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Tom Shafer

Why do my windows get condensation?

January 13, 2015

When the temperature outside drops, do droplets of water drip off the glass of your windows? Or maybe a light coating of frost? Do they leak? And most of all, is this normal or does it mean your windows have a problem?

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To start, let's picture a cold glass of iced tea on a hot, humid day. The glass gets coated with moisture for the same reason your windows do: dew point. The dew point is the temperature where water vapor in the air begins to form water droplets. It happens when the temperature outside the glass - the sweltering weather, for instance - begins to influence the temperature inside the glass - so where our iced tea is. When the inside surface of the glass is cooler than the dew point of the air, moisture forms. The same thing happens when the outside weather influences the glass on your windows.

The reason this happens inside your home is because the air is filled with water vapor from showers, boiling water, humidifiers, and furnaces. Atmospheric conditions also contribute tot his. When a hot humid day is followed by a quick temperature drop, the air inside your home is still filled with humidity. The cooler outside air hits your window glass, the high humidity in the home means a high dew point, and you wind up with condensation.

To stop this from happening, glass and window manufacturers have reduced the amount of heat that transfers from outside to inside by using window materials that transfer heat poorly. Insulated glass with lower u-values keeps the inside glass warmer and reduces the chance of condensation forming.

Labels on a window can indicate the ability of a window to avoid condensation. The National Fenestration Ratings Council (NFRC) window label rates this ability as Condensation Resistance on a scale of 1-100. Tthe higher the number the better the ability to avoid condensation. The windows get tested and are then labeled to meet NFRC test and performance criteria.

How do you reduce the potential for condensation?

  • Lower the humidity in your home - run fans in shower areas, ventilate a kitchen when boiling water,and lower the settings on a furnace humidifier.
  • Purchase windows with a low-e coating.
  • Buy windows with a thermally improved frame material - wood, vinyl, aluminum with thermally broken aluminum and fiber glass.

Condensation is a pesky problem that has been very effectively solved and eliminated. If you're seeing this a lot in your home, it might be time for replacement windows.


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