We used spray foam in two very different applications on our Thornhill home last year — rim joists in the basement and a cathedral ceiling section over the sunroom. Different products, different code requirements.
Rim joist application (closed cell, 2 inches):
Our 1988 house had fiberglass batts stuffed in the rim joist cavities — R-14 at best, and leaking air around all the penetrations. A contractor came in with closed-cell SPF and applied 2 inches around the full basement perimeter (about 155 linear feet).
Cost: $1,650 (no drywall needed in unfinished mechanical room). R-value: approximately R-13. Air sealing benefit was immediately noticeable — the basement went from drafty to calm in the first winter.
Cathedral ceiling application (closed cell, 5.5 inches):
Our sunroom addition has a cathedral ceiling — no attic above, just rafters. The previous insulation was fiberglass batts jammed in rafter bays, R-22, with a suspicious moisture stain in one corner. We had the foam contractor apply 5.5 inches of closed-cell to the underside of the roof deck.
At 5.5 inches: R-35.75. Meets OBC for a rafter assembly.
The thermal barrier issue: This is where homeowners get caught. Ontario Building Code Section 9.10.17 requires any exposed foam plastic (including SPF) to be separated from the interior by a thermal barrier — minimum 12.7mm (half-inch) drywall. Our SPF contractor explicitly flagged this. Drywall over the cathedral ceiling (100 sqft): $1,400 installed.
Total costs:
- Rim joist SPF: $1,650
- Cathedral ceiling SPF: $3,200
- Drywall over cathedral SPF: $1,400
- Total: $6,250
Sunroom is now usable in March. Previously it was unusable until May.