Winter Window Condensation – Sweaty Windows

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What Causes Window Condensation

window condensation 225x300 Winter Window Condensation   Sweaty WindowsEach winter I’m asked repeatedly why new energy efficient windows have condensation on them. I’ve written several articles about window condensation, Winter Window Condensation Problems, Window Condensation – Part II, and More On Winter Window Condensation, but I thought it was worth reviewing the issues because it seems to be so common today in newer homes.

Window condensation is due in large part because of how your home is built and not really a result of faulty windows. Each year homes are being built better with improved insulation, better vapor barriers and overall improved air infiltration. However this has resulted in very “tight” homes. “Tight” homes are great for helping us reduce how much energy it takes to heat and cool them however it makes it much harder to remove water laden air and contaminated air.

The number one reason you see condensation on your windows is because you have a high amount of moisture (humidity) in the air. Humidity (water vapor in the air) is a measure of how much moisture is in the air. We produce water vapor in all kinds of ways. We all create water vapor when we breathe and sweat. Plants and pets all produce water vapor in our homes. On top of that we create a tremendous amount of water vapor from showering, bathing, cooking, washing the laundry and cleaning the dishes.

Condensation is the process in which water vapor (gaseous state) comes in contact with a cool surface (such as a window) and converts back the the liquid state of water. In order for condensation to form on a window the temperature of the glass must be at or below the dew point of the air touching the glass. So if the dew point in your home is 60 F and the glass temperature hits 50 F then you’ll see condensation on the windows. Even with great new energy efficient windows the glass temperature can be slightly lower than the dew point causing condensation on windows.

Older homes don’t seem to have as big of a problem with window condensation. The reason this is true is because older homes are much draftier. Air leakage helps keep the home “breathing” and therefore allows excess moisture to leave the home and it also reduces the dew point. However, older homes typically have windows that are far less energy efficient and therefore have colder glass that is more prone to condensation problems due to colder glass temperatures.

How To Reduce Window Condensation

If you’re having condensation problems in your home then I suggest you start by testing the relative humidity in your home. Measuring the relative humidity in your home is actually fairly easy. All you need to do is buy a hygrometer, Timex Thermo-Hygrometer, which measures humidity. You can also use a dehumidifier in conjunction with good ventilation to reduce the humidity in your home.

Ideally you’ll want to make sure your home is around 40% relative humidity. If you’re home is higher than that then you really need to evaluate your exhaust systems. The Home Ventilating Institute (HVI) recommends a minimum of 8 air changes per hour for bathrooms and 15 air changes per hour for kitchens. So calculate the cubic feet of the room and compare it to the CFM rating on your exhaust fan. This will tell you how long to run the exhaust fan in order to achieve the recommended air exchange. I recommend you install an electronic timer switch on your exhaust fans. This way you can set the appropriate time and achieve the necessary air exchanges. If you’re looking for a new bathroom exhaust fan then I highly recommend the Panasonic WhisperFit Low Profile Ventilation Fans – 17 Watts [ Model FV05VF2 ], I’ve used this in both of my homes and it’s the quietest on the market.

Once you properly exhaust the moisture from your home the condensation problems should stop. Understanding what causes condensation will save you time and money. So stop blaming window manufactures and solve the problem by controlling moisture.


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  • Anonymous says:

    After reading this post, as well as all the links to previous posts on winter window condensation, I'm wondering why the recommendations are always to exhaust warm humid air — rather than just to remove the excess moisture with a dehumidifier? Wouldn't it be better in these energy-saving times just to remove the extra moisture from the warm air, rather than exhausting both the moisture and the HEAT? Maybe I'm being too 'tight', but I hate the idea of sending warm air out through an exhaust fan, especially after I've spent so much time & money sealing all the leaks in my house (built 1996). What's wrong with just using a dehumidifier after showering, rather than an exhaust fan — is it a huge electricity hog?

  • Todd says:

    I agree and disagree with you.

    First off let me say that one problem with new homes is how tight they are and the lack of outside “fresh” air that enters the home. By using the exhaust fans we create a negative air pressure in the home and therefore allow fresh outside air into the home.

    I often work with energy star raters on houses we build and they typically say that 3500 to 4000 sf is the cut off between two approaches. For houses under that size it’s appropriate to use exhaust fans to remove the moisture. Above that size home you should be using some type of heat recovery ventilation in order to exchange sufficient air.

    If you can get sufficient air exchanges in combination with de-humidifiers then that would be ideal. So the long answer is it’s more than just moisture.

  • Awesome post and timely – it happens to me every year

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