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July-August 2002

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Thinking Clearly
How to Reduce Window Condensation 
by Ken Shelbourn

Anyone who has been camping is familiar with the condensation that covers grass in the early morning. As a Boy Scout leader, I’ve spent numerous summer mornings taking the troop across the wet grass, ruining many new sneakers. Except for a few unhappy parents, condensation is not an issue outside. However, condensation inside the home is another matter. This condensation results in leaky roofs and windows, drywall damage, paint peeling on the exterior walls, brick spalling and mold. One of our primary goals is to reduce moisture and condensation on windows—a leading cause of mold.

The most effective way to reduce the risk of mold is to control moisture and humidity. In windows, better insulating glass can reduce moisture significantly in the form of condensation. The better the edge of glass insulates, the less condensation will form. By reducing condensation, the risk of mold is also reduced.

Types of Window Condensation
There are four types of condensation that are related to insulating glass (IG) and windows:

    1. Interior glass surface condensation (4th surface condensation);

    2. Secondary condensation (condensation that forms inside the glazing pocket due to moist air that has entered the glazing pocket from the interior side due to a breached or ineffective glazing air barrier);

    3. Interpane condensation (fogged unit or 2nd- or 3rd-surface condensation);

    4. Exterior glass surface condensation (1st-surface condensation).

Causes of Condensation 
When the temperature of air drops the maximum amount of moisture vapor that can be held by the air is reduced. As an example, at 70 degrees Farenheit, one liter of air holds 9,910 parts per million (ppm) of moisture vapor and is at 40-percent relative humidity (RH). This same volume of air and the same 9,910 ppm of moisture vapor at 45 degrees Farenheit is at 100-percent RH and therefore condensation will begin to form on surfaces that are 60 degrees Farenheit or cooler. Indoors this cool surface may be your interior window glass surface. Outdoors it would be any surface cooler than the dew point of the humid air. Often the smaller, thinner material (grass) tends to lose heat most rapidly and subsequently exhibit the first signs of condensation.

The larger the temperature differential between the interior and the exterior surfaces of a window, the greater the likelihood that condensation will occur. A thin film of air at the interior glass surface is losing heat to the exterior by conduction and radiation. When this thin film reaches its dew point, the moisture vapor condenses onto the glass surface.

Educating Consumers 
In addition to promoting the condensation reduction characteristics of their own windows, manufacturers should also educate consumers on what they can do to reduce condensation in their homes by following these simple guidelines.

Keep indoor temperatures lower. By doing this, the amount of moisture able to be held by the air is less, thus creating less condensation. In addition, a cooler indoor environment will reduce the differential between indoor and outdoor temperatures during cold seasons.

Keep the interior glass surface as warm as possible by keeping drapes blinds and interior shutters open to allow room air to circulate over the window and allow radiant indoor heat to reach the glass 

Ensure the IG unit has a warm-edge spacer system to minimize the thermal transfer at the glass edge between the lites of glass, and an inert gas fill (such as Argon) in the IG unit cavity will also reduce heat loss due to thermal transfer across the IG cavity. Again, by reducing heat transfer between the interior and exterior lites of glass, the surface interior is kept warmer.

When buying new windows, check or ask to be shown that the windows have an interior air barrier as part of the glazing system that will prevent moist indoor air from entering the glazing pocket, which can result in secondary condensation forming in the glazing pocket. Most people do not see secondary condensation but it can contribute to interpane condensation, which can be a significant concern to a homeowner.

As a manufacturer, you not only need to design your products to reduce condensation and moisture, but also need to educate your customer on ways to control condensation. 


Ken Shelbourn is a senior technical specialist with TruSeal Technologies’ Technical Services Group, based in Beachwood, Ohio.

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