Window condensation forms as a result of excess humidity in your home. The glass provides a cold surface on which humidity can visibly condense – similar to the condensation that forms on the glass of a cold drink. Condensation usually occurs on windows first because glass surfaces have the lowest temperature of any of the interior surfaces in the house.
When condensation occurs on your windows and doors, it is a sign that you should reduce the indoor humidity level because it causes problems in your home – like mildew, mold, damaged paint surfaces, rotting wood or moisture spots.