Why Do My Home Windows Fog Up in the Winter?

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Condensation on your home windows is most likely to happen in winter when there is a substantial difference between the temperature outdoors and in the house. When your windows fog up, it indicates that excess moisture is in the air indoors. Once you've identified what is causing high humidity in your house, there are steps you can take to reduce the amount of moisture.

Why It Happens

  • Warm air contains more moisture than cool air. When warm, humid air approaches or comes into contact with a cool surface, its temperature decreases. As the air cools, it can no longer hold all the moisture it carried while it was warm. It gets rid of the excess by depositing it on a cool surface nearby, usually a window. This condensation process fogs up the window.

Causes

  • Cooking, washing dishes, watering indoor plants, doing laundry, and taking showers and baths release moisture into the air. Overusing a humidifier can also send too much moisture airborne. Groundwater can seep into walls or foundations. Dampness can collect in a basement or a dirt floor crawl space. Humidity may also get trapped in energy-efficient homes and escape from plaster, wood and cement in new homes.

Solutions

  • Ensure adequate ventilation by installing an air-to-air heat exchanger to siphon moist air out of the house, heat vents underneath patio doors, vents in the attic and in crawl spaces, a vent for the clothes dryer and exhaust fans in bathrooms. A dehumidifier in the basement can help. Consider investing in energy-efficient windows as well as in gutters, flashing and downspouts that direct water away from the house's foundation. If you have a newly built or newly remodeled home, initially you'll need extra ventilation until the building materials dry out.

Fog-free Windows

  • A hygrometer, which you can purchase in any hardware store, can help you determine how much humidity is in your home. If your house is heated to 70 degrees in winter, you can eliminate window condensation by keeping the relative humidity at 25 to 30 percent whebelow freezing outside. If the temperature is colder, the windows will stay clear with less humid air; if it's warmer, more humidity is needed to keep windows unfogged.

Long-term Consequences

  • If your home windows fog up regularly, it's a sign that moisture may be accumulating and doing damage elsewhere. Problems that can result from excess moisture include mold or mildew, rotting and warping wood, ice building up on the roof, damp insulation, blistered or discolored paint and dampness inside the walls or attic.

References

  • Photo Credit window image by Marek Kosmal from Fotolia.com
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