Humidity affects comfort in a number of ways, both directly and indirectly. The evaporation of water from mucous and sweating surfaces and its diffusion through the skin affect the energy bal­ance and subsequently body temperatures and thermal sensations. When evaporation processes of the skin are compromised or enhanced, skin temperatures change, which is directly sensed by the temperature sensors of the skin. For example, a 30 percent change in relative humidity has the same effect on thermal balance and thermal sensation as a 2°F (1°C) change in temperature with regard to a sedentary person.

Low humidity, or dry air, absorbs moisture from the skin at a rapid rate and produces a chilling effect that can only be offset by increasing air temperature. Dry air also makes fabrics feel smoother and more pleasant, and the air is perceived to be fresher, less stale, and more acceptable. At a given temperature, decreased humidity results in occupants feeling cooler, drier, and more com­fortable, but low humidity also can adversely affect comfort and health. Dry nose, throat, eyes, skin, and other mucous surfaces typ­ically occur in low-humidity conditions, usually when the dew point is less than 32°F (0°C). Excessive drying of the skin can even lead to lesions, skin roughness, and discomfort, and impair the skin’s protective functions. Dusty environments can further exacerbate low-humidity dry-skin conditions.1

On the other hand, high humidity helps our bodies retain heat. In warm conditions, however, thermal discomfort increases with humidity. The discomfort appears linked with skin moisture, as persons rarely judge themselves comfortable in situations where skin wetness is above 25 percent.1 The discomfort associated with skin moisture could be due, in part, to friction between skin and clothing. When fabrics ranging from rough burlap to wool, cotton, polyester, and smooth silk are pulled across the skin, the measured pull force increases with humidity and perspiration, as does the fabric’s perceived texture or roughness.

Thermal comfort is also directly related to the manner in which heat flows through or about building materials, whether by means of convection, radiation, or conduction. While convection may be most noticeable in the form of a breeze or draft, radiation may seem to cause more dramatic comfort variations. Standing next to a large window is an obvious example. These concepts will be discussed further in Chap. 3.