Design & DIY

Cottage Q&A: What’s causing my basement dampness?

The damp interior walls of a basement By cunaplus/Shutterstock

We have a cottage built on a slope. The unfinished basement floor is always damp, which results in high humidity levels inside the cottage, as well as a musty smell. We’re looking for a way to reduce the humidity and get rid of the smell. Would covering the basement floor in plastic sheeting and topping it with fine gravel be a solution?—Mieke Foster, via email

Yes, it would be a solution. But it might not be the best solution if you don’t figure out what’s making the basement damp in the first place and fix that problem before covering the floor.

Moisture can get into basements in all kinds of sneaky ways, but an obvious one, given that the cottage is on a slope, is that surface water is draining into the basement through “the inevitable cracks in your basement walls,” says Kim Pressnail, an associate professor emeritus in civil and mineral engineering at the University of Toronto. If that’s the case, yay for you! “Surface rainwater is often the easiest to control,” he says. Use topsoil and gravity; regrade around the perimeter of the cottage so that the ground slopes away from the walls instead of towards them. Create a “one in 12 slope—the soil drops one centimetre for every 12 cm from your cottage,” says Pressnail. “That will more than do.” And make sure that your eavestrough downspouts are directing water well away from the building. 

Is the cottage below the water table? Then both rainwater and groundwater could be getting in through those basement wall cracks. “Water also may be entering through the walls because they’re in contact with moist soil,” says Pressnail. “It’s known as ‘capillary wicking.’ The soil moisture can ‘wick’ through your masonry or cast concrete below-grade walls and evaporate into the interior basement air.” Or, water may be evaporating up from the unfinished floor. “It’s like a soggy cracker,” says Pressnail. 

So much water! These issues are more complicated to diagnose and treat, and you’ll probably need expert help (from, for example, a contractor). But once you get rid of the moisture source, “the musty smell should gradually diminish,” says Pressnail. “Operating a dehumidifier in the basement during the warmer months may help.”

The article was originally published in the June/July 2022 issue of Cottage Life magazine.

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