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How to Fix Window Condensation

You may have noticed that some of your windows have started to look like a cool can of Coors Light on a hot day, covered in fog or even beads of condensation. You may be wondering what causes this, and what you can do to prevent it. Here\’s how…

The time has come; winter has arrived. With temperatures dipping to the freezing mark, and an honest-to-goodness weather system scheduled for the GTA today, it no longer feels like fall. You may have noticed that some of your windows have started to look like a cool can of Coors Light on a hot day, covered in fog or even beads of condensation. You may be wondering what causes this, and what you can do to prevent it.

What Causes Condensation on Window Glass?

Firstly, rest assured that condensation is NOT a sign of faulty windows or seal failure. If you’re not certain whether what you’re seeing is condensation or moisture between your window glass panes, call us. Glass, even with extremely high insulating values, is usually one of the coldest surfaces on a building envelope, and just like that cold can of beer on a hot summer day, humidity in the air will condense on a cold surface.

If you’ve forgotten your Grade 9 science class lesson on gases and pressure, here is a good primer. For those that don’t want the Bill-Nye-the-Science-Guy lesson, skip the next two paragraphs, and go right to the solution;

The way we often measure moisture content in air is by its relative humidity, defined as the partial pressure of the water vapor in the mixture divided by the vapor pressure times 100%. So a relative humidity of 100% means the partial pressure of the water is equal to the vapor pressure at that temperature, which also means that adding any more water or reducing the size of the container (room) would result in condensation.

Since relative humidity is the ratio of partial pressure to vapor pressure, both the partial pressure of water and the vapor pressure increase when you increase temperature. However, the vapor pressure increases more rapidly than the partial pressure (see Ideal gas law and Antoine equation for quantitative relationships). The result is that for a given increase in temperature, the vapor pressure increases more than the partial pressure, thus lowering the relative humidity, and resulting in air that can “hold” more moisture. As the air is cooled, it can no longer hold as much water vapour. This leads to deposition of water on the cool surface. This is very apparent when central heating is used in combination with single glazed windows in winter. (

How to Minimize Window Condensation:

Depending on what kind of home you live in, condensation might never be an issue. Older double-brick homes with little-to-no insulation have very porous building envelopes that allow air and moisture to circulate quite easily. Condensation typically affects more modern homes that better separate indoor and outdoor air. During the hotter, more humid summer, your house absorbs moisture from the atmosphere. Newly constructed houses, or even newly renovated homes, will hold even more moisture, as various materials (wood, drywall) release small amounts of additional water vapour.

In many homes, condensation problems are limited to the first few weeks of colder weather, as homes ‘dry out’ in the colder, less humid winter air. For houses with better air sealing, the key to minimizing window condensation is controlling humidity, and ensuring adequate air circulation around window glass. Open window coverings to allow for better air circulation immediately around windows, and in extreme cases, use a dehumidifier, though this is rarely necessary in the GTA’s dry winter air. Finally, run range vents or bathroom fans a little longer to allow moisture to escape.

Modern, highly-insulated thermal windows are less susceptible to condensation than older single-pane glass, since the inside temperature is typically much warmer, so if you’re still having condensation problems after following the tips above, installing new windows will help to alleviate your condensation issues AND save you money on your hydro bill. If you’re interested in new windows or doors for your home, or simply have a question, call us at 416-533-4331 or email us at

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2018-08-20T08:04:35+00:00 February 23rd, 2017|