Condensation + Humidity Information


Condensation is the change of water vapour in the air to liquid water on a cold surface. The lower the temperature the less water vapour the air can hold. 


Humidity is defined as the presence of water in the air. It is present as an invisible gas, in nearly all air.


Relative humidity is the ratio of the amount of water vapour actually present in the air to the greatest amount possible at the same temperature.


Condensation Resistance
Inert gas between glazing. Energy Efficient windows are less likely to have condensation. Their resistance to humidity depends on the indoor humidity level.

The shading of windows, either from exterior drapes and curtains, or from trees, will influence the amount of exterior solar gain. On sunny days during the winter, keep the drapes open to admit as much solar gain as possible.

Curtains + Blinds
Curtains and blinds must not prevent air from moving around the surface of the window. Any restriction of air movement will increase condensation on the window.

Hot Air Registers + Heaters
Windows that have heat sources located below them are less susceptible to condensation than other windows. When the heating system is in operation, the air temperature will be higher in the vicinity of the window, resulting in less condensation. However you must take care to keep hot air from registers or base board heaters from flowing directly onto the glass since it could cause the glass to break.


  • Disconnect humidifiers.
  • Make sure that the clothes dryer is vented to the outside.
  • Use bathroom and kitchen fans that vent to the outside when bathing or cooking.
  • Make sure that your basement is well drained and protected against excess moisture, and make sure that gutters and the slope of the land around the house drain water away from the house.
  • Store firewood outside.
  • If you have mechanical ventilation system, make sure that it is working properly and that it is turned on.


What causes exterior condensation?

Exterior condensation occurs when moist air comes into contact with cool surfaces, such as glass. This type of condensation appears when the dew point in the air is higher than the temperature of the glass. This occurs when a cool night follows a warmer day, most typically during the spring and fall seasons.

What causes condensation on the inside glass of windows?

Whenever there is excess humidity in a home, it manifests itself in the form of condensation on the coldest area of a wall, which is normally the windows. The warmer the air, the more moisture it will retain.

Do windows cause condensation?

No, condensation on windows is not the fault of the window. However, by replacing drafty windows and doors or installing a new roof or siding, you are reducing air flow in your home and making it tighter. Tighter homes actually retain more humidity.