Rim Joist Insulation.
Sentence 9.36.2.6.(2) says the effective thermal resistance of rim joists shall be not less than that required for above-ground walls. In Zone 7A that is effective RSI 3.08 (R-17.5), on the one part of the envelope that is hardest to seal with anything else.
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The rim joist band is where building science and plain carpentry disagree. It is a cold surface with warm, moist indoor air on one side, framed as a series of small pockets with a joist end in each — condensation-prone and effectively impossible to air-seal with a batt.
Closed-cell foam at 50 mm is its own vapour barrier under the code's own permeance limit, which is exactly what a rim joist wants — the thickness answer and the measured numbers behind it are set out separately.
Why This Band Is Different.
Building Science Corporation describes rim joists as one of the canonical applications where spray foam earns its price. The reason is geometry, not marketing: every joist end interrupts the insulation plane, and a fibrous material cannot stop air movement around it.
- Effective RSI 3.08 (R-17.5) with no HRV, RSI 2.97 (R-16.9) with one — the same figures as an above-ground wall, imported by Sentence 9.36.2.6.(2).
- Closed-cell foam applied to the rim board, the sill and the joist ends as a single continuous plane, so the air barrier is not interrupted at every 406 mm.
- Air barrier continuity where the code actually demands it: Sentence 9.36.2.9.(2) requires the system to be continuous across junctions between different materials and assemblies, which is precisely what the sill plate to rim board to subfloor sequence is.
- Vapour control from the same material at 50 mm, since CAN/ULC-S705.1 sets its 60 ng/(Pa·s·m²) permeance pass/fail on a 50 mm specimen — and Sentence 9.25.4.3.(3) is satisfied because in this assembly the foam is on the warm side by construction.
- Fire block and thermal barrier questions answered in place, since a rim joist in a finished basement is no longer in a concealed space once the ceiling is open.
- Foam kept clear of heat sources — manufacturers require 75 mm of separation from any device with a surface temperature above 80 °C, and rim cavities are full of dryer vents, flues and mechanical penetrations.
- Attention to the joist ends themselves, since wood framing measured at 30 to 36 ng/(Pa·s·m²) dry cup is already inside the code's vapour barrier limit and is not the leak — the gap beside it is.
- Applied in lifts of no more than 50 mm with cooling between passes, per CAN/ULC-S705.2, which in a shallow rim cavity usually means two thin passes rather than one thick one.
Doing the Band Properly.
- 01
Open every bay
Existing batts come out. A batt stuffed into a rim pocket is worse than nothing, because it holds indoor air against a cold surface long enough for it to give up its moisture there.
- 02
Check for existing damage
Rim boards that have been condensing behind batts for a decade sometimes show it. That is a repair conversation before it is an insulation conversation, and it is far cheaper to find now than under new foam.
- 03
Spray the plane, not the pocket
Sill, rim, joist ends and the underside of the subfloor at the perimeter, in continuous passes, so the air barrier crosses every junction rather than stopping at each one.
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Detail the penetrations
Dryer vents, gas lines, hose bibs and electrical are all in this band. Each is a hole through the air barrier and each is sealed individually, with clearance held where a surface runs hot.
Frequently Asked Questions About Rim Joist Insulation
What does the Alberta code require at a rim joist?
The same as an above-ground wall. Sentence 9.36.2.6.(2) of NBC(AE) 2023 states that the effective thermal resistance of rim joists shall be not less than that required for above-ground walls in Table 9.36.2.6.-A or -B, as applicable. For Climate Zone 7A that is effective RSI 3.08, about R-17.5, without a heat recovery ventilator, or effective RSI 2.97, about R-16.9, with one. There is no separate rim joist table.Why not just use batts in the rim joist?
Because the problem there is air, not conduction. A fibrous batt is vapour-open and air-permeable, so warm indoor air reaches the cold rim board behind it and condenses on the way. The rim band is also interrupted by a joist end every 406 mm or so, which means a batt solution has a joint at every bay. Closed-cell foam solves both at once: it is an air barrier material at roughly 25 mm and a vapour barrier at 50 mm, applied as one continuous plane across the joist ends.How thick should rim joist foam be?
Enough to reach effective RSI 3.08 for the assembly, and at least 50 mm if the foam is also to serve as the vapour barrier, because that is the thickness at which CAN/ULC-S705.1 sets the 60 ng/(Pa·s·m²) permeance pass/fail. Published 50 mm values for Canadian closed-cell products run from 13 to 41 ng/(Pa·s·m²), comfortably inside the limit. Some products also pass at 25 mm, but that is a result off a specific CCMC report rather than something the standard guarantees.Is spraying the rim joist worth doing on its own?
It is one of the few insulation jobs where a small scope has a disproportionate effect, because the rim band sits at the bottom of the stack effect and runs the full perimeter of the building. It is not a substitute for the rest of the envelope, and it will not by itself change a heating bill in any way we would put a number on. What it reliably does is remove a condensation-prone surface and a continuous perimeter air leak.Can this be done in winter?
The rim band is usually one of the easier winter jobs, because it is inside the building and the substrate can be brought into range. The rules still apply: substrate at least 3 to 5 °C above the dew point, frost-free, at or below 19 percent moisture content, ambient relative humidity under 80 percent, and inside the product's published temperature range. In an unheated basement in January that means heating the space first — an ordinary engineering step, not an optional extra.