Why Do My Windows Sweat in the Winter?

Cause of Condensation

• In the winter, the warmer interior air is generally filled with water vapor, or moisture, called humidity. When this warm water vapor contacts the cold, interior surface of the window, the window surface acts like a dehumidifier, removing the moisture from the air and transforming it into the liquid that you notice on the window pane. Also, "tighter" houses that have reduced air flow and fewer drafts trap more humidity inside, where it can cause window condensation. Houses with slight drafts allow humid interior air to be ventilated to the outside, with dryer, colder air coming inside to lower the relative humidity.

Causes of Air Moisture

• The humidity in the interior air comes from a variety of sources, including typical daily activities in the home, from cooking, bathing and laundering clothes to simply breathing. If you have a lot of indoor house plants, they also contribute to the moisture, as do pets. Using ventless gas fireplaces and furnaces, which don't use a vent pipe or chimney, increases interior air moisture. The water vapor from these and other sources simply floats invisibly through the air until it meets a colder surface, such as a wall or window.

Moisture Concerns

• This "sweat" condensate on the windows has to go somewhere, and it often runs down the glass to the windowsill and also collects along the window frames and walls. This moisture can be responsible for different problems, such as peeling paint and rotting wood, and also contributes to the growth of mold. Even in a well-insulated, newer home, the moisture can destroy the insulation's effectiveness.

Fixing the Problem

• If you do encounter window condensation, or sweating, there are ways to monitor the relative humidity inside and control it. Purchase an inexpensive hygrometer, a device used to measure humidity, to keep track of the moisture level changes in the room air. Making sure that areas of your house are well-ventilated and using a dehumidifier can reduce humidity. When you cook, bathe or take showers, use the corresponding kitchen or bathroom exhaust fans to properly vent the moist air from your home. Also, consider installing thermal-paned windows, which keep the window pane surfaces at a constant temperature all year long.