Understanding condensation

Condensation on the interior or exterior surface of the glass generally occurs when excessive moisture or humidity in the air comes into contact with a cooler glass surface and may not mean that the window needs replacement.

Interior condensation

When interior humidity is high and the outdoor temperature is low, condensation can occur on interior glass surfaces. To help protect your home and home furnishings, reducing the relative humidity indoors to lessen interior moisture helps alleviate interior glass condensation. Also consult with experts to help ensure that your home has an adequate ventilation system. To temporarily reduce interior condensation, like after a shower, turn on an exhaust fan or open a window to assist with moisture escape.

To better understand indoor condensation visit our Understanding Indoor Condensation web page.

Exterior condensation

One way exterior condensation on windows can happen is when the glass temperature is below that of the outside temperature, like on a hot humid day. When differences in interior and exterior temperatures and high exterior humidity levels occur, moisture condensation can build up on the exterior surface of the glass. It’s the same type of condition that causes a cold glass of your favorite drink to sweat on a hot summer day.

Another way exterior condensation can happen is when environmental conditions outside promote condensation on the glass. Highly insulating glass units allow the outside pane to be colder than the inside pane. If the outer pane is colder than the dew point of the air, condensation will occur. This could happen even when it isn’t that cold outside. This usually occurs when there is little or no wind or air movement and is more likely during the early hours of the morning. A similar example is the windshield of a car left outside overnight, which develops condensation in the early morning.

Tips for caring for and maintaining your residential windows

To help you care for and maintain your residential windows, see this FGIA guide for more tips.