What causes window condensation?

Condensation is a visible condition that can be characterized by fogging and ice formation on the inside of windows. In Ontario, we most often see condensation on windows in extreme temperatures during the winter months.

The rate of condensation is dependent on the temperature and humidity level in your home verses the temperature outside. Condensation can be short-term during a severe cold spell, or may be limited to a localized area in the home such as bathroom or kitchen where humidity is highest.

When hot met cold

Condensation occurs on windows when warm inside home air containing water vapor (humidity) comes in contact with a cold surface. As the warm air touches the cold surface it is cooled causing the airborne moisture to condense into a liquid. The larger the differentiation between inside and outside temperatures, the more excessive the results of condensation. With fluctuating inside/outside temperatures and humidity levels, condensation typically goes through a cycle of freezing and thawing, leaving puddles of water on the window frame, sill or on walls and floors.

Small amounts of condensation appearing on a window surface may not necessarily be a problem, depending on the amount of moisture that forms, how long it stays, and whether it accumulates in an area that can be damaged by water.