AN607

Rev. 1.7

21

8. Humidity Control for Thermal Comfort

8.1. General Considerations

Humans are sensitive to humid air, because the human body uses evaporative cooling as the primary mechanism

to regulate temperature. When the relative humidity and dew point are high, the rate of perspiration evaporation

from the skin decreases because the amount of water vapor in the air is already close to saturation. Because

humans perceive the rate of heat transfer from the body, rather than temperature itself, we feel warmer when the

relative humidity is high. Relative humidity is a useful indication of how hot the weather “feels” and is more intuitive

and easier to measure than other quantifications of water vapor in the air. Air conditioners are designed to maintain

between 40–60% relative humidity in the occupied space.

Humans tend to react with discomfort to dew points >61 °F (16 °C). The body perspires and produces sweat to

cool down. High relative humidity and consequently a high dew point prevent the evaporation of sweat and reduce

evaporative cooling. The body may overheat, resulting in discomfort.

Lower dew points, <50 °F (10 °C), generally correlate with lower ambient temperatures requiring the body to be

less dependent on evaporative cooling. A lower dew point can be achieved with a high temperature at extremely

low relative humidity allowing for effective cooling.

Table 1. Human Reaction to Humidity*

Dew Point Temp (°F)

RH at 90°FHuman Perception

> 75

> 62%

Extremely uncomfortable

70 to 74

52% to 60%

Quite uncomfortable

65 to 69

44% to 52%

Somewhat uncomfortable

60 to 64

37% to 46%

Comfortable but humid

55 to 59

31% to 41%

Comfortable

50 to 54

31% to 37%

Very comfortable

< 49

< 30%

A bit dry

*Note: See Lawrence, Mark G., “The Relationship between Relative Humidity and the Dewpoint Temperature in Moist Air—A

Simple Conversion and Applications”, American Meteorological Society February 2005.