What causes Window condensation?

Window condensation will often occur during colder weather when the moist air inside the home comes into contact with cooler surfaces. The moisture in the air can condense to form water droplets or even frost. The risk of condensation increases as the weather gets colder and/or the inside humidity rises.

For windows, doors and skylights, condensation can form:

  • on the glass
  • on the frames and sashes
  • near the weather seals
  • on the walls around a skylight
  • between the glass panes if the seal is broken

Window Condensation and Humidity Levels

Air contains moisture in the form of water vapour.  The warmer the air, the more water vapour it is able to hold.  Relative humidity (RH) is the measure of much moisture the air holds in comparison with the maximum amount the air can hold at that temperature.  For example, air at 50% RH holds about half the moisture it is capable of holding.

Health Canada recommends that your home’s relative humidity be kept between 30-55% in the winter.  Lower levels will aggravate skin allergies and respiratory infections, and high levels increase the spread of mould, bacteria and viruses; dust mites spread when humidity is above 50%.

Moisture and Air: Window Condensation

More often than not, many household moisture problems can be solved if one or more of the following steps are taken:

  • Find out where the moisture is coming from.
  • Adopt strategies to prevent excess moisture in the home.
  • Perform maintenance or minor repairs to address leaks.
  • Hire a qualified contractor to make major repairs.
  • Monitor after the remedial work has been done to ensure the problems have been solved

PDF DownloadCMHC Moisture and Air Guide