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The facts about window condensation and what can be done about it.
During the winter months, homeowners often encounter condensation on the windows in their homes. The goal of this article is to define the problem, the causes, and the solutions to this bothersome and ever present potential harm.

If you are troubled during the fall and winter by condensation on the windows of your home, you aren't alone. It is a common problem in cold climates. The typical family produces a lot of moisture indoors (washing, showers, cooking, just breathing, etc.) and it can be difficult to reduce it.

Understanding the causes of the problem is the first step in solving it. Condensation and ice form on windows because the window surface is below the dew point for the air near the window, so some of the moisture in the air condenses on the glass. The higher the relative humidity of the air near the window, the higher the temperature of the dew point.

For example, if the thermostat is set at 20 C degrees, the temperatures on the surfaces of your windows will be much lower, how much lower depends on the outside temperatures and the insulating value of the window. When the outside temperatures are at the freezing point, temperatures on the inside surface of a double glazed window could easily be 10C degrees lower. As the temperature of the air near these cold surfaces drops, its relative humidity climbs to the point where condensation can occur. To eliminate condensation, homeowners either need to raise the temperature of the window surface or reduce the relative humidity in the room. 

If the condensation occurs on the prime window and the window is double-glazed, that usually means the relative humidity is too high for the temperature conditions. With a thermostat set at 20C degrees, condensation should not occur on double-glazed windows until outside temperatures drop well below zero, if relative humidity is kept below 40 percent. If you have additional glazing, including Low-e coatings and argon gas filled spaces, it takes even colder temperatures before condensation occurs.

Several conditions can increase condensation problems. If you close drapes over windows, this can cause the glass temperature and the temperature of the air between the drape and the glass to become cooler, making condensation more likely. If the thermostat setting is dropped substantially at night, this will cause an increase in relative humidity and may increase condensation. This condensation should vanish once the temperature rises in the morning.

The best way to deal with condensation on high-quality, double-glazed windows is to reduce indoor relative humidity to no more the 40 percent. The best method to control indoor humidity is with ventilation. Run range hood, kitchen and bathroom vent fans whenever you are using those rooms. The best solution is to install a whole-house heat recovery ventilation system. These save heat from the indoor air before it is exhausted outdoors and transfers this heat to the incoming fresh dry air. These systems also improve indoor air quality inside your home. The fans will be more effective if you open a window elsewhere in the house to provide replacement air. Be sure that the fans you use exhaust to the outside, instead of going to the attic or just recirculating air within the house.