We close our cottage over the winter, and when we open up again in June, there is mould along the ceiling boards. (We used to get mould elsewhere until we started pulling the furniture away from the walls.) Our cottage is raised up on jacks and has an insulated floor. How can I stop the mould?—Susan Rode Wiet, via email
Based on your description and your previous experiences with mould on the walls, Don Fugler, an independent building scientist in Ottawa, believes this may be a case of diurnal moisture cycling. “This largely theoretical process involves the house heating up with solar gain in the spring, accompanied by high humidity,” says Fugler. “As the house cools down overnight, the moisture condenses on the coldest interior surfaces.” And that moisture leads to mould.
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It was a good call to move the furniture; that at least allows air to circulate into the other mould-prone areas of the cottage. But that won’t help the ceiling, and it won’t reduce moisture inside the cottage.
Assuming diurnal cycling really is to blame, “there are two potential solutions,” says Fugler. Solution one: add ventilation. This is much more effective if you do it mechanically, by getting fresh air into the cottage through fans ducted to the outside, for example. But that would mean leaving your power on all winter. You can instead increase ventilation passively, of course. If you have a pest screen over the chimney, you could leave the damper open; you could cover the screens of your windows with hardware cloth and leave them open a crack. You could also install fresh-air intake ducts in the cottage. Solution two—and this doesn’t involve adding holes to your building: “Keep the windows shaded through the winter and the spring so that solar gain is negligible,” says Fugler. Shading the outside using shutters will have a larger impact than interior blinds or shades.
This article was originally published in the Winter 2023 issue of Cottage Life magazine.
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