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

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?

The truth may be surprising. Condensation is actually a sign that your windows are doing their job and holding heat inside of your home. High-performance windows make the most of the sun’s heating rays during cold months and maintain a remarkable temperature difference between the indoor and outdoor panes of glass. When this temperature difference increases and the humidity in your home reaches certain levels, windows and doors are the first place that condensation gathers. Just like the drops of water that gather on your cool glass of water on a hot summer day, condensation on windows isn’t indicative of a leak—it’s simply moisture from saturated warm air collecting on a cooler glass surface.

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.

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.

Posted by:Marvin

Marvin is a fourth-generation family-owned and -operated business, headquartered in Warroad, Minnesota, with more than 5,500 employees across 15 cities in North America. The Marvin portfolio of products for builders, architects and homeowners is designed to provide exceptional solutions for any project with a focus on creating better ways of living. Marvin products are distributed nationally through a network of independent dealers and are also exported internationally. Visit Marvin.com to learn more.

16 replies on “The Truth About Window Condensation

  1. Near the end of your comments under the heading of Know Your Humidity Levels, you state: “The Better Business Bureau advises maintaining a 25 to 30-degree relative indoor humidity.” This should be 25 to 30 “percent” instead of “degrees”. Humidity is measured in percent not in degrees.

  2. Related to indoor humidity:
    in cold weather climates when it is too high in new homes – especially during very cold weather – it is often because the home’s HRV or ERV system that was designed to bring in fresh air and remove humidity is not set correctly. This is unfortunately not uncommon during the first year a house is built when installers who are putting in those systems sometimes put them in “backwards”. It is easy to fix, but not always obvious from looking at it.

  3. Not happy with inspector results. We have air flow between the windows molding. The inspector used is heat sensor on walls left and right of the windows saying poor insulation.Our old windows had no air flow between the windows molding.They took out all the insulation between the windows ,they must of mist some spots with the spray insulation.

  4. We have an excessive amount of condensation on our windows that has apparently worn down the stain and now there seems to be a buildup of mildew. Do you have any recommendations on how to deal with that?

    1. Hi Lindsay, If you want to be sure you have mildew and not just dirt accumulation (especially on the exterior), you can confirm by placing a drop of household bleach on the suspected mildew area. If small gas bubbles develop in the droplet of bleach and the area bleaches out, mildew does exist and should be removed. You can try this basic solution for controlling exterior mildew problems, which is safe for use on all wood components on the interior or exterior of your windows.
      • 1/3 cup (79 ml) powder laundry detergent
      • 2/3 cup (158 ml) trisodium phosphate (TSP)
      • 1 quart (946 ml) household bleach
      • 3 quarts (2839 ml) water
      Apply solution with a soft bristle brush using medium pressure. Rinse well with clear water after cleaning. Avoid using any chemicals on parts that contain vinyl, like a weatherstrip. If you have mildew on a replaceable component such as a weather-strip, or even in some cases, interior trim – it may be easiest to replace that component if the mildew proves difficult to remove.

  5. Hi, my problem isn’t with condensation on the inside of the windows. My brand new Marvin windows collect the moisture on the OUTSIDE of the windows and are in a square pattern not taking up the entire pane. Seems no matter what the weather is, they do this…eventually they dry out if it’s sunny outside, but it’s a few hours in the morning before I can look out at the view. Thank you, Janet Drew

  6. Hi Janet, We’d love to connect you with someone on our service team to chat with you about the issue. You can contact us at 1-888-262-4192. Thank you!

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