Myth Buster: The Real Reason for Window Condensation & Tips to help you Control it

What is window condensation?

Common household condensation or “sweating” on windows is caused by excess humidity or water vapor in a home. When this water vapor in the air comes in contact with a cold surface such as a mirror or window glass, it turns to water droplets and is called condensation. All homes have occasional condensation, such as a little fogging on the windows, which is no cause for concern.

Window Condensation

Window Condensation

On the other hand, frost, peeling paint and even moisture spots on ceilings and walls can be signs of excessive condensation and be potentially damaging problems in your home. We tend to notice condensation on windows and mirrors first because moisture doesn’t penetrate these surfaces. Yet they are not the problem, simply the indicators that you need to reduce the indoor humidity of your home.

Fact or Fiction: Windows cause condensation…

It’s a myth that window’s cause condensation. You may be wondering why your new energy-efficient replacement windows show more condensation than your old drafty ones. Well, your old windows allowed humidity to escape. Now that your new windows create a tighter seal, the extra moisture in your home is unable to escape, therefore making you more aware of excess humidity. Windows do not cause condensation; instead they prevent humidity from escaping and provide an easy surface for condensation to collect.

Where does indoor humidity come from?

All air contains a certain amount of moisture, even indoors. And there are many common things that generate indoor humidity such as your heating system, humidifiers, cooking and showers. In fact, every activity that involves water, even mopping the floors, contributes moisture to the air. Condensation is more likely to occur in homes where January temperatures drop below 35°F because there are greater temperature extremes affecting the glass in the home. It is very normal to experience condensation at the start of each heating season. During the humid summer months your home absorbs moisture and then perspires when you turn on the heat. This is only temporary though, after the first few weeks of heating your home should dry out, reducing, if not eliminating condensation. You’ll notice the same scenario if you’ve completed remodeling or building projects. Due to the high levels of moisture in wood, plaster and other building materials, your home will temporarily sweat during the first few weeks of the heating season. Another factor in the condensation equation is progress. With today’s modern insulation, moisture-barrier materials and air-tight construction, we all enjoy a more thermally efficient home – one that blocks the cold out, yet traps the moisture in, producing higher humidity levels and . . . more condensation.

They key is to reduce humidity inside your home:

The best way to reduce condensation is by eliminating excessive humidity. So, how much humidity is too much? The following table* illustrates the recommended or comfortable levels of indoor humidity during the winter months. By eliminating excessive humidity in your home you may very well eliminate most, if not all, of your condensation problems.

Outside Temperature Recommended Relative Humidity
+ 20° F and Above 35% to 40%
0°F 25%
-10°F 20%
-20°F 15%

Let the experts help you control the indoor humidity in your home:

6 easy steps:

  1. Make sure all sources of ventilation to the outside are functional and use kitchen, bathroom and laundry room exhaust fans during and after humidity-producing activities to vent excess moisture.
  2. Air out your home periodically. Opening windows for just a few minutes a day lets the stale moist air escape and the fresh dry air enter without compromising your heating.
  3. Check your humidifier settings. Use the humidity comfort levels provided in the table to correctly set and balance the humidity level in your home.
  4. Be sure that all louvers in the attic or basement are open and large enough. You can even open your fireplace dampers to allow excess moisture to escape.
  5. If you have a large amount of house plants, try to concentrate them in one area and be careful not to over water.
  6. If troublesome condensation persists, see your heating contractor about an outside air intake for your furnace, venting of gas burning heaters and appliances, or installation of ventilating fans.

*Recommended humidity levels courtesy of Blue Flame Gas Association.


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