Water vapour becomes a problem when it condenses into liquid water. This happens at the point of 100 percent relative humidity (known as the dew point), which is when the air cannot hold any more water vapour. Warm air can contain far more water vapour than cold air.

Condensation on windows is a typical example. When air contacts a cold window, the air loses heat. The air can then no longer hold all the water vapour and some condenses out onto the surface of the window. If the window is extremely cold, the condensation will appear as frost. Condensation is more likely to occur in humid areas such as the kitchen, bathroom and some basements and crawl spaces.