Building Science at UC Berkeley: Research

ASHRAE Investigation of Thermal Comfort at High Humidities

Edward Arens, Department of Architecture
Fred Bauman, Center for Environmental Design Research
Charlie Huizenga, Center for Environmental Design Research

Environmental Analytics

When first publicly released, ASHRAE Standard 55-1992 set an upper limit of 60% relative humidity (RH) for its human comfort zone. This limit was lower than the limit specified in the previous version of Standard 55 (1981) for most temperatures within the comfort zone. The basis for the change was not clearly expressed in the Standard, and attracted the objection that it was based on biotic/health considerations outside the Standard’s scope. The Standard 55 Committee subsequently developed an addendum to Standard 55-92 establishing an interim upper humidity limit based solely on comfort considerations. The Committee suggested, however, that new research was needed to better define the level and shape of the upper humidity boundary. This led to the current ASHRAE-funded research project investigating thermal comfort at high humidities.

A total of 395 human subjects were tested under steady-state conditions in a controlled environment chamber configured to resemble a modern office space. The subjects were exposed to a full range of temperatures (20 to 26°C ET*) and relative humidities (60 to 90%). During the test, subjects repeated a series of step-exercises to simulate three different activity levels (1.2 met, 1.6 met, and 4-5 met), representative of a range of typical office work. Subjects wore typical informal office attire with an insulation value of either 0.5 clo (summer) or 0.9 clo (winter).

We found no significant psychological or physiological differences in human response to exposures between 60 to 90% RH for the temperature range 20-26°C ET* while sedentary. Non-sedentary activities produced differences in overall acceptability, thermal sensation, skin dampness feeling, and air humidity acceptability. These differences are the result of the onset of sweating and the increase in skin temperature that occurs when metabolism is increased. There was no variation of this discomfort mechanism with variations in humidity level. On one subjective scale, direct assessment of "air humidity acceptability," the subjects indicated that 90% relative humidity was universally unacceptable at all temperatures. None of the other subjective scales indicated consistent statistically significant effects. We can say generally, though not with statistical support, that (1) the 90% RH condition was typically the least favorably rated, (2) the 80% RH condition was not apparently worse than the 60% or 70% condition, and (3) the 70% RH condition was frequently more favorably rated than the 60% condition. The data are consistent with the ASHRAE 55-95a Addendum. Finally, for metabolic rates of 1.6 met and above, it was concluded that no reasonable limit on humidity will lower the percentage dissatisfied below 25%.

This project provided support for one Ph.D student.