Window Condensation and Humidity Levels

Learn more about 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 how 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%.

Relative Humidity and Health Effects

The combination of indoor moisture sources, air exchange rates and cold surfaces determine how much condensation will take place in your home.  Cold surfaces can also cause condensation. Think of droplets of water that form on a glass of cold water (or beer) on a hot summer day.  The air immediately adjacent to the glass becomes chilled to the point where the relative humidity is 100% and condensation occurs.  Your home’s the coldest surface is usually your window which explains why condensation occurs during cold days – in the form of fogging, frost or water droplets.


Condensation can be expected to form on windows, doors and skylights under the right conditions.  The higher the relative humidity, the warmer a surface temperature needs to be in order to avoid condensation.  To reduce the likelihood of condensation, the chart below provides recommended maximum percentage of indoor relative humidity based on varying outdoor air temperatures with an indoor air temperature of 70o F or 20o C:

The chart below summarizes recommended humidity levels in your home.

Recommended Indoor Relative Humidity

Tips to manage your indoor humidity include:

  • Ensure your home is properly ventilated
  • Turn on ceiling fans and run exhaust fans as needed to circulate air and remove excess moisture
  • Open curtains and blinds to allow air circulation – keeping them closed increases the likelihood of condensation forming
  • Other items may increase your moisture level, like plans, aquariums or certain construction/renovation projects including fresh paint and new masonry
  • Adjust the output of your home’s humidifier if you have one
  • It may ne necessary to run a dehumidifier to remove excess moisture from your home


  • Natural Resources Canada – ecoENERGY: Moisture Problems
  • American Architectural Manufacturers Association – Understanding Indoor Condensation in Your Home
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