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142 Appendix C lesser amount of water vapor diffuses into and out of the building through the building materials themselves. Figure C-1 illustrates locations of moisture entry. The ability of air to hold water vapor decreases as the air temperature is lowered. If a unit of air contains half of the water vapor it can hold, it is said to be at 50% relative humidity (RH). As the air cools, the relative humidity increases. If the air contains all of the water vapor it can hold, it is at 100% RH, and the water vapor condenses, changing from a gas to a liquid. It is possible to reach 100% RH without FIGURE C-1: Moisture Gain in a Building changing the amount of water vapor in the air (its "vapor pressure" or "absolute humidity"); All that is required is for the air temperature to drop to the "dew point." Relative humidity and temperature often vary within a room, while the absolute humidity in the room air can usually be assumed to be uniform. There- fore, if one side of the room is warm and the other side cool, the cool side of the room has a higher RH than the warm side. The highest RH in a room is always next to the coldest surface. This is referred as the "first condensing surface," as it will Courtesy of Dean Wallace Shakun, Clayton State College, Morrow, GA