Interior Condensation

This is the most common form of window condensation that people experience. This is caused by cold exterior temperatures lowering the interior temperature of the internal pane of glass and then the moisture in the warmer air inside is drawn to the colder glass surface and condensates. This can also lead to “frosting” of the windows. Triple pane windows with LowE and Argon gas will be your best defence against this. Double pane glass with LowE and Argon glass will improve this but will not give as good of performance as the triple pane.

Exterior Condensation

This is the lesser occurring of the two condensation types. It is becoming more prevalent as time goes on due to higher performing glass types (eg. Triple pane glass).In this instance the warmth from inside the house is unable to escape through the glass enough to keep the moisture in the exterior environment from drawing to the surface of glass. This will typically dissipate as the sun rises. This is a sign that your high efficient windows are working exactly as they should.

Condensation between Glass Panes

This is a sign that the seal on insulated glass unit has failed and the glass unit will need replacing. This is covered under warranty by Western Western glass warranty.

Recommended Humidity Chart

(Inside Relative humidity of 21°C with Double Glazing)
Outside Air Temperature* Indoor Relative Humidity
-28°C (-18°F) or below
-28°C (-18°F) to -23°C (-9°F)
-22°C (-8°F) to -17°C (-2°F)
-16°C (-1°F) to -12°C (10°F)
-11°C (11°F) to -6°C (21°F)
-5°C (22°F) to 4°C (40°F)
not over 15%
not over 20%
not over 25%
not over 30%
not over 35%
not over 40%
*Including Wind Chill

Controlling Humidity

    • Varying Humidity Levels: remember that the humidity in some rooms, kitchens and bathrooms in particular, can be higher than in other areas of the home, thus encouraging condensation to form on the windows in these rooms.
    • Exposure: some windows in the home may be protected by other buildings, trees, etc., which form a windbreak while others are totally exposed to the wind and, therefore, colder.
    • Ventilation is the most effective means to remove moisture from your home. By exchanging some of the drier outdoor winter air for warm humid interior air, the moisture level inside the house will be reduced.
    • Open a door or window for short periods to allow moisture to escape.
    • Gas appliances produce moisture from combustion. Cooking adds even more moisture so be sure to run your kitchen fan while cooking. Vent the kitchen fan directly to the outside to achieve the desired effect.
    • Showers are a high source of moisture. Keep bathroom doors shut and run the exhaust fan to eliminate moisture.
    • Clothes dryers should be vented to the outside. Hanging up wet clothes to dry inside your home can add significant moisture to your home.
    • Shut off the furnace humidifier and any other humidifying device in the home.
    • If you have a fireplace, open the damper occasionally to allow moisture to escape.
    • Free air circulation is important. Do not cover hot or cold air registers with furniture or appliances.  Leave bedroom and bathroom doors open.
    • Keep the temperature of all rooms at a minimum of 10 C (50 F), even if unoccupied. Condensation will occur in an unheated room.
    • Wipe up any tracked-in snow before it melts and evaporates.
    • Floors wet from mopping can add large amounts of moisture. Run exhaust fans (bathroom and kitchens) while floors are wet. Avoid washing floors on extremely cold days.

  • Most builders install a fresh air intake into the cold air return duct of the heating system. Make sure that the intake damper is open and that there are no obstructions to prevent airflow. If you do not have a fresh air intake, have one installed.
  • Drapes and blinds should be left open during the daytime. At night, raise blinds at least 100mm (4″) up off the frame to allow air to flow against the glass.
  • Certain fabric blinds that are designed to fit from jamb to jamb must be left partially open to ventilate the window cavity. Failure to do this can cause extreme temperatures between the blinds and the glass.  Glass breakage or warping of PVC can occur, which can void warranties.

CMHC Moisture and Air Guide