May 2009

Volume 18, Number 5

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Adaptive Thermal Comfort

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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.

Thermal comfort depends on far more than just air temperature—it also includes the environmental factors of mean radiant temperature (the average temperature of the surfaces surrounding an object or individual), relative humidity, and air velocity. And it depends on highly variable personal factors such as the amount of clothing being worn, a person’s resting metabolic rate, and the level of physical activity. Many of these factors can be adjusted by individuals—one can put on a sweater, for example—but the complex interaction of all these factors can add up to a significant challenge for building designers.

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.

The most recent (2004) version of ASHRAE Standard 55 recognizes the role of adaptive factors in establishing thermal comfort, but using these factors to design buildings remains a challenge. The standard allows a wider range of temperatures in naturally ventilated buildings, which are necessarily more susceptible to outdoor conditions. Unfortunately, it does not yet account for adaptability when buildings are in mechanical heating mode or in any building with mechanical cooling—which includes most buildings in climates with pronounced seasonal swings. With its broadened definition of thermal comfort, however, Standard 55–2004 gives greater legitimacy to naturally ventilated buildings and paves the way for significant energy savings.


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April 29, 2009