By Andrea Ward, Jessica Boehland, and Nadav Malin
Reprinted by permission from BuildingGreen.com
Thermal comfort is hard to define and even harder to achieve. The most common complaint about workplace environments is that they are too cold. This would be a fairly simple problem to fix if the second most common complaint weren’t that the same spaces are too hot.
According to ASHRAE Standard 55, which defines thermal comfort in commercial buildings, success means that a building meets the needs of 80% of occupants. The conventional way to meet that threshold is to create a highly predictable, controlled environment using energy-intensive mechanical equipment.
However, as concerns around energy efficiency and indoor air quality have led to more interest in ventilating buildings naturally, the concept of adaptive thermal comfort has emerged. The theory suggests that a connection to the outdoors and control over their immediate environment allows humans to adapt to—and even prefer—a wider range of thermal conditions than is generally considered comfortable.
Adaptive thermal comfort broadens our understanding of the human comfort zone by taking into account the ways that people’s perceptions of their environment change based on seasonal expectations of temperature and humidity as well as their capacity to control the conditions in a space. On a hot summer day, for example, people may be more accepting of warmer temperatures indoors if they can open a window. This not only invites breezes, which reduce the perceived temperature, it also orients occupants to the conditions outdoors, improving productivity and overall occupant satisfaction. Installing devices like fans near workstations also gives building occupants more control over the conditions in their immediate environment.
Conventionally Recognized Factors
Most of us would agree that 85°F (29°C) feels fine outdoors, in the shade, with a breeze blowing, but miserable in a sealed office building. We have known for a long time that thermal comfort is affected by far more than just air temperature. Air temperature, the degrees in Fahrenheit or Celsius of the ambient air, is the most obvious of the physical factors, one category of conditions that affect comfort. Also important is mean radiant temperature, the average temperature of all nearby surfaces, weighted according to the emissivity of those surfaces.
Another variable in the comfort equation is humidity. Absolute humidity refers to the amount of water vapor in the air, usually expressed as pounds of moisture per pound of dry air. As air temperature rises, it can hold more moisture, and relative humidity (RH) is used to express the amount of water vapor in the air as a percentage of the total amount it could hold at that temperature. Although the role of humidity at temperatures within the comfort range is relatively small, its influence on temperatures outside the comfort range can be great, making already warm temperatures unbearable.
The saying “It’s not the heat; it’s the humidity” highlights the role of moisture in thermal comfort. Since air with low RH can absorb more moisture, under these conditions sweat will evaporate from our skin more quickly, cooling us more effectively. As RH rises, sweating becomes a less effective means of cooling the body until, at 100% RH, the air can absorb no more moisture, rendering sweating ineffective. Since it encourages mold growth, high humidity also poses a threat to indoor air quality, sometimes at levels below the upper limits recommended for comfort. Although low humidity is never a problem for thermal comfort, it can cause other problems, such as dry eyes and static electricity.
A fourth variable is air velocity. Fans and breezes make us feel cooler not because they introduce cooler air to the space, but because they move air across our skin, causing heat loss by convection and inducing evaporative cooling. The effect of air velocity on comfort varies with the other factors, but ASHRAE Standard 55 predicts that air moving at 100 fpm (0.5 m/s) can offset temperatures of 2°F to 4°F (1°C to 2°C) above the normal comfort zone, and an air velocity of 250 fpm (1.2 m/s) can offset temperature increases of 4°F to 10°F (2°C to 5.5°C). In his book Heating, Cooling, Lighting: Design Methods for Architects (see EBN Vol. 10, No. 5), Norbert Lechner gives a frame of reference: a gentle breeze outdoors is around 900 fpm (4.5 m/s), he says. Indoors, 200 fpm (1 m/s) is the upper limit for comfort in air-conditioned spaces, and a good speed for natural ventilation in hot, dry climates; 400 fpm (2 m/s) is good for natural ventilation in hot, humid climates, according to Lechner.
The fact that airflow can expand our comfort range is behind the design of “dogtrots,” or open hallways, in American southern-climate buildings. A dogtrot takes advantage of the Venturi effect to speed air through the narrow portions of the building, thereby making the occupants feel cooler. Air velocity can make us uncomfortable for other reasons, however; if the velocity is too great, for example, it will blow papers from desks. Both the 1992 and 2004 versions of ASHRAE Standard 55 call for an upper airspeed limit of 160 fpm (0.8 m/s), significantly lower than the recommendations of Lechner and others.
Adaptive Comfort
Physical and personal factors interact in complex ways, affecting the comfort of building occupants. But thermal comfort—and designing spaces to make people comfortable—is more complex than even all of this suggests. “Occupants drive comfort much more than the environment in which you place them,” says Walter Grondzik, of Florida A&M University. Engineers are just starting to recognize the role of adaptive factors, the third component of the thermal comfort conundrum. Gail Brager, Ph.D., associate professor in the Department of Architecture at the University of California at Berkeley and vice chair of the ASHRAE Standard Project Committee 55, and Richard de Dear, Ph.D., faculty member in the Division of Environmental and Life Sciences at Macquarie University in Sydney, Australia, have led the effort to convince ASHRAE to recognize adaptive comfort. In their 2000 ASHRAE Journal article “A Standard for Natural Ventilation,” they describe three elements of adaptive thermal comfort.
The most well-documented and widely accepted of these newly recognized variables is behavioral adaptation, or control. Behavioral adaptation, according to Brager and de Dear, includes both conscious and unconscious actions that we take to adjust our thermal environment. Examples include changing clothes or activity levels (which are also considered personal factors), as well as adjusting the environment itself, by turning on a fan or opening a window, for example. “In a naturally ventilated building, occupants are more in the driver’s seat,” says Grondzik. “Given some choices and opportunities, people are willing to adapt and expand the comfort zone.”
The second component of adaptive comfort is physiological adaptation, also called acclimatization. Physiological adaptation refers to biological changes caused by “prolonged exposure to characteristic and relatively extreme thermal conditions,” according to Brager and de Dear. People physically adapt to hot climates, for example, by beginning to sweat at lower temperatures. While acclimatization may come into play in extreme conditions, Brager and de Dear report that it is insignificant for typical office conditions.
The third component of adaptive comfort is psychological adaptation, which “refers to an altered perception of, and reaction to, physical conditions due to past experience and expectations,” according to Brager and de Dear. These expectations—relating to weather, season, routine, and culture—actually shift our feelings of comfort. If the weather is cool, for example, building occupants are comfortable at a slightly lower range of temperatures.
Another aspect of psychological adaptation is our dislike of unchanging conditions, which results in lethargy and listlessness—the phenomenon called thermal boredom. In his foreword to the 1996 Sustainable Design Guide of the Japan Institute of Architects, Amory Lovins, cofounder and CEO of Rocky Mountain Institute, complains that “the typical Western mechanical engineer would strive to eliminate every pesky trace of variability with thermostats and humidistats and photosensors, to render the human experience uniform and constant down to the last lux of light and molecule of air—as if people were dead machines, not dynamic organisms.” In his 1980 book Indoor Climate, Donald McIntyre captures the importance of fluctuating indoor temperatures: “It can be argued,” he says, “that achieving a steady optimum temperature is akin to finding the most popular meal at the canteen and then serving it every day.”
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| The Personal Environments© system from Johnson Controls is one way to provide building occupants with individual workstation-level controls over airflow, temperature, and other conditions. This control over mechanical-system based comfort is not addressed by the adaptive thermal comfort option in the new ASHRAE Standard 55—2004. |
ASHRAE Standard 55—2004
Revised roughly once each decade since its release in 1966, Standard 55 was updated in June 2004. Standard 55—2004 represents a major shift in our understanding of thermal comfort. “The old standard assumed that any person exposed to a given set of conditions responded the same way, regardless of the context,” Brager told EBN. The adaptive comfort option in the new version, according to Brager, “allows for a wider range of conditions that would float with outdoor temperatures.” It can be applied to whole buildings or to portions of buildings, and it is optional—designers may rely on the more familiar model even for naturally ventilated spaces if they wish.
This new method for determining acceptable thermal conditions applies to a limited range of situations. First, the optional method applies only to spaces where the occupants are nearly sedentary and have the freedom to adapt their clothing to the thermal conditions. The thermal conditions of the space must be regulated primarily through the opening and closing of windows; the windows must open to the outdoors and must be readily operable and adjustable by the occupants. Mechanical ventilation is allowed, but only in situations where windows provide the primary means of regulating the thermal conditions of the space. Mechanical heat is also allowed, but the adaptive comfort method applies only when the heating system is not in operation.
The new method does not apply, however, to any space that has a mechanical cooling system, even during times when that system is not being used. That means that the new method does not apply to mixed-mode spaces, where natural ventilation is used during swing seasons but supplanted by mechanical air-conditioning during the summer. While this may appear overly restrictive, some evidence suggests that it may be appropriate. According to Peter Alspach, an engineer with Arup in San Francisco, a U.K. study found that mixed-mode buildings rate lower in occupant satisfaction than either fully naturally ventilated or fully mechanically conditioned buildings. One theory for this result is that occupants of mixed-mode spaces come to expect the consistent conditions provided by the mechanical system and are then dissatisfied when windows are open and conditions fluctuate more. “Over time I would like to see the applicability of the adaptable comfort model expanded,” Brager told EBN, “but I was not arguing hard for that with the 2004 version. It’s important to move slowly and to get more data on those building types.”
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| From ANSI/ASHRAE Standard 55-2004, p. 10. |
Figure 2 shows the acceptable operative temperature ranges for naturally conditioned spaces, as laid out in Standard 55—2004. Note that the range of both acceptability and preference rises in accordance with outdoor air temperature. Since it is based on field studies, not laboratory simulations, the adaptive comfort model automatically takes into account factors such as humidity, air velocity, and occupants’ clothing. “The adaptive model is much simpler,” Brager said, “because it doesn’t have so many inputs.”
Not everyone agrees that this simpler approach is dependable, however. Architect and engineer Dan Nall of Flack & Kurtz Consulting Engineers in New York City notes that Brager and de Dear’s data doesn’t adequately cover temperate humid climates (including the United States’ Eastern Seaboard and Midwest). He argues that factors such as humidity and available breezes need to be factored into the equation, since under humid or still air conditions opening windows won’t do much to make occupants more comfortable.
Checklist for Some Low-Energy Thermal Comfort Solutions
Many factors affect thermal comfort in buildings. Some of these can be addressed, at least in some conditions, without conventional, energy-intensive technologies. Here are a few examples:
- Provide a high-performance thermal envelope to control radiant temperature (hot or cold surfaces). In large buildings, an efficient curtain wall or other wall system can eliminate the need for perimeter heating, even in relatively cold climates. In homes, well-insulated walls and windows can eliminate the need for delivering heating and cooling to the outside walls.
- Provide an airtight building envelope to minimize drafts and unwanted latent heat gain.
- Expose high-mass building elements to even out temperature swings. This strategy, often coupled with night-flushing of a space to precool the mass, works best in dry climates when nights are cool and days are hot.
- Give occupants some control over their immediate environment. When people have control, whether it is the ability to open a window, adjust a thermostat, or change the airflow from a diffuser, they tend to tolerate less than “ideal” conditions. This flexibility on the part of the occupants can translate into less energy spent maintaining a fixed temperature set-point, and, in some cases, smaller mechanical systems. Occupant control must be carefully integrated with the overall comfort strategy, however, to avoid situations in which energy is wasted. For example, sometimes occupants change thermostat settings drastically in a misguided effort to get quick results. It can also be a problem if users open windows when outside conditions are not conducive to comfort, and the mechanical system fights to overcome the influx of outdoor air.
- Use air movement to increase comfort. Ceiling fans, desk fans, and outdoor breezes can all make people comfortable in conditions that would otherwise be too hot, which can lead to savings in energy use for cooling. Airflow must be managed to avoid discomfort from too much wind, however, and fans should be turned off to save energy when not affecting occupants directly.
- Use displacement ventilation. Introducing cool air at the floor and exhausting it at the ceiling removes heat from the ceiling plane rather than mixing it back into the space. Because lights generate heat and warm air tends to rise, removing this warm air from high in the space and exhausting it to the outdoors can reduce the amount of cooling needed.
- Capture humidity in ventilation air. Humidity is difficult to control without using a lot of energy, but, in mechanically ventilated buildings, devices such as enthalpy wheels can transfer moisture from incoming air to outgoing air (or vice versa), reducing the need to dehumidify or add moisture and allowing cooling equipment to be downsized.
- Exploit the benefits of evaporative cooling. It takes heat to evaporate moisture, so if a space can tolerate added humidity, direct evaporative cooling is an easy way to convert hot, dry air into cooler, more humid air. In other situations, indirect evaporative cooling may be an option.
- Encourage seasonally appropriate clothing. With the right corporate culture (or household culture), occupants can make themselves comfortable in a range of temperatures by adjusting their clothing. This flexibility can allow higher set-points during the cooling season, and lower ones during the heating season.
- Consider the effect of furniture and furnishings. In office buildings, which tend to operate primarily in cooling mode, chairs with mesh backs and seats will reduce heat buildup and keep people comfortable at higher temperatures than those with foam cushions. Many other aspects of an interior space, even color choices, can affect people’s perception of warmth or coolness.
This article is adapted from Expanding the Engineers’ Comfort Zone: Working with Adaptive Thermal Comfort and Adaptive Thermal Comfort Available at www.BuildingGreen.com.