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What Causes Window Condensation?

By Susan Grindstaff
Updated May 16, 2024
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Window condensation is usually caused by water that forms when cool air meets warm air. Window condensation can occur on either the inside or the outside of the glass. Outside condensation most often occurs during spring or fall, when the daylight hours are significantly warmer than those at night. Indoor window condensation occurs when very warm air inside a room meets window glass that is cold. The warmer the air, the more humidity it contains, and this excess humidity turns into water when it meets cold glass.

For most people, heating their homes is necessary during cold weather, which makes indoor condensation hard to avoid. Many homeowners use treated window glass specifically designed to cut down on condensation. Glass is normally considered a very good conductor of heat and cold, but treated window glass drastically limits this conduction.

With proper installation, treated window glass can greatly reduce window condensation problems. This type of glass is usually referred to as Low-E glass, and in many cases, it can reduce temperature transference by as much as 50%. This reduction may decrease decay caused by moisture both in and around the window frame.

Most of the time, condensation will occur all over the window. As the water slowly reaches the same temperature of the surrounding air, it begins to slide down the surface of the glass until it finally rests at or near the bottom of the window. If the window does not have a rim at the bottom, the moisture generally pools on the surface of the window sash. If the sash is wood, and the moisture is left on the sash, it may eventually lead to mold or rot.

There are a few steps homeowners can take to help reduce window condensation in the interior of their homes. Humidity control is one of the most obvious methods, and can usually be achieved by purchasing a home dehumidifier. These units are available at most home improvement centers in varying prices and sizes. For very large homes, it may be necessary to purchase more than one unit to dehumidify the entire home.

Some other methods for cutting down on window condensation include using exhaust fans in rooms where moisture tends to collect. This includes kitchens, bathrooms, and laundry rooms. Homeowners should also take care that their clothes dryers and gas burners are vented to the outdoors. It may also be a good idea to open windows for a few minutes each day to allow humid air to exit and be replaced with fresh air.

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Discussion Comments
By anon285655 — On Aug 16, 2012

Is indoor or outside window condensation possible with dual pane windows? Four of my new double pane windows developed condensation. I figured it was between the panes since the decorative metal sections inside made the condensation look as if it were on several sections of glass instead of just the one.

Also, in the same three part window, only the center section showed condensation. The company has denied my claim saying that it was probably on the inside or outside. I had not heard of this before so did not check. My previous home had dual pane windows which had so much condensation between the panes over the years that they became hard to see through. --Jackie

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