The temperature at which water vapor will condense from the air at any specific relative humidity is called its dew point. As mentioned earlier, water vapor within dwellings normally is present only in its invisible gaseous state. However, when vapor-laden air comes into contact with a cold surface, at or below its dew point, such as a window pane in winter or an article removed from the refrigerator, it promptly condenses into water droplets (dew). If the surface it contacts is below 32°F (0°C), the vapor becomes ice crystals. Air with a high relative humidity always has a higher dew point than does drier air. A given temperature and relative humidity determine the dew point.
Temperature differences between indoor and outdoor environments create conditions that promote condensation. The difference between outdoor temperatures and indoor thermostat set points is typically greater in the winter than in the summer. This leads to a greater likelihood of condensation occurring in the winter season.
In the design and construction of the thermal envelope of buildings (the enclosure of desired temperatures and humidities), the behavior of moisture must be considered, particularly the change of state from vapor to liquid (condensation). Problems arise when moisture comes into contact with a relatively cold surface (temperature below the dew point), such as a window or within outdoor walls or under-roof ceilings. Excessive condensation within indoor walls that enclose cold spaces must also be considered.