Top 10 Repair Questions

8. Sweaty Windows

"Condensation has been a persistent and often misunderstood problem associated with windows," says Dariush Arasteh, staff scientist at the Environmental Energy Technologies Division of Lawrence Berkeley Laboratory in Berkeley, California. In cold climates, condensation collects on windows when the temperature of the glass drops below the dew point of the inside air. Minor sweating is normal with most windows, but excessive condensation can contribute to the growth of mold or mildew in or on the walls, damaged paint surfaces and eventually rotted wood components, says Arasteh.

Newer low-e, double-glazed windows can reduce frost and condensation because interior glass surface temperatures are warmer. In an existing home with older windows, the simplest and most effective way to control condensation is by reducing interior humidity. Using vent fans (in the kitchen and bathrooms) and dehumidifiers also helps lower interior humidity levels.

The National Fenestration Rating Council, an organization that tests the energy efficiency of windows, is developing a new condensation resistance rating that will appear on window labels; however, until such a rating is available, a lower U-value generally indicates greater condensation resistance.

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