Basement Insulation.
Foundation walls in Climate Zone 7A carry effective RSI 3.46 (R-19.7) under Table 9.36.2.8.-A — a different table, and a different number, from the walls above them.
- 01Diagnose
- 02Price
- 03Repair
Get a Free Estimate
Name and phone is all we need to call you back. Takes ~20 seconds.
24+ five-star reviews
A basement wall is the only part of a house with soil on one side and living space on the other, and concrete is not a vapour barrier. That is the whole reason the below-grade requirements are written separately from the above-ground ones.
Closed-cell foam becomes its own vapour barrier at a specific thickness, and on a foundation wall that matters more than anywhere else — the 50 mm answer, with the measured permeance values is worth reading before choosing a system.
What a Basement Wall Has to Do.
Applying closed-cell foam directly to the interior face of a below-grade concrete wall is mainstream, code-recognised practice in Canada. What varies is how the rest of the assembly is built around it.
- Foundation walls to effective RSI 3.46 (R-19.7) in Zone 7A from Table 9.36.2.8.-A; the widely published with-HRV figure is RSI 2.98, which we treat as secondary rather than primary.
- The above-grade portion of the foundation wall follows the below-grade table under Sentence 9.36.2.8.(3), not the wall table — the opposite of what most people assume.
- A basement slab is separate again: unheated floors below the frost line may be uninsulated, above the frost line the figure is effective RSI 1.96 (R-11.1), and a heated floor is RSI 2.84 (R-16.1).
- Vapour control from the foam itself at 50 mm, the thickness at which CAN/ULC-S705.1 sets the code's 60 ng/(Pa·s·m²) permeance pass/fail — with position still governed by Sentence 9.25.4.3.(3).
- Sentence 9.25.4.2.(2), which uniquely permits variable-permeance vapour barriers on thermally insulated foundation wall assemblies: 60 ng/(Pa·s·m²) or less dry cup and more than 300 ng/(Pa·s·m²) wet cup.
- The rim joist band at the top of the wall taken as part of the same continuous plane, since that is where the air actually moves.
- Thermal barrier planned for a finished basement: once studs and drywall go up, taped gypsum board from Subsections 9.29.4 to 9.29.9 is the ordinary answer, and it needs to be in the drawing rather than discovered at inspection.
- Closed-cell rather than open-cell against concrete — the closed-cell standard caps water absorption at 4 percent by volume, while open-cell product data explicitly rules out below-grade use and contact with bulk water.
Before Anything Gets Sprayed.
- 01
Solve water first
Seepage, efflorescence, a failed weeping tile or a grading problem gets fixed before insulation. Closed-cell foam bonds hard to concrete — published adhesion to concrete is around 207 kPa against a 2.6 kPa requirement — so treat it as a permanent decision.
- 02
Set the target and the thickness
Effective RSI 3.46 for the wall. If the foam is also the vapour barrier, 50 mm is the code-anchored minimum for that job, and the two requirements are checked separately rather than assumed to coincide.
- 03
Carry the plane through the rim
The foundation wall and the rim joist band above it are one assembly. Stopping the foam at the top of the concrete leaves the leakiest part of the perimeter untreated.
- 04
Plan the covering into the framing
A finished basement gets taped gypsum over the assembly, which satisfies Article 9.10.17.10 without argument. Where a coating is proposed instead, confirm it with the insurer first — that substitution has been refused in Canada before.
Frequently Asked Questions About Basement Insulation
How much insulation does a basement wall need in Alberta?
Effective RSI 3.46, about R-19.7, for a foundation wall in Climate Zone 7A, from Table 9.36.2.8.-A of NBC(AE) 2023. Municipal 9.36 compliance checklists give RSI 2.98 as the with-HRV figure from the -B table; we treat that as secondary because the -B column was not transcribed from primary code text. The part of the foundation wall that sits above grade follows the same below-grade table under Sentence 9.36.2.8.(3).Do I need a poly vapour barrier over basement foam?
Generally not, and adding one can be actively unhelpful. At 50 mm, closed-cell foam conforming to CAN/ULC-S705.1 meets the code's 60 ng/(Pa·s·m²) dry-cup limit by definition of the material standard, and Sentence 9.25.4.3.(3) expressly contemplates the same product serving as both insulation and vapour barrier where it sits close enough to the warm side to prevent condensation at design conditions. Putting a second low-permeance layer inboard of it removes the assembly's remaining inward drying path. Alberta also permits variable-permeance vapour barriers specifically on insulated foundation wall assemblies, under Sentence 9.25.4.2.(2).Can foam be sprayed directly onto the concrete?
That is the normal method, and it is what makes the assembly work: the foam is bonded to the concrete with no gap for air to circulate in and no cold cavity behind the insulation for indoor moisture to reach. Published adhesion figures for closed-cell foam on concrete are around 207 kPa against a required minimum of 2.6 kPa. The corollary is that removal is mechanical and difficult, so the wall needs to be dry and sound before it is covered.Does basement foam have to be covered with drywall?
In a finished or occupied basement, yes. Article 9.10.17.10 requires foamed plastics forming part of a wall or ceiling assembly to be protected from adjacent space, and a finished basement room is adjacent space. The listed options are an interior finish from Subsections 9.29.4 to 9.29.9 — taped gypsum board being the usual one — qualifying sheet metal, or a thermal barrier meeting Sentence 3.1.5.15.(2). If a sprayed intumescent coating is being proposed instead of board, confirm it with your insurer before committing: a Nova Scotia mutual insurer refused exactly that in 2018, at a documented cost of about $3,700 to the homeowner, even though the coating complied with the code.Is spray foam better than batts against a foundation wall?
For this particular assembly the case is strong, and it is about moisture rather than R-value per dollar. Concrete is vapour-open and cold; a fibrous batt in a stud wall in front of it lets warm indoor air reach that cold surface, which is how framed basement walls end up with mould behind them. Closed-cell foam bonded to the concrete removes the air path and the condensing surface at once. Where cost matters, a hybrid — foam against the concrete for air and vapour control, fibre in the stud bay in front of it for the rest of the R-value — is a common and defensible answer.