Center for the Built Environment

There has been considerable discussion of what the maximum allowable relative humidity (RH) should be in occupied spaces. ASHRAE Standard 55, 'Thermal environmental conditions for human occupancy', has had three different maximum limits in the last 10 years. The mechanism by which high levels of humidity affect comfort is not yet fully understood. To determine what humidity levels are acceptable, a literature review and laboratory tests were carried out. This paper examines how human subjects evaluated comfort level and comfort acceptability under a range of high humidities, while holding their thermal heat balance constant. Human subjects were tested in a controlled environment chamber configured to resemble a modern office with typical furniture and partitions. Wearing informal office attire, the subjects imitated a series of different activities while being exposed to constant thermal conditions with relative humidities in range from 50% up to 80%. The temperature setpoints were along the new effective temperature (ET*) line at 26.1°C. By subjective and physical measurements, the study attempted to explore the relationship of subjective comfort and humidity around the upper boundary of comfort zone specified in ANSI/ASHRAE Standard 55-1992.