The following discussion addresses the mechanisms of moisture movement and condensation, the effects of interior moisture on exterior finishes, and methods of controlling this movement to prevent problems in houses. Condensation problems in houses were of little consequence prior to the introduction of thermal insulation and attempts to more closely control the indoor environment. When builders began using thermal insulation during the 1930's, condensation problems developed because insulation resulted in lowering the temperature of wall sheathing below the dew point temperature of indoor air. The use of vapor retarders near the warm face of walls was proposed to prevent moisture from migrating to the exterior. Initially, these vapor retarders provided adequate control, but the energy crisis of the 1970's resulted in much more insulation and more airtight houses.
Condensation can cause several problems for exterior finishes.
Water soaking into the back of siding causes it to 'swell and consequently stress the film-forming finishes. In some case condensation occurs at the interface of the siding and the finish film, and causes blisters. Water in siding may also leach extractives from certain extractive-rich woods and cause staining. Wet siding also promotes mildew growth on the surface. A certain amount of condensation occurs in most houses.