Design & DIY

Do I need to vent my crawl space?

Rafters and joists

My cabin has a five-foot crawl space that is insulated and heated. (The floor of the crawl space is covered in plastic.) I plan to use the cabin in the winter. Do I need to vent this space to the outside?
—Kenneth P., Long Lake, Sask.

For now, you don’t need to do anything except pat yourself on the back, because you have a properly designed crawl space. (Many cottages don’t.)

“Hallelujah—he’s done the right thing,” says Kim Pressnail, an associate professor of civil engineering at the University of Toronto. Thanks to the insulation, the plastic on the floor to keep moisture from coming up from the ground, and the heat on in winter, “it’s a dry space,” he adds. “There’s no need to ventilate. I can’t imagine he’s going to have problems.”

Don Fugler, a building science researcher in Ottawa, agrees. Assuming your crawl space hasn’t shown any signs of excessive moisture so far, he recommends you wait a year, then reassess.

If, on the other hand, you’ve already seen signs of dampness, staining, or mould (and you’ve reduced or ruled out any sources of moisture, such as leaks), your first defence would be a small exhaust fan that draws the air from the crawl space and sends it outside, says Fugler. Choose an exhaust fan suitable for a bathroom (20 to 30 cubic feet per minute). “It doesn’t need to be big, but it does need to run continuously.” This should be enough to get rid of excessive humidity in the winter. Another, even more efficient ventilation option, according to Pressnail, is an air-to-air heat exchanger (a.k.a. a heat-recovery ventilator). “This is basically an exhaust fan, but it captures some of the heat from the outgoing air to warm the fresh air that it brings in.” In the meantime, regardless of whether or not you’ve noticed any moisture, it may not hurt to keep the humidity down (lower than 70 per cent), by installing a dehumidifier and running it through late spring, summer, and fall.