Altek - portes et fenêtres en aluminium - Aluminum windows and doors

Overcoming Window Condensation

condensation sur une fenêtre


Condensation on the Interior Surface of the Glass:

Replacing old windows with new, more efficient windows usually leads to improved airtightness of a building’s envelope. This results in substantial energy savings and improved occupant comfort.

However, when the household’s daily routine and level of moisture produced (due to showers, baths, cooking, plants, etc.) remain the same and the air exchanged between the interior and exterior of the home is significantly reduced, condensation may form on the interior of the new windows at certain times due to excessive relative humidity.

This condensation may appear around and even in the centre of a thermal unit, especially when the thermal glass covers a large area. The two panes that make up the sealed unit become closer to each other in cold weather. With less space between the two panes, the interior glass becomes colder. The home’s warm, moist air therefore causes it to fog up more easily. A lack of air flow on the surface of glass pane will cause condensation all around it.

Controlling moisture inside the home is therefore very important for keeping the interior surface of the glass from fogging up. The table below shows acceptable levels of relative humidity depending on the exterior temperatures. Glazed surfaces will obviously always be the first to fall prey to excessively high moisture levels and ironically, windows or doors will be the first to be blamed.

The National Research Council recommends the following relative humidity levels in winter:

Outdoor Temperature

Maximum Indoor Relative Humidity

-30 °C or colder

15 %

-30°C to -24°C

20 %

-24°C to -18°C

25 %

-18°C to -12°C

30 %

-12°C to -6°C

35 %

-6°C or warmer

40 %


Here are some tips to better control moisture inside a building and explanations to help you understand the causes:

  • New homes are often prone to high levels of humidity that cause condensation issues. Allow at least one year for the moisture levels to stabilize.
  • In any season, it is recommended to keep the maximum level of relative humidity inside a home at 40%.
  • In winter, remove the inner screens from your windows. The air will flow more freely on the glass, which will prevent condensation especially in the bottom corners. If your windows are equipped with blinds or curtains, open them regularly.
  • Avoid hanging laundry to dry indoors and ensure the air from the dryer vents outdoors.
  • When cooking, the range hood should be turned on to help control moisture.
  • When bathing or showering, the bathroom fan should be turned on. If you do not have a fan, open the bathroom door to increase air flow.
  • Avoid placing plants near the glazing as plants produce water vapour.
  • Install heat sources under the windows.
  • Avoid drying firewood inside your home. A cord of wood can release more than 270 L (60 gallons) of water.
  • The time of day affects condensation. Condensation inside the home is more common early in the morning.
  • Applying stickers to the inner glazing creates a thermal break that could damage the glass.


Efficient air flow and maintaining the relative humidity levels recommended by the National Research Council are most important for ensuring outstanding performance.

Condensation on the Exterior Surface of the Glazing

Condensation issues on the exterior surface of the glazing first appeared with the introduction of high-performance windows. In winter, this kind of condensation can even show up as frost.

You have never noticed this occurring with your old windows? That is because your new windows are very efficient. Traditional windows allow the heat in your home to escape through the glazing. This warms up the exterior surface of the glass, leaving it free of condensation.

Exterior condensation may also be caused by these other factors:

  • The building is located near a lake;
  • Plants and foliage grow under the windows;
  • No wind;
  • The window is facing west, south or north – east-facing windows are not generally affected because the glass is warmed up by the rising sun;
  • Moisture in the air;
  • Clear weather conditions.