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Personal: Residential

Fenestration Products –
Condensation and Humidity

What is condensation?
Prevention techniques

Humidity in your home
Maximum humidity chart
Controlling the relative humidity of indoor air
Humidity reduction tips

What Is Condensation?

Condensation on windows occurs when the surface temperature of the glass (glazing), sash or frame is lower than that of the humid air around it. The moisture vapour in the air changes into liquid water on contact with these cold surfaces. Condensation will often occur at the edge of the glazing because this is where cold air is more likely to seep through. Condensation can reduce the amount of natural light that comes through the window, affecting visibility, and also cause stains, mould and peeling paint on surfaces near the window.

Exterior Condensation

Energy-efficient windows will occasionally have condensation on the exterior glass surface. This usually happens during periods of warm weather and high humidity near the time of sunrise. Because energy-efficient windows greatly reduce the amount of heat escaping to the outside (which is good during the long periods of cold weather), the exterior glass surface is cool enough to allow condensation to form for a short period of time.

Prevention Techniques

You may have to consider one or more of the following techniques to reduce or stop condensation:

Condensation Resistance

Manufacturers use several techniques to increase the condensation resistance of windows. These include:

Figure 27

Energy-efficient windows are least likely to have condensation. Their resistance to condensation, however, depends on the indoor humidity level.


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

Remember that the type of trees and shrubs you plant near your windows may affect the winter solar gain potential of the windows. For southern exposures, select deciduous trees that have thin branching characteristics. They will provide shade in the summer but will allow more sunlight through after they lose their leaves in the fall.

Curtains and Blinds

Interior window accessories such as curtains, blinds and valances must not prevent air from moving around the surface of the window. Any restriction of air movement will increase condensation on the window.

Figure 28

Hot Air Registers and Heaters

Heat loss through windows is greater than through the adjacent walls, even if the windows are energy efficient. This is why 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 than in the centre of the room, resulting in increased condensation resistance. You must take care, however, to keep hot air from the registers or baseboard heaters from flowing directly onto the interior surface of the glass. Resulting thermal stress problems could cause the glass to break.

Humidity in Your Home

A certain amount of humidity in your home is desirable for comfort during the colder months. Windows, doors and skylights that are not energy efficient will often collect condensation or frost even when the indoor humidity is at a reasonable level. Besides obscuring the view, this condensation can lead to mould formation on the frames and sashes. With an energy-efficient ENERGY STAR® qualified product, it will take a higher indoor humidity for condensation to form.


With well-controlled humidity in your home, you may even be able to turn down your thermostat and still feel comfortable. Because humidity levels are normally higher in bathrooms and kitchens, consider installing an appropriate ENERGY STAR labelled product in these rooms to reduce or eliminate condensation on windows.

Condensation Chart: Maximum Humidity
Before Window Condensation Occurs
Outside Temperature Standard Window Energy-Efficient Window
C 50% 63%
–10°C 38% 50%
–20°C 26% 40%
–30°C 18% 30%
–40°C 12% 23%

Be aware that high indoor humidity can lead to the formation of mould in areas of the house other than the windows, doors or skylights.

Controlling the Relative Humidity of Indoor Air

If there are signs of excessive humidity in your house, you should increase ventilation. If you have a problem only once or twice during the winter, you can reduce or eliminate the problem by briefly opening two windows located on opposite walls or by turning on a kitchen or bathroom exhaust fan. If your home has persistent condensation problems, think about installing a controlled mechanical ventilation system. Try to use a system that incorporates a heat recovery unit and controls relative humidity control.

Figure 29

Humidity Reduction Tips

  • Disconnect humidifiers that are either fixed to your heating system or are portable.
  • 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.
  • If you have a crawl space under your house, cover the beaten earth with 0.15 mm (6 mil) polyethylene. The crawl space may have to be ventilated during the summer.
  • 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.
  • Some airtight houses were built without mechanical ventilation systems. If high humidity is a chronic problem, consider having a heat recovery ventilator system (HRV) installed.
  • If you have a mechanical ventilation system, make sure that it is working properly and that it is turned on.

The ENERGY STAR name and symbol are administered and promoted in Canada by Natural Resources Canada and are registered in Canada by the United States Environmental Protection Agency.