Condensation and frost on the inside of windows occur when warm, humid air inside comes in contact with a pane of glass chilled by the outside air, according to the American Society of Home Inspectors. This reduces the temperature of the air in contact with the glass, forcing condensation of moisture. If the outside temperature is cold enough, this moisture can freeze on the glass and create interior frost.
A number of factors can contribute to window frost. Single-pane glass or poorly insulated windows can allow interior heat to escape, chilling the interior surface of the glass panes. High humidity levels in the home can allow extra moisture to condense on these cold surfaces, contributing to window frost.
Removing excess moisture with exhaust fans or a dehumidifier can help reduce interior frost, as can keeping blinds or curtains open and allowing air to circulate across the surface of the glass. This prevents the air at the window from becoming too cold, thus reducing the chance of condensation and frost. Increasing the heat inside the home can also help dry out the air as well as keeping warm air circulating against the windows, which prevents the glass from becoming cold enough to form frost.