What Causes Condensation on Windows in Your Home?

A window expert reveals why you shouldn’t panic if you see moisture or frost on the inside of your windows.

November 10, 2023
When temperatures drop and beads of moisture or a thin layer of frost develops on your windows, it’s easy to jump to conclusions about the window’s quality or effectiveness. If windows are meant to keep the elements out of your home, why do fluctuations in temperature cause water to collect on your window panes?
Our resident expert Lyle Kvarnlov, Product and Services Manager for Marvin, offers insight into the surprising factors that can contribute to window condensation and steps you can take to improve or prevent it.

Airflow and Window Condensation

Never underestimate the power of airflow. If there’s one takeaway for reducing potential window condensation, it’s making sure that the air is moving in your home. Use fans to help promote circulation, and make sure that your bathrooms and kitchen are properly ventilated with exhaust fans that are vented to the outside. The same rules apply to laundry areas, as the exhaust from a dryer can emit large amounts of water vapor in a short period of time if vented into your home.

Identify unexpected culprits. There are many surprising items in your home that can contribute to excessive moisture in the air. Do you store unseasoned firewood inside? Do you have many indoor plants, especially on window sills? Do you have a large family? All of these factors can mean a moister environment where condensation could occur. You probably didn’t know that normal breathing and perspiration by a family of four adds a half pint of water to the air each hour—and cooking can add another four or five pints.

Be aware of bigger problems. Groundwater seeping through the foundation of your home can be a major cause of excess moisture. Ensure your home has proper gutters, flashing and downspouts and make sure that water is being channeled away from your foundation. Dirt floor crawl spaces can also be a source of moisture and should be vented or covered with plastic to create a vapor barrier. A building professional can help you diagnose and address these potential causes.

Consider the age of your home. Excessive moisture is certainly not a symptom of older homes alone. Building materials like new wood, plaster and cement from a remodel or new construction contain a great deal of moisture. When the heat is turned on for the first time, this moisture will flow out into the air and settle on your windows. Expect this to disappear after the first cool season you spend in your home.

Limit heavy window treatments and raise your shades when it’s coldest outside. It may seem counterintuitive, but heavy window drapes and blinds can trap warm air against the cold glass, leading to more condensation than you would see if the window was completely exposed to the interior of the room. Keep window coverings open during the day when it’s cold outside and avoid heavy drapery when window condensation is an issue.

Know your humidity levels. Testing the relative humidity in your home is a simple way to understand why you might be seeing moisture on your windows. The Better Business Bureau advises maintaining a 25 to 30-percent relative indoor humidity. You can easily test your home’s levels with a hygrometer available at most hardware or home improvement stores and many of today’s smart home thermostats will also monitor your home’s humidity levels.

Your old windows weren’t “better,” they were less energy efficient. Sometimes, after replacing older or original windows with new ones, you might see more condensation than you remember seeing before. Does this mean your original windows were simply better quality? Not necessarily. It actually means that the insulation and weather-stripping in your older windows allowed the house to breathe (more air in and out around the window seal) and more easily exchange drier air with more humid air. Today, windows and the houses they are placed in are more energy efficient, meaning that humid air can be more easily trapped inside of a home.

Understanding where moisture comes from and how to reduce it is the first step to diagnosing your condensation issues, but you don’t have to take our word for it. For additional tips on controlling humidity and to learn more about the causes of window condensation, read more from home expert Bob Vila or the American Society for Home Inspectors.

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