Indoor Environmental Specialists
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Temperature & Humidity Thermal comfort in the indoor environment is defined by ASHRAE as; "…the condition of mind that expresses satisfaction with the thermal environment; it requires subjective evaluation."1 The perception of an acceptable thermal environment is dependent on the heat one produces as a result of the activities one is engaged in, how easily that heat is allowed to transfer to the environment, and the physiological adjustments one's body makes to aid in the heat transfer (perspiration or chills). Because of individual differences, it is impossible to specify a thermal environment that satisfies everyone. Therefore, it is generally understood that thermal comfort is satisfactory when it is acceptable to at least 80% of building occupants.

Because of the seasonal clothing habits of building occupants, the temperature range for comfort in summer is higher than in winter. Therefore, the operative temperature range in a building in which the majority (~90%) of occupants are performing light, primarily sedentary activity at 50% relative humidity is:

  • 68F - 75F during the winter
  • 73F - 79F during the summer

The winter and summer comfort zones overlap in the 73F to 75F range. In this temperature zone, people in the summer type clothing may tend to feel slightly cool whereas those in winter dress may tend toward feeling slightly warm.

Regarding humidity, the evaporation of water from the moist surface of the skin directly affects the body temperature and thermal sensations. Humidity can also indirectly affect one's perception of the quality of the indoor environment. Odors may be stimulated by elevated relative humidity or the olfactory senses may be hindered by low relative humidity. Cooler, drier indoor air is sometimes perceived as being "fresh" and free from contamination. In a clean, non-odorous, and well-ventilated area the perceived freshness of the air may be said to decrease with the increase in relative humidity.

RH Impact

Temperature and humidity do affect comfort of building occupants and the indoor air quality. At a given temperature, decreased humidity results in occupants feeling cooler, drier, and more comfortable. "For a sedentary person, a 30% change in relative humidity has the same affect on thermal balance sensation as a 2F change in temperature."2

The upper and lower limits ASHRAE recommends for the indoor environment are based on considerations of dry skin, eye irritation, respiratory health, microbial growth, and other moisture related situations. According to ASHRAE 55-1995, 30% to 60% is the acceptable range of relative humidity.

Temperature and relative humidity in the indoor environment impacts energy costs and equipment performance and occupant comfort and health. Monitoring and maintaining these levels within recommended guidelines generally provides a satisfactory thermal environment and does not encourage the development of undesirable microorganisms.

The optimum range of indoor humidity is given below and its impact other factors than can affect human health.

1ANSI/ASHRAE 55-1995, Thermal Environmental Conditions for Human Occupancy
2Comfort and Humidity, Larry G. Berglund, Ph.D., P.E., August 1998 ASHRAE Journal


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