Fixit: How to eliminate ice buildup on inside of windows

  • January 19, 1999 - 10:00 PM

Q: We have a real problem with ice on the inside of the windows in our home. It accumulates about 5 inches up from the bottom, especially overnight. What can be done about this?

A: Once it gets cold in the fall, houses with air that's too moist or that have low-quality windows will start to have condensation on their windows. In winter, the condensation turns to ice.

This happens because water vapor in the air condenses to liquid on cool surfaces and turns to ice on really cold surfaces. A little window condensation in the fall is normal, but it should not persist or become an ice problem in the winter.

There are two basic ways to prevent window condensation problems: Install quality windows, or lower the indoor humidity.

Windows with poor thermal performance are prone to condensation and are considered to be of low quality. Thermal performance is measured as u-values and the lower the u-value, the better the window. When buying windows, look for a u-value of three-10ths or less. Typically, such windows are double-paned, gas-filled, warm-edge windows.

High-quality windows or low u-value windows can handle higher indoor humidity levels, which is desirable for comfort and health.

But buying new windows just to solve a window condensation problem may not be economically feasible.

A simpler solution may be to lower the humidity in the home. That can be tricky but basically involves increasing ventilation.

Opening windows on a daily basis is one way to do that. But that can be a nuisance chore as well as expensive and uncomfortable.

Another way to increase ventilation is to operate bath and kitchen exhaust fans part of every day to expel excess moisture. This is in addition to the time the exhaust fans are used when bathing and cooking. Be sure that exhaust fans blow directly outside and not into attic space.

Also, it's important that exhaust fans don't interfere with the correct operation of the furnace and water heater and that the air they pull into the house doesn't come from an attached garage.

For information on how to operate exhaust fans safely, call the Minnesota Department of Public Service Energy Information Center at 612-296-5175 or 1-800-657-3710.

The best way to lower humidity in a home and add ventilation to control excess window condensation is to install a balanced mechanical ventilation device with heat recovery, commonly referred to as an air-to-air heat exchanger.

Such a unit is smaller than a furnace and is mounted in the basement. It uses the forced-air ducts to exchange the air in the house every three hours, which controls excess moisture and supplies occupants with fresh air. For information about this type of ventilation, where to get it and its cost, call the Energy Information Center at the numbers listed above.

Also, if you have a humidifier, turn it off. Many people run humidifiers or add them to their furnaces even though most new homes and many improved older homes in Minnesota need little or no humidification.

  •   Important: If your window condensation problem is sudden and you have not made any other changes to your home (installed new insulation, siding, windows, furnace, etc.), then the icy windows could be a sign that a combustion appliance in your home is malfunctioning. That can be dangerous.

    Appliances such as the furnace and water heater produce moisture as they operate. Normally, this water, along with the exhaust created by these appliances, goes out of the house via the chimney or vent. If there is a malfunction in the appliance or chimney, moisture and dangerous exhaust fumes will enter the home. The first clue that this is happening often is excessive moisture demonstrated by condensation or ice on windows.

    For your protection, do two things right away: Mount a carbon monoxide alarm in your home to warn you of deadly invisible, odorless exhaust fumes, and call your utility or heating contractor to verify that your appliances are working properly.

    -- Send your questions to Fixit in care of the Star Tribune, 425 Portland Av. S., Minneapolis, MN 55488, or call 612-673-9033, or e-mail Past columns are available at Sorry, Fixit cannot supply individual replies. Fixit appears every day except Friday.

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