What Causes Frost to Form Inside Home Windows?

Condensation

• In winter, the home’s air temperature is much warmer than that of the outdoors. But window glass is typically much colder than the temperature of the air inside the home. A cold window is often all that’s needed to turn water in its gaseous state -- vapor -- back into its liquid state. The result is condensation, or water droplets that form on the inside of the window in much the same way as a glass of ice water sweats on a hot, humid summer day. When outside temperatures plummet further, bringing the temperature of the window pane down with them, the condensation turns to ice, which appears as frost on the window.

Windows

• The windows themselves may be a cause of water vapor that turns to frost. Single-paned windows with aluminum frames aren't effective at blocking out cold air. Even if moisture levels in the house are low, an icy single-paned window will often cause condensation and frost. Wood-framed, double-paned windows are more energy efficient because the inside pane is relatively warmer than the outside pane. While frost may still form, outside air temperatures must fall much lower than they would to cause frost on single-paned windows.

Internal Moisture Sources

• Moisture is the other cause of condensation and frost on windows. If the humidity levels inside the home are too high, even energy efficient windows will collect moisture. Run bathroom and kitchen fans that vent outdoors, not into attic spaces. You'll also want to ensure the dryer vents outside as well. Showers, cooking moisture and damp laundry areas contribute much of the water contained in a home’s air. Check humidifiers to make certain they are not adding too much moisture to the home.

External Moisture Sources

• Most homes have some air flow from the outside into the home’s interior. That air is often laden with moisture that can get trapped inside the house. Vapor barriers in crawl spaces and insulated floors help minimize the problem of moisture from exterior sources.