Every fall for the last decade I’ve received emails from homeowners frantic about condensation forming on their windows as the weather gets colder. But this year something’s different. There are more requests for help than usual. Many more.

The most perennially asked home repair question I receive is “How do I fix my window condensation?”

I don’t know why, but I do know that the problem of sweaty windows is widespread across Canada and it’s not getting better. In fact, the issue of window condensation and the resulting mold growth is the single most perennial question I receive, despite years of writing about successful solutions. And besides being common, window condensation also indicates more of a problem than meets the eye.

Think of window condensation as the proverbial canary in a coal mine.

Window condensation is a canary in a coal mine.

Whenever a window develops beads of water on the inside during cold weather, it means that the surface of the glass has cooled enough to cause the moisture carried in the indoor air to condense. The air itself is the source of the mysterious moisture, and dealing with the air is where the best remedy will be found.

A little bit of non-running condensation around the edges of a window is normal during winter and harmless, but when it advances enough to require a rag to mop up the water, you need to find a solution. Just don’t be fooled by appearances. Your windows probably aren’t the cause of the problem.

New homes and recently renovated ones include features that keep warm air in and drafts out. That’s good. What’s not so good is the other stuff that tight construction holds in: airborne contaminants and moisture. If your windows sweat a lot, it indicates inadequate ventilation. There’s too much water in the air, and probably too many off-gassed contaminants, too. My favourite way of solving the water problem automatically leads to much better indoor air quality.

The best window condensation solution is a Heat Recovery Ventilator (HRV)

Eliminate window condensation by installing a Heat Recovery Ventilator (HRV).

A Heat Recovery Ventilator (HRV) is a ventilation appliance that draws fresh outdoor air into your home, expels moist, stale indoor air outside, while also retaining about 80% of the heat energy invested in the old air. Outdoor wintertime air becomes bone dry when it’s brought inside and heated up, and this is why HRVs are so effective at reducing sweaty windows. They lower indoor humidity levels like nothing else can. Leading HRVs also feature a replaceable HEPA-rated filter that helps lower indoor levels of pollen and some pollutants.

Although HRVs are the technology of choice for eliminating window condensation, they’re not cheap. The unit itself typically costs $1000 to $1500, with installation by a ventilation technician costing an additional $1000 or more. All this is why some folks try to solve their wet window problem using a dehumidifier. And while this seems logical, there’s a problem. Two problems, in fact. Dehumidifiers can’t lower relative humidity levels enough to prevent window condensation during winter, and even if they could you’d still only have dry, stale air.

It’s understandable that the blame for wet windows would be cast on the windows themselves, but this is rarely correct. In fact, one of the classic queries I get comes from the homeowner who had new windows installed, only to find that condensation is the same or worse. Condensation that gets worse after installing modern, multi-pane windows is actually a good sign. It usually means that the new windows are sealing better than the old ones. Drafts have been reduced, and indoor humidity levels have risen as more moisture is being retained.

Will an HRV work in a home that has no ducts to distribute the fresh air? I wondered the same thing 15 years ago. After failing to find anyone who would give me a definitive answer, I went ahead and installed my own HRV in a second-story storage area, with one stale air intake pipe going through the floor to draw air from the level below, and one fresh air outlet leading into the second story room just outside the storage area. The result has been excellent performance, despite no ducts. Having the inlet and outlet on separate floors forces house-wide circulation. Even in single-story homes, by strategically locating your HRV in the basement (perhaps with a small amount of ducting) you can expect pretty good results.

Before you go ahead and commit to an HRV, check on a few things first. Is the humidifier on your furnace turned off? No point in adding more moisture the air if you’ve already got too much. Shutting off the humidifier may solve your problem. Also, if your current windows have only one pane of glass, they may still sweat even with an HRV on your side. With such low insulation properties, the glass surface will remain a potent source of condensation because it gets so cold.

You must also realize that unless you have windows with very good insulation properties, you’ll probably have to make your home uncomfortably dry in order to completely eliminate condensation. During very cold weather you’ll need to discover the balancing point between comfort and a tolerable and harmless amount of window condensation.

One thing that’s easy to get used to is breathing fresher indoor air delivered by an HRV. Experience it for yourself and you’ll be glad your wet windows told you there was more of a problem than just damp glass.

Thinking of installing a Heat Recovery Ventilator in your home? Connect with Verified by eRenovate™ HVAC Contractors by using the eRenovate Post My Project tool or the eRenovate App.


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 Steve Maxwell’s Site.


Write A Comment