Eliminate window condensation and enjoy better air, too

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. 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. It’s like the proverbial canary in a mineshaft.

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.

All of this is why your best solution to window condensation is a heat recovery ventilator (HRV). It’s 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.

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