort design for HVAC engineers and others responsible for delivering thermal comfort.¹¹

Adaptive Comfort Model

The adaptive comfort model was developed with an understanding that individuals adapt to their surroundings through behavioral and psychological adjustments. The original adaptive processes of interest were outlined as physiological (e.g., acclimatization), behavioral (e.g., opening windows), and psychological (e.g., expectation based on outside air). Principles of thermal adaptation can be attributed to research across many fields dating back to the 1950s.¹² It has been suggested that adaptive comfort is mainly achieved through a person shifting their thermal expectations as a result of a higher level of perceived control (windows, clothing, shade). This theory suggests that comfort is a state of mind, partially shaped by individual perceptions and expectations, in a way that complements the physical parameters.¹³ As a result, by taking these perceptions into account, buildings can be designed to offer a great diversity of thermal experiences.¹⁴ Compared to the static PMV/PPD model, use of an adaptive comfort model can provide a greater level of satisfaction for people in buildings, even at higher temperatures (Figure 4).

The adaptive comfort model is based on field measurements and surveys done in real buildings where people actively interact with and adapt to their environments, rather than in carefully controlled laboratory conditions where human subjects merely react to the environment.

In naturally conditioned spaces where people are used to natural temperature swings and have control over windows, they adapt to variation and prefer a wider range of temperatures over the year compared to people in sealed buildings.¹⁴ In response to this research, ASHRAE has updated Standard 55 to allow for the application of these alternative adaptive comfort requirements in naturally conditioned spaces, better predicting what will make people thermally comfortable.^{14 1} Because they are generated from field data, adaptive comfort requirements for naturally ventilated buildings take into consideration contextual factors beyond the six widely recognized factors used in the PMV/PPD model, and show that people’s subjective thermal preferences are linked to expectations of their environment, and the degree to which they have personal control over them.¹⁴ These contextual influences are not easily replicated or measured in controlled laboratory settings. Because this model accounts for an individual’s interactions with the environment, it can be viewed as a more dynamic representation of thermal comfort as opposed to a static heat balance.

While the adaptive comfort model accounts for human behavior in the workplace, there are several notable restrictions in its allowable applications. For example, preconditions set in ASHRAE Standard 55 state that no cooling system may be installed if a building is to be classified under the adaptive comfort model.¹ The tenants of the adaptive comfort model are defined as:

• A person’s metabolic rate is between 1.0 and 1.3;
• A person may freely adapt their clothing;
• The space must be equipped with operable windows that open to the outdoors and can be readily opened and adjusted by people within the building;
• Opening and closing of windows must be the primary means of regulating the thermal conditions in the space;
• Does not apply when the heating system is in operation;
• Not applicable when mean monthly outdoor temperature is less than 10°C [50°F] or higher than 33.5°C [92.3°F].

Buildings that use a combination of natural ventilation and HVAC, known as mixed mode, a method that will be discussed at greater length later in this WELLography^TM, rely on adaptive comfort as well as free address of operability to achieve relatively high success rates of thermal comfort.⁹

Thermal Comfort in Existing Buildings

Evaluation of thermal comfort in existing buildings is usually conducted by building operators and/or HVAC technicians. Thermal comfort evaluation can be performed at multiple building operation phases, such as upon completing the design phase and prior to occupancy, during operation and maintenance phase as part of HVAC system balancing, as a result of occupant complaints, or as part of evaluating building performance in order to maintain and/or achieve building certification. In each of these cases, there is a set of questions which need to be answered in order to assess whether thermal comfort conditions are complying with design criteria or standards.

Methods for evaluating thermal comfort in existing buildings can be classified into the subjective measurements (surveys) of occupants’ perception of the thermal environment and the physical measurements of indoor environmental conditions. The former approach aims to determine thermal satisfaction and sensation of occupants by means of statistical analysis of the results from occupants’ surveys. The latter approach relies on thermal comfort models described above to assess satisfaction and sensation of occupants based on measured parameters. In some cases, such as building diagnostics or for research purposes, these two approaches can also be used in combination with each other.

Occupant Surveys

ASHRAE 55 Standard reports that the satisfaction with that thermal environment and its acceptability can be evaluated via direct responses from the building occupants.¹ The survey needs to be designed in a way to meet the thermal comfort limits using standard response scales in the surveys. These scales and limits can vary contingent on whether the survey is short- or long-term, i.e. whether the survey is a point-in-time or a satisfaction survey targeting long-term evaluation of comfort, respectively.

Satisfaction survey, also known as long-term survey, asks the building occupants about their feelings towards thermal environment and about any potential causes of their dissatisfaction. For evaluation of thermal comfort conditions over a significant amount of time, the results of occupancy surveys need to be correlated with long-term measurements of environmental parameters. The two main components of the satisfaction survey are the satisfaction scale that ranges from “very satisfied” to “very dissatisfied”, and diagnostic questions to identify causes of dissatisfaction.

“Point-In-Time” survey, also known as short-term survey, asks the building occupants about thermal sensation and acceptability of temperature. This type of survey utilizes short-term recordings of environmental parameters in order to evaluate thermal comfort conditions. The two main components of the “point-in-time” survey are the acceptability questions on a continuous scale (seven points) ranging from “very unacceptable” to “very acceptable”, and sensation questions using ASHRAE’s seven-point thermal sensation scale: “cold, cool, slightly cool, neutral, slightly warm, warm, hot”.¹ In this type of survey, we are essentially interested in specific thermal conditions that building occupants find acceptable.

Physical Measurements

Depending on whether the building is mechanically conditioned or naturally conditioned, there are different criteria for prediction of thermal comfort based on measurements of environmental parameters. Both types of spaces utilize the same thermal comfort models described earlier, with a key difference that existing buildings rely on actual measurement data of environmental parameters taken during building operation during occupancy hours, while building in the design stage rely on predicted environmental factors from energy simulations.

In case of mechanically conditioned spaces, the ASHRAE Standard 55 requires the user rely on graphical method of determining thermal comfort.¹ In this method, user can rely on graphical representation of the range of thermal comfort factors which can result in a comfortable thermal environment while taking into account personal data of a representative occupant. Thereby, there is a list of personal and environmental variables (both observed and measured) that the user needs to know:¹

• Air speed – measured.
• Air temperature – measured.
• Humidity – measured.
• MRT – measured.
• Metabolic rate – adopted from Tables based on human activity level.
• Clothing level – observed.

In case of naturally conditioned spaces, the ASHRAE Standard 55 requires that the following parameters are measured in the space being evaluated:¹

• Indoor air temperature.
• MRT.
• Outdoor air temperature.

Properties of Thermal Comfort

The following environmental and personal variables play a central role in thermal comfort: dry bulb temperature, humidity, air speed, mean radiant temperature, metabolic rate and clothing insulation.¹

These first six factors are inputs to the most commonly used model, the PMV/PPD model, and are used to predict whole body (general) comfort based on steady-state, uniform thermal environments. ¹⁰ However, not all thermal environments are considered uniform and thus, thermal comfort under these dynamic conditions cannot be predicted with the same model. Non-uniform conditions occur when there is a temperature gradient across a space and as such, may result in local thermal discomfort. Local thermal discomforts can result from many factors including colder air settled under warmer air, asymmetric radiant temperature fields, draft, or through surface contact. ^{1 4}

Dry-bulb Temperature

The dry-bulb temperature (DBT) is the air temperature in a specific space. More precisely, it is the temperature of air measured by a thermometer freely exposed to the atmosphere, with no consideration of radiation moisture, or temperature gradient.⁴ When people refer to the temperature of a room, they are generally referencing the DBT. In terms of thermal comfort, temperature is the easiest variable for people to describe.

Humidity

With regards to thermal comfort, we focus on relative humidity (RH) and dew point. Relative humidity is the ratio of the amount of water vapor present in the air the maximum amount (saturation point) of water vapor the air can hold at a given temperature. ¹⁶ Dew point is the temperature that the air temperature must reach in order for it to become saturated at a constant pressure and moisture content. ¹⁶ When DBT falls below the dew point temperature, moisture will condense on surfaces. ¹⁶

Humidity is more noticeable to the skin in warmer environments, primarily because it reduces the rate of evaporation thereby decreasing the efficiency of sweating, one of the body’s natural cooling mechanisms. ¹⁷ When the human body sweats, moisture and salts are released from sweat glands in the skin. This moisture covers the skin and transfers heat from the body to the atmosphere as the sweat evaporates. As the humidity outside increases, the process of evaporation slows, and the body retains heat for a longer period, resulting in a less comfortable environment. ¹⁸

The rate of water evaporation from an object depends on the difference in vapor pressure between the ambient air and the vapor pressure at the surface of an object. This translates to higher rates of evaporation when vapor pressure differences are greater. Assuming a constant temperature of 24°C [75.2°F], a RH drop from 50% to 20% would increase the evaporation rate from 32.5 mL/hr [1.1 oz/hr] to 38.5 mL/hr [1.3 oz/hr] for a person wearing pants and a long sleeve shirt. ¹⁹ This increase in the evaporation rate is a noticeable sensation that most would describe as feeling as though they were in a dry room. Overall, the added energy loss from evaporation will result in the individual feeling cooler in the lower humidity environment because of a decreasing skin temperature. ¹⁹

Air Speed

Air speed is the rate of air movement at a single point, without regard to direction. ¹ Despite its simplicity, air movement can considerably influence an individual’s thermal comfort, especially in naturally ventilated spaces. Air movement has been shown to be an important variable in perceived thermal comfort. ²⁰

Movement of air can be achieved through several methods, either intentional or accidental, such as fans, operable windows, heating ventilation and air conditioning (HVAC). Any given air speed produced by these methods can be perceived as a comfortable sensation or a nuisance, depending upon the coincident thermal parameters, and individual preferences. In addition, the context of how that air movement is produced can also make a difference. For example, when subjects were exposed to air movement from artificial sources that mimic natural air movement, the sensation was found to be less pleasurable than when the source was natural. ²¹

When individuals are feeling slightly warm, air movement is generally perceived as positive at velocities up to 0.8 m/s [1.8 MPH]. ²⁰

On the negative side, a draft is defined in ASHRAE Standard 55 as “unwanted local cooling of the body caused by air movement.” ¹ Given that drafts cause an unwanted cooling effect, it is likely that the environment in which they occur is already cool, or approaching the lower bound of thermal comfort.

Mean Radiant Temperature

Mean radiant temperature (MRT) is the uniform surface temperature of an imaginary black enclosure in which a person would exchange the same amount of radiant heat as in the actual non-uniform space. ²² ASHRAE Standard 55 further defines it as “a single value for the entire body expressed as a spatial average of the temperature of surfaces surrounding the occupant weighted by their view factors with respect to the occupant.” ¹

MRT ultimately depends on the surface material’s ability to absorb or emit radiant heat, the extent to which the surface area is exposed to the person (view factor), and the temperatures of the surrounding objects.

Radiant heat can positively affect the thermal comfort of an individual, especially when a person feels slightly cool. One example could be the warming effects of a hot stovetop element from across the room or the warmth of a fireplace from across the room. ²³ Radiant heating systems are designed to affect MRT, and thus the heat exchange with the people in the space, by supplying heat directly to the surrounding surfaces of the floors, walls, and ceilings.

Operative Temperature

The combined effect of the body’s convective heat exchange with the air (described by DBT and the radiative heat exchange with surrounding surfaces (described by MRT is described in terms of “operative temperature.” Operative temperature is defined as “the uniform temperature of an imaginary black enclosure and the air within it in which a person would exchange the same amount of heat by radiation plus convection as in the actual non-uniform environment. ¹ In other words, it can be thought of as the effective temperature actually experienced by the person in a room. This measure takes the average of the MRT and DBT weighted by their respective heat transfer coefficients. ⁴ The body is able to take all temperatures into account in an integrated way, and has been shown to be capable of recognizing changes that differ by as little as 0.003°C.²⁴

Metabolic Rate

Metabolism is the transformation of chemical energy into mechanical energy, releasing heat as a by-product within an organism. Metabolic rate measures the speed of the metabolism, which varies widely across individuals and is largely determined by activity level and is commonly described in terms of equivalence units or metabolic equivalents (MET). One MET equates to 3.5mL of oxygen used per kilogram of body weight per minute, which is the energy produced by an average person seated at rest.^{25 1} This metric makes it easy to draw direct comparisons of metabolic activity involved in specific activities. One MET is equivalent to 58.15 W released per m² of body surface. An average adult, with a body surface area of 1.7 m², and an activity levels of 1 MET will therefore have a heat loss of approximately 100W. This situation is a good representation of heat loss for an average person, at rest. Human metabolism is at its minimum during sleeping hours (0.8 METs) and at its maximum (10 METs) during high-intensity physical activities. A commonly adopted MET rate is 1.2, which corresponds to a normal office work when sitting. For more accurate evaluation of the metabolic rate of an occupant, it is recommended to use an average value for physical activities performed during the last hour. This is because the body’s heat capacity reflects the results of the activity levels performed within the last hour.

Metabolic rate is important when assessing thermal comfort, as the heat from an individual changes the dynamic of the room and directly affects the rate of cooling or heating for a specific individual.

Clothing Insulation

Clothing provides a barrier of insulation, providing resistance to heat loss. Clothing insulation units (Clo) are similar to R-values, assigned to home and commercial insulation (1 Clo = 0.88R) and provide an easy-to-understand rating system that provides information about expected warmth of a garment. ²⁶ A Clo is defined as 0.155 m²°C/W where C is temperature in Celcius (can be substituted for Kelvin) and W corresponds with whole body heat flux . A change of 1 Clo is equivalent to a change in 5°C [9°F] at rest and 10°C [18°F] while exercising. ^{1 27} Thicker articles of clothing generally have higher Clo values, indicating better insulating properties. Figure 5 shows some common clothing types and their respective clo values. As seen, the Clo values are designed so that a naked human body has a Clo value of zero, while a regular business suit weights one Clo.

The Clo value can be calculated by knowing two parameters: the occupant’s dress layout and the Clo values for the individual clothing garments. The results are obtained by summing up the Clo values. This calculation method usually provides a sufficient accuracy. When calculating Clo values, it is important to include the effects of upholstered seats, car seats, beds, beddings or others surfaces in direct contact that provide additional body insulation and reduce the overall body heat loss.

Local Thermal Discomfort

The criteria for local thermal discomfort has been developed to keep the low levels of estimated number of dissatisfied building occupants (~10%) due to local discomfort factors. Local thermal discomfort factors are not included in determining average operative temperature range required by the Standard. The goal is to satisfy as many as possible building occupants by combining average and local discomfort factors.

One of the major challenges with local thermal discomfort guidelines is that ASHRAE only applies the requirements when the people within the building have clothing insulation of 0.5–0.7 Clo with metabolic rate between 1.0–1.3 METs. ⁴ Despite the narrow range of applicability, the requirements are very rigid and encompass radiant temperature asymmetry, draft, vertical air temperature differences, and floor surface temperatures.

Radiant Temperature Asymmetry

Asymmetric thermal radiation is just one example of non-uniform conditions that can cause local discomfort. It is setting-dependent and caused by a variety of factors including cold windows, un-insulated walls, and improperly sized heating panels. In workplaces, non-uniform thermal radiation may result from cold or warm products, equipment, or machinery. ⁴

The graph below (Figure 6) shows that warm ceilings are least comfortable and lead to the most dissatisfaction among people within a building. On the other hand, cool ceilings and warm walls have much lower impacts on a person’s dissatisfaction.

Thermal radiation may be non-uniform due to multiple surfaces of varying temperature and direct sunlight. ASHRAE Standard 55 notes that this asymmetry is likely to cause local thermal discomfort and reduce the space’s thermal acceptability. ¹

Draft

Draft is defined as unwanted movement of air that causes excessive local cooling and it is known to be the most frequent cause of complains in thermal comfort studies. Draft complaints are present when the body thermal sensation is below neutral (cool), and therefore, it applies only at operative temperatures below 22.5°C [72.5°F]. The two main determinants of draft discomfort are the average air velocity and turbulence intensity. High fluctuation in turbulence is known to cause the fluctuations of the skin temperature, which leads to sensation of draft discomfort. Draft sensation is usually the higher at body parts that are uncovered, particularly in the region of head, neck, shoulders, legs, ankles and feet. However, the ASHRAE 55 Standard does not recognize variations among the human body parts, and it requires consideration of the overall Clo value only (applicable only when Clo is equal to or greater than 0.7).¹ When Clo is less than 0.7, MET levels are less than 1.3, and operative temperature is below 22.5°C [72.5°F], then the maximum average air speed provided by the HVAC system cannot exceed 0.15 m/s. Nevertheless, if occupants are given a control over their thermal environment, the maximum average air speed can go up to 1.2 m/s [2.68 MPH].

Vertical Air Temperature Difference

Building occupants will often feel local discomfort when they feet feel cold and their head is hot. ASHRAE 55 and ISO 7730 report that the maximum air temperature discrepancy between the head (1.1 m [3.6 ft] height for sitting; 1.7 m [5.6 ft] for standing) and ankle (100 mm [10 cm] height) should not exceed 3°C [5.4°F].^{1 3} The vertical thermal gradient can arise owing to heat sources that display the warmer air upwards and thus remaining the cooler air at the lower room levels. One representative example is the human thermal plume that moves the air upwards around the human body.²⁹ Typical buildings where the vertical air temperature difference needs to be carefully considered include those equipped with displacement ventilation and underfloor ventilation. This requirement is not applicable to occupants whole MET rate exceeds 1.3. For instance, this requirement does not apply to walking occupants whole MET rate is approximately 1.7.

Floor Surface Temperatures

When the surface of the floor is too high or too low, a direct contact between the feet and floor can cause local thermal discomfort. The major indicator of the local discomfort due to uncomfortable floor temperature is the heat loss from the feet. The heat loss from the feet is determined by the surface temperature, conductivity and heat capacity of the flooring material, as well as the thermal insulation provided by footwear, if applicable. ISO 7730 recommends the acceptable floor surface temperatures in the range from 19 °C [66.2°F] to 29°C [84.2°F].³ Note that most of thermal comfort standards and guidelines do not address the floor temperature requirements for occupants not wearing shoes or for occupants sitting on the floor.

Thermal Comfort and the Human Body

While thermal comfort is defined by ASHRAE 55 as a subjective state of mind, it is impacted by a number of factors, including skin temperature, skin wetness, and CBT, all of which are outcomes of the body’s net heat exchange with the environment, and its own thermoregulation systems.³¹

As actual comfort is not the same as homeothermic or static heat balance, addressing it requires an understanding of biology, as well as situational perspective. The components that make up the thermal environment and the biological reaction to that environment can be defined and are quantifiable; it is the subjective interpretation of them that makes actual thermal comfort situational and personal. The following is an overview of the biology involved in processing thermal signals.

Thermoregulation

Thermoregulation is an organism’s ability to keep its body temperature within certain boundaries through heat gains and losses. In humans, the core temperature is approximately 37°C [98.6°F], a temperature that is thought of as an evolutionary defense mechanism designed to protect against fungal infections. ³² When maintained at 37°C [98.6°F], the body is warm enough to protect against the growth of harmful fungi, but not so warm as to raise the metabolism to a level associated with exercise, or extreme physical exertion 38-41°C [100.4-105.8°F]. ³¹ The human body uses two main systems to achieve thermoregulation: the integumentary system, which is made up of the skin and appendages (hair and nails) and serves as a temperature-sensing shell, and the endocrine system, which controls chemical regulators passing between the skin and brain. As the outward facing membrane, the skin comes in contact with a large range of temperatures and acts as the first line of defense in thermoregulation; in turn, the hypothalamus regulates the physiologic response to external stimuli with the endocrine system responding by releasing hormone, making the whole process seamless and efficient.³¹

Integumentary System

The skin is the largest of the body’s organs, measuring in at close to 1.8 m² [20 ft²] for the average person.³³ As a macro defense mechanism, the skin protects the body’s organs and tissues from the environment. As a micro defense mechanism, intact skin provides a tight outer seal against viruses, microbes, and other potentially harmful pathogens.³³ The skin is comprised of three distinct layers: the epidermis (outermost layer), the dermis (middle layer), and the hypodermis (sub cutis layer), with free nerve endings scattered and reaching up through the dermis and epidermis (Figure 7). ³³

Nerve endings in the skin can be grouped into cold and warm receptors, with further subdivisions of noxious and innocuous sensory receptors.³³ The sub grouping of receptors based on temperature range stems from the skin’s dual function as both a temperature regulator and as an indicator of imminent danger (i.e., burning skin versus a gentle warming).³⁴ Generally, thermo-receptors found in skin with no hair are more densely packed and are sensitive to immediately hazardous stimuli (hot pan, cold liquid), while thermo-receptors found in skin with hair are more disperse and used for signaling overall thermal comfort ³⁵ Once innocuous sensors register a discomfort signal, a message is sent to the skin’s blood vessels to either expand (vasodilation) or contract (vasoconstriction), causing an individual to either sweat or shiver. ⁵ In this way, the skin acts as both an indicator and mediator of thermal comfort (Figure 8). While the specific response generated by a thermal stimulus varies and is dependent upon the stimulus’ duration and severity, the skin always serves as the first line of defense.³⁶

Endocrine System

The endocrine system consists of glands and organs that regulate a number of bodily functions through the secretion of hormones (Figure 9). The control center of the endocrine system is located in the hypothalamus The hypothalamus is divided into three main regions: the tuberal, the anterior, and the posterior, the latter two of which are directly involved in thermoregulation The anterior hypothalamus predominantly acts as a heat loss controller.³⁸ A rise in the body’s average general temperature causes nerve impulses to activate vasodilation and sweating mechanisms to lower the body temperature. Higher metabolic activities such as exercising cause a rise in internal temperature, and the hypothalamus responds appropriately with heat loss mechanisms. The posterior hypothalamus is generally a heat generation controller for the body when it is cold. The cold sensors in the skin relay their signals to the posterior hypothalamus to initiate heat-generating mechanisms like shivering, and prevent further heat loss by vasoconstriction. ^{31 36} Additional endocrine mechanisms include breaking down adipose tissue into energy for heat, and hormonal regulation of the sympathetic nervous system by the thyroid.³⁹

Heat Gains

There are two primary sources for heat gains in the body: internal metabolic functions and external influences from the person’s environment.

The production of heat from food via metabolism is the body’s primary form of non-locomotive heat generation. The amount of heat generated from any other activity depends on physical exertion and other biological factors like illness, chronic inflammation, or menstrual cycle.

The brain, heart, liver, and active skeletal muscles are the main sites of internal heat generation in the human body.³¹

The heat gained from metabolic functions and through exercise is transferred to the skin for warming through blood vessels and tissues. Metabolism in the brain is estimated to account for about 20% of the total metabolism in the body, and most of this is expended in ion transport. ³⁶ This is especially important to consider when assessing thermal comfort, as research shows temperature modification to specific zones of the body plays a very important role in overall perception of comfort. This concept is described using what is known as the multi-node model, which breaks the body down into specific sub-sections to better quantify the behavior and interaction between the separate parts contributing to overall thermal comfort.⁹

External heat gains from the environment are transmitted through the skin. The three potential mechanisms for heat gains in the body that operate through the skin are conduction, convection, radiation and respiration When indoors, long wave radiative heat gain will be most common from either warm radiation passing through windows onto surfaces that are receiving direct solar gain or from radiant heating systems that are intentionally creating warm surfaces.⁴¹ Hot environments, where the air temperature is warmer than skin temperature, also foster convective heat gain, but such conditions are less likely to occur indoors where comfort is the primary goal. Throughout the environments in which people live, work, learn, and play, there are many intentional (conditioning of air or radiant systems) and unintentional (waste heat from electronics, lighting, or other appliances) sources of heat.⁴²

Heat Losses

To be comfortable, CBT needs to stay in a neutral range. This happens when the energy generated in the body by metabolism is equal to the net amount of heat lost from the body.

There are five mechanisms for heat loss in the body: conduction, convection, radiation, evaporation, and respiration.

Radiative heat loss occurs in all objects with a temperature greater than absolute zero (0°K). The concept is fairly simple: Electromagnetic radiation (photons) is emitted freely into the environment from the body. This phenomenon can often be felt when moving from hot outside temperatures to cooler inside temperatures.

Conduction is the loss of heat to an adjoining object or surface, such as a chair or table. The body will naturally transfer heat to items that are at a lower temperature than the skin. The transfer of heat from a low to high gradient (diffusion) does not happen very efficiently with air, so body heat losses from conduction generally occur with other objects (i.e., a table, a chair, clothing) as air is generally a poor conductor of heat.

Convection is the transfer of heat from a body to the surrounding air due to motion (air currents). Air currents carry the heat away from the skin and into the surrounding air. The convection of heat from the body can have a noticeable effect on room temperature when there are multiple people in a room.

In order to maintain CBT the body also perspires, cooling the body by removing heat from the skin through evaporation. As sweat is released to the surface of the skin, it provides a cooling effect by transferring heat from the skin to the atmosphere through evaporation. As sweat evaporates from the skin, it cools the body by taking the excess heat being generated and using it to convert liquid sweat into vapor. The amount of energy required for this conversion at normal skin temperature is 2,427 kilojoules/gram, which translates to significant cooling.⁴³

Respiration losses involve a combination of convective and evaporative heat losses. The air coming into the respiratory tract becomes nearly saturated with water vapor and is then warmed by the body’s heat before leaving at a slightly lower temperature than the core. The exhaled air is warmer and more humid than that which was inhaled, thus representing both a sensible and latent heat loss from the body.^{36 4}

Circadian Cycle

The body is constantly working to maintain a stable CBT However, CBT fluctuates in response to many factors including the body’s natural circadian cycle. As our metabolic rate slows during sleep, CBT decreases; similarly, as our metabolic rate rises during waking hours, so too does our CBT.³¹ During sleep, the body’s surface skin temperature increases as the body dissipates heat and cools CBT. ⁴⁴ Upon waking up, the reverse process occurs. ⁴⁴

Although the CBT fluctuates according to the circadian cycle, ambient temperature can disrupt natural regulation, leading to disturbances in sleep.⁴⁵ Research has found that in general sleep quality is best when a state of thermal neutrality is reached. The optimal temperatures associate with thermal neutrality for sleep range from 16–19°C [60–66°F] when wearing pajamas and using a sheet, and 30-32°C [86-89.6° F] in an open room.⁴⁶ Furthermore, it has been shown that altering the temperature at the surface of the skin to warmer conditions by ambient heating (HVAC), or insulating (clothing), can induce the onset of sleep.⁴⁵

Elements of Thermal Comfort

This section outlines three elements of thermal comfort as they relate to human health and comfort and include air temperature, air speed, and humidity.

1. Air Temperature

There are many ways to measure temperature, each with its own merits. The overall perception of room temperature is usually measured in terms of dry bulb temperature when in fact operative temperature is really the best measure. Current ASHRAE standards recommend an indoor air temperature range of 20–27°C [67-80°F] when people are wearing 1.0 Clo (warmer clothing) and 24–26.1°C [75–79°F] for 0.5 Clo (cooler clothing). ¹ It’s important to note, however, that actual clothing patterns will vary significantly between individuals in a particular building and will also depend on that location’s actual climate or the workplace culture and dress code. When indoor air temperatures exceed or fall below these ranges, PPD is likely to rise unless mitigated by other variables of thermal comfort (e.g., increased air movement to compensate for warmer temperatures).

Health Effects

Respiratory System

Lung function. One study examining the effects of temperature and asthma found that indoor air temperature is significantly associated with acute lung function decrements in children with pre-existing asthma.⁴⁷ Additionally, the American Lung Association lists exposure to cold air and sudden temperature change as triggers for asthma in adults.⁴⁸

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Respiratory symptoms. A systematic review of indoor thermal factors and building-related symptoms (stuffy or runny nose; sore or dry throat; dry, itching, or irritated eyes; and dry, itching, or irritated skin) in office workers for 100 office buildings is outlined in the EPA’s Building Assessment Survey and Evaluation Study.⁴⁹ The EPA found that acute building-related illness symptoms were most prominent in the winter months when indoor temperatures were at the higher end of the comfort range and to a lesser extent in the summer months when indoor temperatures were below the comfort range. This suggests that lowering the winter indoor temperature will reduce the effects of acute building-related symptoms and conserve energy while maintaining thermal comfort. Additionally, raising the summer indoor temperature would increase thermal comfort and reduce building symptoms and energy usage.⁵⁰

Comfort and Focus

Productivity. Indoor thermal discomfort has been associated with satisfaction and is tenuously linked to productivity. ^{9 4} The general approach to link productivity with thermal comfort has been to measure work output, since this provides a quantifiable measure of task-based output that can be monetized.⁵¹ It has also been shown that productivity is likely related to overall comfort and satisfaction, with a poor correlation to temperature alone.⁵² However, there is no agreement that the measures chosen for this research accurately evaluation productivity, as repetitive tasks may be impacted, while tasks involving critical thinking tasks may remain unaffected.

The current body of literature provides several contradictory examples of the link between some variant of productivity and thermal comfort.

For example, one study showed that schoolchildren perform better on tests when in 20°C [68°F] classrooms as opposed to the maximum end of the suggested comfort scale of 25°C [77°F].⁵³ Another study showed that productivity losses occur at indoor temperatures above 25°C [77°F].⁵⁴

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Other research has found that office productivity for computer users increased as indoor temperature increased from 20°C [68°F] to 25°C [77°F].⁵⁵

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The idea of an optimal temperature for productivity for everyone is somewhat contentious and any research dealing with comfort and focus parameters needs to be interpreted contextually.

A review of performance-based studies with an expanded set of data shows that there is more scatter among productivity impacts of temperature than suggested by narrow readings, and that there is no obvious optimum temperature for productivity within the wide range of approximately 21–27°C [70–80°F]. While certain measures of productivity have been shown to decline in warmer conditions, it should be noted that there was no elevated air movement in those warm condition tests, which may have served to restore comfort and productivity.⁵⁶

Productivity in the workplace is of major concern to employers as labor costs can easily be the largest controllable expenditure for a business, even trumping energy costs.⁵⁷ For the purposes of this WELLography™, thermal comfort and productivity are addressed as a whole. Given the current research, it is not possible to tie measures of productivity to any specific variable within thermal comfort.

Solutions

1. Radiant Heating and Cooling

The use of radiant heating and cooling greatly reduces the number of allergens circulated in the air as this type of system does not use forced air to distribute heating or cooling. Additionally, the use of radiant systems is easily scaled to match the area being covered, ensuring proper heating and cooling capacity.⁵⁸

2. HVAC Design

Equipment design that takes into account total occupancy is an effective method of improving Indoor Environmental Quality (IEQ).⁵⁹ This ensures that the proper volume of air can be adequately conditioned to suit the needs of the people within the building. Correctly sized HVAC equipment is essential for optimal thermal comfort. Oversized equipment is prone to causing more duct leakage due to higher operating duct pressures and increasing use of fan power.⁶⁰

3. Personalized Control

Where zoning allows, individually-accessible thermostats that enable users to set their own thermal conditions independently of other zones should be used. In larger spaces such as open offices, it may be necessary to provide localized control to people who work in cubicles and other work areas. Additionally, the use of personalized equipment, such as desktop fans or heating pads, gives people the ability to better control their sensation and comfort.

4. Mixed Mode

Mixed mode conditioning of air is a hybrid approach to space conditioning that uses a combination of natural ventilation from operable windows (either manually or automatically controlled) and mechanical systems that include air distribution equipment and refrigeration equipment for cooling.⁶¹ Spaces utilizing this method of regulating temperature by providing people with the ability to open windows, or access control mechanisms to adjust the temperature, have proven effective at alleviating temperature and providing satisfaction to people within buildings, especially in milder climates.^{62 63}

2. Air Speed

The study of air movement indoors has shifted from viewing air movement as a nuisance to identifying the positive effects. Initial research and modeling on the subject linked the idea of thermal comfort at various temperatures to differing air velocities, with increased velocities being viewed as positive in warm temperatures and as negative in cooler temperatures. This initial research suggested air velocities remain low 0.2 m/s [0.44 MPH] to avoid having an environment be perceived as too drafty.⁶⁴ Based on extensive field studies, rather than the artificial conditions of controlled laboratory experiments, researchers have identified that increased indoor air speeds of up to 1.02 m/s [2.28 MPH] are viewed positively in milder warm temperatures, thereby potentially extending the range of temperatures people perceive as comfortable.^{65 66}

Health Effects

Comfort and Focus

Comfort. A total of 119 subjects were recruited (57 females, 62 males) and subjected to a range of warm temperatures in an environmental test chamber. ⁶⁵ Subjects were allowed to adjust air movement to suit their individual preferences. Over 80% of subjects with a MET of 1.2 were comfortable up to 29°C [84°F] and those with a MET of 1.0 up to 31°C [87°F] when air speeds up to 1.4 m/s [3.1 mph] were chosen.⁶⁵

Solutions

1. Draft Prevention

Drafts in commercial buildings are commonly caused by a person’s location relative to air supply diffusers, and by cold windows that cool the adjacent air, which then moves downward across the glass and into an open space. Taking steps, such as sealing doors and windows, can help prevent the unwanted cooling by uncontrolled currents of air.

3. Humidity

Humidity (most commonly measured as relative humidity (RH)) affects personal thermal comfort both directly and indirectly. The effects of humidity impact the body’s energy balance, thermal sensation, discomfort, perception of air quality, and overall health.¹⁹ The current recommendations for indoor humidity from ASHRAE suggest that in buildings with dehumidification capability, the RH level not exceed 65%. There are currently no recommendations for minimum humidity levels based on thermal considerations alone. ⁶⁷Extremely low levels of humidity would be likely to cause dermal, mucosal, and ocular irritation, which would warrant close attention to these factors even though they would not affect general thermal comfort conditions and therefore would not be within the scope of ASHRAE Standard 55. ¹⁹

Health Effects

Respiratory System

Respiratory symptoms. When people are chronically exposed to conditions of either high or low humidity, it is likely that the effects observed are due to conditions becoming conducive to accumulation of dust mite reproduction and fungal growth.⁶⁸ This increased likelihood of mold growth can lead to the agitation of the respiratory system, resulting in allergic reactions, asthma and other respiratory conditions.⁶⁹

Integumentary System

Mucous membrane irritation and dryness. Insufficient evaporative and convective cooling of the mucous membranes in the upper respiratory tract may cause local discomfort and a sense of stuffiness and staleness when the temperature and humidity are high.³¹

It has been noted that office workers, particularly those with contact lenses, mention dry air as a source of ocular discomfort.⁷⁰ Dry eye discomfort results from the thinning or sometimes rupturing of the pre-corneal tear film. This film protects the surface of the eye from environmental exposure.³¹ Additional studies show that dry eyes can be caused by high temperature, low RH, and indoor pollutants.⁷¹

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Skin moisture and irritation. At an optimal humidity of 30–50%, ⁷² human skin will not lose moisture to the environment until perspiration occurs. At very low levels, research shows that lower RH causes ocular dryness, skin irritation, and dry inflamed mucosal membrane dryness. ^{70 73}

Solutions

1. HVAC Design

Equipment that is designed properly and takes into account total occupancy is an effective method of improving Indoor Environmental Quality (IEQ). ⁵⁹

Explanations of Solutions

HVAC Design

ASHRAE advises that both residential and commercial buildings comply with the requirements spelled out in its Standard 55. Building HVAC systems should be designed to monitor and control for variations in indoor temperature, radiant heat transfer through the building envelope, relative humidity, and air movement. Ultimately, the design should enable the people who live, work, learn, and play to easily make system adjustments. Systems should always be designed with rational and person-centric thermal zoning in mind, helping to optimize the system’s thermal performance.

Correctly sized HVAC equipment is essential for optimal thermal comfort. Oversized equipment is prone to causing more duct leakage due to higher operating pressures and the blowers’ increased use of fan power ⁷⁴ For residential equipment, using Air Conditioning Contractor’s Association of America (ACCA) manuals for sizing will help to ensure optimal comfort and energy efficiency.

Draft Prevention

Drafts may cause the people within a building to experience discomfort with cool or cold indoor temperatures. In cold heating conditions, a noteworthy drawback of increasing air temperature to offset draft discomfort is higher energy usage. Currently, only unwanted drafts of cold air are defined as an air movement related nuisance, as people generally prefer air movement in neutral or warm conditions. The body is more sensitive to localized thermal sensations and will perceive them as warmer if the body is colder, and conversely, colder if the body is warmer.⁷⁵

Drafts in commercial buildings are commonly caused by a person’s location relative to air supply diffusers, and by cold windows that cool the adjacent air, which then moves downward across the glass and into a space. These problems may be addressed by integrating the design of the HVAC and workspace layout, and by using insulated window glazing appropriate for the climate. In residential buildings, infiltration during cool weather may provide an additional draft source. A properly sealed building envelope is ideal to prevent draft and to better manage airflow. All openings and joints should be properly sealed and maintained in order to optimize airflow.

The overcooling of buildings in summer is a commonly observed and persistent problem in offices and other commercial buildings.

This issue has important comfort and energy implications. A study of 95 U.S. office buildings shows that buildings are actually cooler in summer than in winter, and the average temperature in summer is below the ASHRAE recommended comfort zone.⁵⁰ The reasons are not fully understood, although thermostat settings that over-correct for a person’s complaints are one posited cause. Another cause may be the result of standard HVAC design practice that requires variable air volume (VAV) airflow minima that are well above what is required for ventilation and cooling in low-load conditions. Field research demonstrated that reducing airflow set points to about 10% of maximum levels resulted in improved summer comfort as well as energy savings. ⁷⁶

Personalized Control

Due to the difficulties in setting temperature levels that suit all individual preferences, thermal comfort conditions should be optimized to ensure baseline satisfaction for the largest number of people. Where zoning allows, individually-accessible thermostats that enable users to set their own thermal conditions independent of other zones should be used. In larger spaces such as open offices, it may be necessary to provide localized control to people who work in cubicles and other work areas. It is important, however, that energy-efficient devices are selected and that employees are educated about how to use these devices effectively and safely. Similar solutions can be implemented in other spaces, such as residences and schools. A significant benefit for users is the ability to control and adjust their environment to maintain comfort. New building control “apps” allow people in commercial buildings to “vote” and to directly influence the operation of HVAC systems, without use of thermostats or intervention by building operators. These systems offer promise to enable highly granular individual control in new and existing buildings. Additionally, when possible, personal control of ventilation should be used, as it is shown to improve self-reported productivity rates and decrease symptoms associated with sick building syndrome (SBS) ⁷⁷

Window controls can also be utilized to improve thermal comfort in “mixed-mode” buildings that combine operable windows and mechanical cooling, discussed in further detail below.

Many buildings are turning toward a mixed mode of operation in an effort to merge the standard comfort model with the adaptive comfort model while providing personal interaction with the outdoor environment.

Mixed Mode

“Mixed mode” is a hybrid approach to space conditioning that uses a combination of natural ventilation from operable windows (either manually or automatically controlled) and mechanical systems, including air distribution equipment and refrigeration equipment for cooling.⁷⁸ Some of the advantages of this particular system include lower energy costs thanks to reduced HVAC usage, increased satisfaction motivated by individuals having direct influence on the surrounding thermal conditions, and increased flexibility in the ability to immediately satisfy thermal comfort demands of the people in a particular building.⁷⁹ Signaling systems that inform building residents about when to open and close their windows (such as red/green lights or lighted signs) can be a low-cost solution to balance the benefits of manual and automatic control if building owners sufficiently educate people in buildings on the tangible benefits of personal window control.⁸⁰

Radiant Heating and Cooling

There are a number of advantages to radiant heating and cooling. First, it can be highly energy efficient compared to air systems. It also takes up much less room, since water has a significantly higher heat capacity than air and therefore the pipes are much smaller than ducts.⁵⁸ In addition, the water temperature can be cooler in heating mode, and warmer in cooling mode, making the production of heated or chilled water more efficient. Radiant heating also does not distribute allergens to the same degree forced air systems do, making it a preferred choice for those with allergies. ²³ Furthermore, a variety of energy sources can be used to heat the liquid of radiant systems, including natural gas, oil-fired boilers, and solar systems.

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