We have an uninsulated, 90-year-old cottage on the Gatineau River, Que. In the fall, we drain the plumbing, install wooden shutters over all windows and doors, and turn off the power. Inevitably it gets slightly musty. What type of passive airflow can I create? The floors are 3/4″-thick original pine. There is a four-foot open crawl space underneath. The A-shaped ceiling is quite steep, but there’s an accessible loft with a window built into one end.—Geoff Ott, Nepean, Ont.
Happy news: “This cottage has been around for 90 years,” says Kim Pressnail, an associate professor emeritus with the University of Toronto’s department of civil and mineral engineering. “That’s a good sign.” And adding ventilation will help to reduce that musty smell, which is ultimately brought on by interior dampness (more on that later).
“Wind, plus a slight, natural ‘chimney effect’ can improve drying,” Pressnail explains. Bring on the chimney effect! Install a roof vent near the roof’s ridge on the prevailing downwind side and a wall vent near floor level on the prevailing windward side, he suggests. Block them with heavy wire mesh, “to keep the furry, four-legged animals out.” Heating the cottage slightly will encourage the up-and-out chimney effect: the warmer air will rise and exit through the vents, says Pressnail. You don’t need to keep the power on for this; leaving some of the windows unshuttered will cause the air to warm up enough naturally through solar radiation.
But wait. The musty smell is caused by mould and mildew, and these fungi need moisture to thrive, says Pressnail. So, for the best results, you’ll also want to eliminate any indoor dampness. “Evaporation from the soil beneath the cottage is likely one of the sources,” he says.
Cottage Q&A: What’s causing my basement dampness?
If water is getting under the building, you want to divert it—you can do this by regrading around the perimeter so that the ground slopes away from, not towards, the foundation. Assuming the crawl space is bare soil, “cover it with a sheet of 6 mil (approx. 0.15 mm) polyethylene plastic and protect it with a 50 mm layer of sand,” says Pressnail. You could also air-seal the floor by adding spray foam insulation around the joists underneath. All of these moves will help to keep moisture from building up inside the cottage. Smells like success!
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This article was originally published in the Sept/Oct 2024 issue of Cottage Life.
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