by factors such as the motion (velocity) of air in the room, the insulation value of clothing, and the metabolic rates of the occupants.
The operative temperature and relative humidity/dew point ranges in ASHRAE Standard 55-1992 are specified for the occupied zone, which is defined as the space approximately 18 to 72 inches above the floor. For thermal acceptability, this standard allows no more than a 5º F vertical difference in air temperature within the occupied zone, a 9º F vertical radiant asymmetry, and an 18º F horizontal radiant asymmetry. As such, the ASHRAE thermal environmental criteria for comfort in occupied spaces have limited influence on the thermal environmental conditions in microenvironments, such as on the surfaces of walls or windows, or within porous furnishings such as carpets, mattresses, and upholstered furniture.
Thermal environmental conditions that are conducive to the growth of some arthropods are well defined, although the actual relationships between ambient and microenvironmental conditions have not been examined. Mites reproduce over an ambient relative humidity range of 45 to 80 percent, with 75 to 80 percent being optimal (Andersen and Korsgaard, 1986). The temperature range of 65º to 80º F is optimal for mite growth. This range just brackets the range of thermal acceptability defined in ASHRAE Standard 55-1992 (ASHRAE, 1993). Relationships between fungal and bacterial growth and relative humidity have been less clearly defined, and probably vary with specific organisms.
Condensation, which will usually lead to fungus growth, will occur on walls, ceilings, and floors when their temperatures are below the dew point temperature of the surrounding air. In cold climates, condensation is often seen on the surfaces of windows, ceilings, and walls. Condensation is especially evident in residences at the inside surfaces of corners of walls, the inside surface junction of the ceiling and the external wall, and at the inside corners of the building that have minimum or no protective insulation (White, 1990). In cold climates, condensation will also occur within the building envelope itself when moist indoor air exfiltrates through leaky construction and encounters cold surfaces on the ''weather side" of the building (Lstiburek, 1989).
In hot and humid climates, condensation can occur on the inside surfaces of exterior walls in air-conditioned buildings when warm moist outdoor air infiltrates through exterior facades and encounters a surface at a temperature below that of the dew point of the infiltrating outdoor air (Morey, 1992). Vinyl and other wall coverings often have low water vapor permeabilities. When these coverings are used in hot and humid climates, they can result in condensation at gypsum board wall covering interfaces at or near room temperatures.
Condensation may also occur on the uninsulated surfaces of chilled water