Relative humidity (RH) is the amount of moisture in the air relative to what the air can "hold" at that temperature. As a ratio, RH is expressed in percent and can be calculated with the following equation. 

RH is used in construction to determine whether a project may need passive strategies and/or mechanical support to regulate interior temperatures. Both RH and Dry Bulb Temperatures (DBT) are used to predict the  thermal comfort of occupants during critical time intervals, which in certain climate zones may determine the majority of a projects Energy Use Intensity (EUI).

An occupants thermal comfort is most affected by humidity. Our skin relies on the air to regulate body temperature. The process of sweating is your body's attempt to keep cool and maintain its current temperature. If the air is at 100% relative humidity, sweat will not evaporate into the air, because the air cannot "hold" any more moisture, thus is saturated. As a result, we feel much hotter than the actual temperature when the relative humidity is high if the DBT is high. If the relative humidity is low, occupants may feel cooler than the actual room temperature because a lower RH means the surrounding moisture density is low, resulting in faster evaporation rates for our sweat. If the RH is too low, the dry air can cause our skin to feel dry and itchy.

Buildings that do not regulate in areas of extreme RH percentages, may facilitate growth of Fungi and bacteria, have high dust mite populations, poor indoor air quality, and serious condensation problems which overtime will compromise many building materials. 

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