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The HRV Solution For Window Condensation

Steve Maxwell
November 2007
The Toronto Star

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 mould 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.

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 to require a rag to mop up the water the water, you need to find a solution. There’s too much water in the air and probably too may 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, or 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. This is why HRV’s are so effective at reducing sweaty windows. They lower indoor humidity levels like nothing else can.

Leading HRV’s also feature a replaceable HEPA-rated filter that helps lower indoor levels of pollen and some pollutants.

Also HRV’s are the technology of choice for eliminating window condensation, they’re not cheap. The unit typically costs $1,000 to $1,500, with installation by a ventilation technician costing an additional $1,000 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 stale 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 the 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 moisture is being retained. 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.