How thick does spray foam need to be? It comes down to your target R-value and which foam you use, since closed-cell (R-6 to R-7 per inch) hits a number in roughly half the depth of open-cell (R-3.5 to R-3.8 per inch). Here are the typical Alberta targets.
Common targets
- Attic R-38: about 6" closed-cell or 11" open-cell. Unvented roof deck: 5-8" closed-cell.
- Basement wall: ~2" closed-cell gives roughly R-12 to R-14 (and becomes a vapour retarder); 3-4" reaches R-18 to R-28 in tight cavities.
- Rim joist: 2-3" closed-cell to seal and insulate the band.
Why thickness is a spec, not a guess
The right thickness depends on the assembly, the target, and the manufacturer's data sheet - which governs the exact number on a real job. A good installer specs to your target R, not a rule of thumb.
Estimate board feet for your target, then get a quote. See closed-cell vs open-cell.