Moisture from inside the home can collect and condense in the wall cavities if not properly controlled. When a house is closed up, as in the winter to retain heat, excessive amounts of moisture can accumulate from sources such as showers, cooking, dryers not vented outside the house and even respiration. The warmer the air, the more moisture it can hold. This moisture will migrate to an area of lower vapor pressure such as a cold wall cavity, not properly protected by a vapor barrier.

The moisture condenses in the cold wall cavity until warming temperatures in the spring and summer cause the condensed moisture to again vaporize and migrate through the siding toward the dry outside air.

Moisture can also enter the wall cavity from outside due to faulty caulking around doors and windows, leaking roofs or gutters and loose flashing. Warming of the air in the wall cavity creates the same conditions described above. As moisture forces its way through the siding to the dryer outside air, it can cause blistering or peeling of the finish and paint discoloration problems. To avoid or correct such problems, it is essential that moisture inside the house be properly controlled with vapor barriers and ventilation.