What Causes Condensation on Windows?

In the winter, freezing temperatures outside and warm, humid air inside a home create a perfect environment for condensation to form on windows. This most commonly happens in kitchens and bathrooms, but can happen virtually anywhere in the house. During cold weather, condensation collects on the inside of windows, while during the summer it collects on the outside. Its cause is hotter and more humid air on just one side of the glass. This can also be seen more when people wear masks with their glasses on!

Photo by Matteo Catanese / Unsplash

Greg Caplan, window and door specialist at Boston Building Resources, explained that windows with a single layer of glass are more likely to have condensation. Windows with double- or triple-pane glass usually do not. Although the type of window plays a part in creating the conditions for condensation, it is rarely the fault of the window and more due to environmental conditions within the house.

Activities that increase humidity levels indoors include showering, cooking, having indoor plants, and leaving containers of standing water. Higher humidity levels can lead to condensation on walls, windows, and other surfaces around the house. The consequences of not taking care of the problem can be costly. This moisture can drip, causing water damage to window frames and insulation, causing paint to crack or peel, warping surfaces, staining walls and ceilings, and causing structural damage to the house. Mold and mildew can also start to grow on wood, carpet, and insulation. When moisture problems are not addressed promptly, damage to the home can become serious and expensive to fix.

Indoor humidity should be between 30% and 50%. To bring down humidity, start by opening blinds and drapes. Turn on ceiling fans to circulate the air, preventing hot air from staying at the top of the room. In winter, reverse the fan direction to blow air up and use the slowest speed. Use exhaust fans when cooking and showering; this will send excess moisture outside the house. Some exhaust fans come with a humidity sensor that will activate the fan automatically at a certain moisture level. Check the dryer vent from time to time to make sure it is clear of obstructions and working properly. Other strategies include reducing the number of houseplants, keeping firewood outside, and not using a rack or line to dry laundry when it is cold outside. You can purchase a humidity gauge to measure humidity level and, if necessary, a dehumidifier to remove moisture from the air.

All of these methods can help get rid of condensation if it is on the inside surface of the window, but if the moisture is between layers of glass in a double- or triple-pane window, that indicates glass seal failure. The only way to fix it is to replace the glass.

If moisture condenses between a regular window and a storm window that has been added to the outside, this is normal. Older wood windows, which are usually supplemented with storms, will never be 100% airtight. Some moist air will inevitably move around the older window sashes and condense on the inside of the storm. Proper storm window installation includes small weep holes at the bottom, allowing any moisture to escape. Otherwise, the window sill can begin to rot.

Taking care of condensation is important to prevent the expensive problems that can result from excess moisture. Your future self will be thankful that you took care of things today!

—Audrey Pappadia