Learn why condensation forms on old and new windows, what it means for your home, and how to control moisture. Practical tips for preventing window fog, mold, and water damage year-round.
Interior condensation on old windows happens because the glass surface is cold enough to reach the dew point of the indoor air. Single-pane and older double-pane windows have poor insulation (high U-factor), so the interior glass temperature drops close to the outdoor temperature. When warm, moist indoor air contacts that cold glass, water vapor condenses into droplets. Upgrading to modern triple-pane or Low-E double-pane windows keeps the interior glass warmer, virtually eliminating interior condensation.
Exterior condensation on new windows is actually a sign that your windows are working correctly. High-efficiency windows with Low-E coatings and gas fills insulate so well that very little indoor heat reaches the outer pane. On cool, humid mornings, the exterior glass temperature drops below the outdoor dew point and moisture forms on the outside -- just like dew on a car windshield. This condensation evaporates as the sun warms the glass and is completely normal.
Condensation between the panes of a double- or triple-pane window means the hermetic seal has failed, allowing moist air to enter the sealed unit. This is known as a 'blown seal' or insulated glass unit (IGU) failure. Once the seal breaks, the argon or krypton gas escapes and the window loses much of its insulating value. The affected glass unit needs to be replaced -- the window frame can often be reused, but the sealed glass assembly cannot be repaired.
To reduce interior condensation: use exhaust fans in kitchens and bathrooms, run a dehumidifier to keep indoor humidity between 30-50%, ensure your home has adequate ventilation, and keep curtains or blinds slightly open to allow air circulation near the glass. For persistent condensation on old windows, upgrading to energy-efficient windows with a U-factor of 0.25 or lower is the most effective long-term solution.
Chronic interior condensation can cause significant damage over time. Water running down the glass can rot wood window frames, sills, and surrounding wall framing. Persistent moisture promotes mold and mildew growth, which can affect indoor air quality and trigger respiratory issues. Paint peeling, wallpaper bubbling, and plaster damage are also common. Addressing condensation promptly -- through humidity control and window upgrades -- prevents costly structural repairs.