Eliminating winter window condensation

How you can stop this common household problem

By Steve Maxwell

window condensation

Photo by Steve Maxwell


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 Canada 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 have a problem. And this problem goes beyond ruined window-frame finishes and mould growth on windowsills. It includes the very real potential for decay within wall cavities and attics, too. Window condensation can also be a sign of low indoor-air quality which affects your health.

Where the water comes from

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 moulds, 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.

While today’s 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.

Open windows a little

This approach is about as easy as they come. Yes, opening windows will cost you a bit more in heating, but it still may be the cheapest way to solve your moisture problem.

Use exhaust fans and proper venting

Bathroom exhaust fans, in particular, should be used during every shower or bath and for at least 15 minutes afterwards.

Installing an exhaust fan in high-moisture areas of your home can help if you continue having minor condensation problems even with your windows opened.

Dryers that vent indoors spew massive amounts of moisture into your home. Proper outdoor venting of your dryer could solve the whole problem.

Install a heat recovery ventilator (HRV)

Although this option will cost $2,000 to $2,500 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, HRVs are so effective and energy efficient that they’re now required by code for new houses in some jurisdictions.

HRVs incorporate fan ventilation with a built-in heat exchanger that typically extracts 75 to 85 per cent 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.

Opt for better-insulated windows

The higher the R-value of a window, the better it can handle humidity and keep condensation from forming. Triple pane windows, for instance, are much less likely to form condensation than double-pane, all else being equal.

Replacing your windows with ones that have better sealing, but the same insulation value as the original ones, can actually increase window condensation because the new windows reduce air leakage and natural ventilation.


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Jan. 21, 2014

7:28 pm

My second floor windows get condensation on them. Not the basement or the main floor. There is a fan in the bathroom and I do not have any heat on in the second bedroom. Only in the master bedroom is the condensation occurring. The windows have two panes of glass, I don't remember what they are called. Not only does moisture collect on the inside panes, but of course, it also freezes. Making it impossible to open the windows in an emergency. Should I put a dehumidifier in the bedroom? I really can't afford much reconstruction or new windows. Thanks.


Jan. 20, 2014

9:25 am

A few years ago we upgraded all the window in our home. Went from old single pane aluminum frame windows to the newer thermo-pane windows in vinyl frames. Still had a bit of a condensation problem, so we purchased a portable dehumidifier and have had very little or no condensation problems since.

As a previous writer pointed out, the dehumidifier should have been added to this article as an option. Just watch the major hardware outlets for sales - you should be able to purchase a larger sized dehumidifier for under $250.00 (including taxes).

It was a good article otherwise.


Dec. 15, 2012

2:07 pm

I have installed a humidex. This unit is installed in the basement and goes from floor to up inside the floor joists. The unit mounts to an outside wall and is vented directly out side. it runs 24 / 7 and uses very little hydro. it sucks the moist air from the floor and blows it out side. Best investment i have ever made. No more problems with windows sweating

Willy in Whitby

Dec. 15, 2012

11:52 am

Blinds that have some thermal value can cause a lot of condensation on windows even when the humidity in the house is at a reasonable level. The air between the blind and the window gets much cooler than the rest of the room, especially in a bedroom which might already be cooler than the rest of the house. If I keep the humidity low enough to prevent the condensation on these windows, it is much too low in the rest of the house.

My solution, poor as it is, is to make sure the inside of the window along the sill is properly caulked and painted to keep the condensation from getting behind the sill. Also, mop up the excess in the morning when you open the blinds.


Jan. 17, 2012

4:52 pm

After significantly upgrading the insulation and seal on our 150 year old home, we began to have significant condensation inside the windows, We cured it by installing a portable dehumidifier on the main level. Over a period it dried everything out nicely. While a central air exchanger comes in at about $2000 to $2500, this cost about $200. I'm surprised this was not mentioned as an option. At one tenth of the cost, it should have been, in my view.

Good article though.

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