Why your windows ‘sweat’ in the winter

Window condensation

Have you noticed moisture on the glass of your windows on the inside when the temperature gets cold in the winter? This is typically caused from excessive humidity inside the home.

As the outside temperature drops, the window glass temperature also drops. When the moist air inside the home comes in contact with the cold glass pane, the moisture condenses and forms water or condensation on the window glass.

Excessive humidity in the air forms as condensation on the window glass. It runs down the window and then rests on the bottom window sash or jamb.

Damage from window condensationThis can lead to moisture damage to the wood window sill, wood jamb, and/or the sheetrock below the window. This moisture can also penetrate the walls, deteriorating the wood framing and reducing the effectiveness of insulation. Mold or mildew can also form near the window glass due to window condensation.

Here are two steps you can take to reduce indoor humidity in the winter.

  • The first is to control the levels of humidity in the home. Using kitchen and bathroom exhaust fans can assist in reducing humidity levels.
  • You can also verify that the humidistat is set appropriately. See the diagram below for the recommended interior relative humidity for the different outside air temperatures.

Humidity chart

When maintaining proper indoor humidity levels in the home, you will lengthen the life of your windows and help avoid any possible moisture concerns due to condensation.