Tom Shafer

How can I stop window condensation?

April 01, 2015

Spring is here, and that means warmer weather - for the most part. You'll still see a cold snap or two. What does this have to do with windows? The variance in temperature can actually impact your window glass. Moisture forms on the surface of it and you wind up with condensation.

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When water vapor in the form of humidity in the air reacts with a cold surface - like your window - the moisture changes from a gas to a liquid which you can then see sitting on your window.

You might see this happen more and more as the weather warms up. Cold air can only hold so much water vapor, which is why you get lower humidity during colder months. Warmer air is denser and holds more moisture, hence the higher humidity during summer. If your windows and doors are opened, that outside warmer air with higher humidity enters the house. Night comes, the outside temperature drops, the glass, which is in contact with outside air, becomes cooler. The warmer air in the house, laden with higher humidity, touches the glass also, and that's when you get condensation.

There is nothing wrong with the window; the insulated glass has not lost its seal. It's just physics. But over time, depending on your window frame material and other factors, it can cause damage.

Here are three quick ideas:

  1. Lower the humidity in the home by using a dehumidifier. Have condensation in the winter, too? This sounds counter-intuitive, but running the air conditioner about a half hour a day will remove moisture.
  2. Open a window a small crack or use a ventilation fan when cooking, or showering, or using a washing machine. Any house activity that adds moisture into the air will cause condensation.
  3. Raising the temperature of the glass prevents condensation. This is where u-value (which measures insulation) becomes important. Glass with a lower u-value prevents thermal transmittance. Low-e glass with an argon infill is the best way to achieve this.

The National Fenestration Rating Council has a great paper on questions about windows and condensation, and it's available online.

Excessive condensation can rot wood, discolor paint, and allow mold growth. Condensation on the windows can be prevented and can keep your windows dry. Replacing your current windows with newer, energy-efficient models can make a big different.


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