Humidity and condensation

Condensation on the inside of windows

If you notice condensation on your thermal windows, you can be sure that their manufacturing and installation are not to blame. Rather, the problem is most likely caused by the humidity level in the house, which you will need to learn how to control. Even if you buy the most efficient windows on the market and have them expertly installed, you’ll still end up with condensation if the humidity level in your home is too high. What’s more, living in a home that’s too humid can be bad for your health.

For more information about the humidity level inside a home and about air quality (CMHC site)

Condensation on the outside of windows

You might notice condensation on your windows in fall or spring. If you look closely, you’ll notice that it’s not the inside of the window that’s fogged up, as is often the case with older windows, but rather the outside.

Download the PDF document from the APCHQ – Condensation extérieure des fenêtres (French only)

Condensation on windows: a normal seasonal occurrence

It’s perfectly normal for condensation to form on your windows in certain weather conditions; this happens when the surface of the window’s outside pane reaches a temperature below the dew point. Since outside window panes are better quality, and therefore more efficient, they let less heat through, causing condensation to form occasionally. This is the same phenomenon that occurs in summer when condensation forms on a glass of ice water.

There is no need to worry—the condensation is not due to a manufacturer’s or installation defect. As soon as the wind picks up or the sun comes out, the condensation will disappear (source: APCHQ).

Here are the proposed humidity level to control condensation (Source: APCHQ):

Outside temperature
in degrees Celcius
Maximum relative humidity level desirable
for an indoor temperature of 21°C
-28 °C or less 15%
-28 °C to -23°C 20%
-22 °C to -17°C 25%
-16 °C to -12°C 30%
-11 °C to -6°C 35%
-5 °C to +4°C 40%


Download the PDF document – Vitrages et casse thermique (French only)