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Educational Consumer Tips

Condensation on Windows

Author: Better Business Bureau

This report is general in nature and is not intended as a reliability report on any company, service or product.

Occasional water on your windows during winter is nothing to worry about. High humidity in the home often causes “sweating” on windows. Condensation is a natural occurrence on all surfaces in the home caused by excess humidity in the air. However, excessive condensation can damage your window frames and sills in addition to your window coverings and the dry wall, plaster, paint or wallpaper surrounding your windows.

During the winter, the insulated glass in your windows will generally be cooler than the rest of the room. When the warm moist air in the room comes in contact with the colder glass surface and frame materials , it immediately cools down. Since warm air holds more moisture than cold air, as the air against the window cools down, it can't hold the same amount of water vapor. The excess moisture condenses into water, frost or ice on the window and frame. This occurs more frequently during the winter because of the extreme difference between the inside and outside temperatures. However, the same phenomenon can occur in the summer. When the outside air is warm and humid and comes in contact with glass that is cold due to air conditioning usage, moisture can form on the outside surface of the glass.

All homes will have occasional condensation from things that add moisture to the air such as cooking, showering and venting the exhaust from a dryer into the house. Newly constructed or remodeled homes may have condensation from the moisture in the building materials which should disappear after the first heating season. You may also see temporary condensation following a humid summer when houses absorb moisture. The house should dry out after a few weeks.

You can correct a condensation problem by reducing the humidity in your home. You can do this by lowering the setting on your humidifier if you have one or by increasing your ventilation by controlling the amount of moisture you release into the air. When cooking or showering, use exhaust fans or open a window slightly. When drying clothes, don't vent the dryer into the house since this air holds a lot of moisture. To help dry out the air, open blinds and drapes during the day. You can also reduce window condensation by insulating your existing windows or installing better-insulated windows and frames. Insulated windows and frames that don't transmit the cold outside temperature very well will decrease the difference in temperature between the window and the air in the room. In some cases, however, you may notice more condensation after installing new windows that fit tighter than your old windows because they allow less airflow around them. If this occurs, you should take steps to reduce the humidity in your home and at the same time recognize that once the house comes into “balance” the condensation opportunities will be reduced.

Check to see how well your window will resist the formation of condensation. The higher the Condensation Resistance Factor (CRF) rating, the better that product is at resisting condensation formation. While this rating cannot predict condensation, it can provide a credible method of comparing the potential of various products for condensation formation. CRF is expressed as a number between 1 and 100 and all windows should have this number readily available.

For more information on the Energy Performance Ratings label and its Condensation Resistance rating, visit The NFRC site will also provide additional information regarding a windows performance rating such as air infiltration and u-values. This is a standardized rating program and permits fair rating comparisons between different window manufacturer’s offerings. In 2012 Pennsylvania adopted the 2009 Internal Energy Codes requiring all windows sold in the state to have a U-factor rating of .35 or lower. The rate of heat loss is indicated in terms of the U-factor of a window assembly. The lower the U-factor, the greater a window's resistance to heat flow and the better its insulating properties.

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Updated 1/15