Does condensation build up on the inside of your home’s windows during the heating season? If it does, you’re not alone. Winter window condensation is a growing problem in regions that get cold winters and its root has a surprising origin. As homes are sealed better against air leakage, natural ventilation to the outdoors is reduced. As a result, indoor air becomes much more likely to contain damaging levels of moisture during winter. If your windows sweat enough during the heating season to require periodic wiping with a towel, then you’ve got a problem. And this problem goes beyond ruined window frame finishes and obvious mold growth on window sills. It includes the very real potential for decay occurring within wall cavities and attics, too. Window condensation can also be a sign of low indoor air quality that affects your health.

Why the Window Condensation Develops

When warm, moist indoor air meets the cooler surfaces of windows during winter, condensation develops on the glass. It’s the same thing that happens on the outside of a drinking glass filled with a cold beverage on a hot summer day. Flaws in your home’s vapour barrier (and there are bound to be some in every home) can allow warm moist air to seep into internal wall cavities, condensing there as it did on your windows, and creating a perfect breeding ground for hidden molds, fungus and other nasties. Breathing, cooking, showering and drying clothes all release huge amounts of moisture into the air. In the good old days, this moisture would make its way outside through all the cracks that were once common around windows and doors. That’s why old, leaky houses are often so dry during winter, with no window condensation at all. And while today’s tighter homes mean lower energy bills, they also demand that we consciously provide some sort of fresh air to vent off all that water vapour. Boosting home ventilation is the key to solving the window condensation problem.

Solving the Window Moisture Overload

Tactic 1: Open your windows a little and use exhaust fans consistently. This approach is all about using what you have to best advantage. Yes, opening windows will cost you a bit more in heat, but it still may be the cheapest way to solve your moisture problem. Bathroom exhaust fans, in particular, should be used during every shower or bath and for at least 15 minutes afterwards. Dryers that vent indoors spew massive amounts of moisture into your home. Proper outdoor venting of your dryer could solve the whole problem for you if it’s not sufficient now.

Tactic 2: Install exhaust fans in high-moisture areas of your home where there aren’t any now. Give this a try if condensation isn’t too bad, but not completely solved by opening more windows.

heat-recovery-ventilatorTactic 3: Install a heat recovery ventilator (HRV). Although this option will cost $2000 to $2500 installed, it will fix the problem once and for all. It will also retain most of the heat that you’d normally lose through open windows and out of exhaust fans. In fact, HRV’s are so effective and energy efficient that they’re now required by code for new houses in some jurisdictions. HRV’s incorporate fan ventilation with a built-in heat exchanger that typically extracts 75% to 85% of the heat out of stale indoor air before exhausting it outdoors. This saved heat is then transferred to a fresh stream of air coming into your home from outside.

One more thing. The greater the insulation value of windows you have, the higher the indoor humidity you can keep your house at without condensation forming. Triple pane windows, for instance, are much less likely to form condensation than double-pane, all else being equal. Also understand that replacement windows that are merely better sealing than you’ve got, without offering a higher insulation value, can actually cause increased window condensation because they reduce air leakage and natural ventilation.


Steve Maxwell is Canada’s Handiest Man. An award-winning home improvement authority and woodworking expert, he’s truly a treasure of home wisdom and the ultimate home GURU. Be sure to visit his site at


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