Have you ever experienced your windows fogging up or getting wet on the inside during the winter? Roofing and windows specialists get lots of calls for this, because some homeowners believe that moisture building up on the inside of the window indicates that the window is not properly sealed or is somehow leaking. The truth is, this isn’t caused by moisture escaping the barrier between your windows and the outdoors. It’s actually caused by moisture that’s already inside.

During the winter, even though the air is drier in general, there is still some moisture in the air in our homes. Every time you cook or take a shower all winter, it releases moisture into the air. Normally, when that moist inside air hits a window that is making contact with winter air of an average winter temperature at about 40-30 degrees, there’s no problem. But if you experience a snap of extreme cold in the 20’s and 10’s, the moisture in the air from your home will condense on your windows. That’s what that water on your windows really is—not leaking snow or rain, but condensation.

The Real Problem with Condensation

Water condensing on your windows is not much of a problem. It’s mostly harmless if you keep it wiped up and off of your windowsills and wall finishings where it could cause warping or staining. The real problem is when you get condensation happening in your attic.

The same condensation that occurs on your windows can also happen in the attic space. Even with the best ventilation possible, when cold air comes in and forms a barrier layer with warm, moist air escaping, condensation forms. The danger in your attic is not warped paint or wallpaper, but mold.

It’s common for homeowners to assume that mold and condensation in the attic is a ventilation problem, but it’s not. Attic condensation in the winter is caused by warm, humid air escaping from your home into your attic. This could be because the sealings are not sealed properly, or there may not be enough insulation to keep the warm air down. Even if you don’t experience attic condensation due to extreme cold, you don’t want to be paying to heat the air in your attic, where your unused Christmas lights hang out.

How to Keep Humidity Down

Lowering the overall humidity in your home is one way to control condensation, and reduce your risk of mold or mildew growing. If you have a humestat, you can control your humidity directly. Some folks tend to keep it too high, because 68 degrees feels warmer at a high humidity. Plug-in humidifiers also add moisture directly to the air, and contribute to excess condensation.

These sources are obvious, but you might not realize that a room full of plants will release humidity as the plants aspirate. Any activity that uses or generates hot water, like cooking on the stove, running a dishwasher, or taking a shower, will also release moisture into the air. It’s worth it to double check the insulation and sealings between your kitchen and bathrooms and your attic to ensure the mildew-harboring moisture isn’t making its way up there unnoticed.

As long as you’re controlling moisture in your attic and other places that aren’t frequently disturbed, you don’t need to worry about a little condensation on the window on a very cold day. It actually means that your window is working the way it should.