What is the rim joist?
The rim joist is the wooden band that sits on top of your foundation wall, where your floor joists end. It's a single piece of wood — often just 2 inches thick — separating your basement from the outside air. In most Ottawa homes built before the 2010s, it's 'insulated' with a fiberglass batt that sits in the joist cavity but does nothing to stop air movement.
Because the rim joist runs the entire perimeter of your foundation, it's a large, continuous surface — and if it's not air-sealed, it's a large, continuous air leak.
Why it's so drafty
Two forces make the rim joist drafty. First, stack effect: warm air rises and escapes through your attic, which pulls cold air in through the lowest leaks in your home — and the rim joist is one of the lowest. Second, fiberglass insulation doesn't stop air movement — cold outside air blows right through it and hits the rim joist, making the wood cold and the basement drafty.
The draft you feel in your basement is often air being pulled in through the rim joist, flowing across the basement, and up into your home. It's also why the floors above the basement feel cold — the cold air pools at the floor level.
What a drafty rim joist costs you
Comfort: Cold, drafty basements and cold floors above. Energy: The air leaking in through the rim joist has to be heated, which raises your bills. Moisture and mold: In summer, warm humid air enters through the rim joist, hits the cool wood, and condenses — growing mold on the rim joist and sill plate. Radon: The rim joist and sill plate are a known radon entry point; a drafty rim joist lets soil gas in.
The fix: closed-cell spray foam
The solution is closed-cell spray foam applied directly to the rim joist and sill plate. It does three things fiberglass can't: it insulates (high R-value per inch), it air-seals (expands to fill every gap, stopping the drafts), and it resists moisture (no condensation, no mold). It's a relatively small, high-impact project — most Ottawa basements are done in a day — and the comfort improvement is immediate.
We remove any moldy fiberglass first, dry and prep the rim joist, then spray closed-cell foam across the entire perimeter. The result: no more drafts, warmer floors, lower bills, and a rim joist that stays dry and mold-free.