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Tue, Jan. 21

Ask the Contractor: Window condensation a sign of indoor humidity

Recessed windows, like bay or bow windows, usually exhibit more condensation than other window styles because bays and bows hang away from the insulated house wall, so these windows are often a few degrees cooler in temperature than other windows. (MetroGraphics)

Recessed windows, like bay or bow windows, usually exhibit more condensation than other window styles because bays and bows hang away from the insulated house wall, so these windows are often a few degrees cooler in temperature than other windows. (MetroGraphics)

Why are our windows forming condensation, most always in the bathroom? Lucy and Paul, Prescott

Window condensation can be a real pain in the glass! This week we have received five calls about condensation, asking “Why is this happening?”

The answer to the question does not relate to condensation caused by seal failure because with these calls, none of the windows are cloudy and opaque; therefore, the seal is not broken.

The moisture issue has to do with the glass and frames. Windows do not create the moisture for condensation; that is a condition of the environment. Condensation occurs on windows when warm, moisture-laden air comes in contact with a cold surface, like a metal frame or glass, when the dew point is reached.

Factors that contribute to window condensation include the following: the climate, outdoor environmental conditions; indoor relative humidity through the window; whether window coverings/curtains are closed for a long time during cold weather; and the Condensation Resistance Factor (CFR) and the windows’ ability to resist condensation. The CRF is a number between 0-100. The higher the number, the greater the resistance the window has for condensation.

To control condensation on widows, control indoor humidity, which should not be above 40-45 percent. In cold weather, lower the humidity levels to prevent condensation.

Often, humidifiers are set in the fall and never adjusted for times of cold weather. High humidity can be caused by humidifiers, as well as by indoor plants, aquariums, lack of kitchen or bath exhaust fans, wet crawl spaces and basement and poorly vented dryer exhausts. Window condensation, as noted above, can be a sign of excessive indoor humidity. That’s why the windows in the bathroom and other high-moisture areas are likely to gather condensation. As soon as the moist, warm air comes in contact with the cold glass, the air is cooled, and moisture is released in the form of condensation.

Wood, plaster, cement and additional building materials that are used in new construction and remodeling usually create moisture.

During the heating season, there may be a significant amount of temporary condensation and after the first few weeks of heating, the condensation will dry out. Sharp drops in temperature can also cause temporary condensation during the heating season.

If you didn’t have as much condensation before replacing your old windows, it’s probably because they were drafty. New windows and insulation all create barriers to air exchange. When combined with the additional water vapor from showers, cooking, or from clothes dryers not vented to the outside, the result is excess moisture and a high relative indoor humidity level.

The type of window can also affect the amount of condensation that appears on a window. Recessed windows, like bay or bow windows, usually exhibit more condensation than other window styles. This is because air circulated around those window types is usually more restricted, and since they hang away from the insulated house wall, bays and bows could be a few degrees cooler in temperature.

Drapes and window shades also contribute to window condensation by restricting the flow of warm room air over the glass surface. Therefore, condensation is more apt to occur when drapes are closed and shades are down. Today’s heavily insulated drapes and tighter shades further contribute to condensation on windows.

Vinyl windows resist condensation and help guard against the damaging effects of condensation because of the higher insulating value of the vinyl. Also, double-glazed windows are far more resistant to condensation than single-pane windows. Single-pane windows allow for a higher percent of the indoor relative humidity.

Some window condensation is normal and should not be a concern. But with excessive condensation on windows, problems can arise, like peeling or blistering paint around the window. That can lead to mold or mildew growth. Excessive condensation can also lead to rot of wooden window seals.


The solution lies in controlling the humidity inside your home.

First, understand where the moisture comes from. During the hot, humid summer, your house absorbs moisture. The same principle applies to a newly constructed or remodeled home due to the abundance of moisture from the building materials used in construction.

To avoid condensation crack open a window or door daily to air out your house. Run exhaust fans longer in the kitchen, bathroom and laundry room. Open drapes and blinds, allowing air to circulate against windows. Install and use a dehumidifier.

If you find condensation between the two layers of glass in an insulated window, the airtight seal has probably been broken and the glass will need to be replaced.

Remember, your windows are an early-warning system to alert you that a source of moisture in your home needs to be controlled. It is important to address such problems directly, either by eliminating the source of moisture or by installing some type of exhaust fan.

If your home is very tight, and you have lots of condensation on your windows, you probably need to install a mechanical ventilation system and use it.

YCCA’s Hammer Time airs twice each weekend, Saturday and Sunday mornings at 7 am on KQNA 1130 AM, 99.9 FM and 95.5 FM or the web Sandy and Mike talk about the construction industry and local community partners and contractors.

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