Air will always contain a certain amount of moisture (water vapor), and the amount of moisture it contains will depend upon its temperature. Generally speaking, warm air is capable of containing more moisture than cold air.
Air will lose moisture in the form of condensation when its temperature falls below its dew point. The dew point is the temperature
at which moisture begins to condense, in the form of tiny droplets or dew. Any reduction in temperature below the dew point will result in condensation of some of the water vapor present in the air.
The attic is one place where condensation often occurs. In the winter, the warm air from the occupied spaces leaks past the insulation on the top-floor ceiling and comes into contact with the cooler air in the attic. This causes the water vapor to condense. The moisture resulting from condensation works its way down through the ceiling and walls, eventually causing damage to the insulation, wood, and other building materials.
The solution to condensation in the attic is twofold. Adequate ventilation should be provided in the attic to remove the humid air. This can be accomplished by installing louvers at each end of the attic or in roof overhangs to provide cross ventilation (Figure 3-2). The louvers should have a total area of 1 ft2 for each 300 ft2 of attic floor space. For example, if the attic has an area of 900 ft2, the total louver area should be at least 3 ft2. In addition to suitable attic ventilation, you should install a vapor retarder between the rafters. This is particularly true if the attic is occupied or used extensively.
Figure 3-2 Two methods of attic ventilation.
Foundation crawl spaces are also subject to condensation. A vapor retarder between the floor joists prevents excess moisture from accumulating and causing dampness and eventually damage to the structure. Ventilating the crawl space so that the humid air can be removed is also very effective. Finally, placing an insulating paper (e. g., roofing paper) on the ground will reduce the amount of moisture released by the soil (Figure 3-3).