Open-Cell vs Closed-Cell Spray Foam — Ontario Guide

The technical differences come from cell structure: closed-cell has tiny sealed cells holding gas (high R-value, water-resistant); open-cell has connected cells filled with air (lower R-value, more flexible, sound-absorbing). The behavioural differences in an Ontario assembly follow directly from those properties.

Open-cell is the right pick when:

1. above-grade walls in a heated, conditioned space — interior partitions and exterior 2x6 walls in homes with proper exterior vapour control
2. attics — when there's good ventilation and you want sound dampening from the roof
3. sound-sensitive partitions — bedrooms, home theatres, or music rooms where the cellular structure helps with STC
4. budget-constrained projects where R-value is achievable with cavity depth alone (e.g., a 2x6 wall hits R-22 with open-cell at full depth, meeting OBC zone 6 minimum).

The cost-per-R-value math favours open-cell when the cavity is deep enough to hit code minimums without compression.

Closed-cell is the only correct choice when:

1. below-grade walls — basement walls always need vapour control against the foundation, and closed-cell handles incidental moisture without growing mould
2. rim joists and band joists — high air-leakage zones that benefit from a thin layer of high-R-value foam
3. shallow cavities — 2x4 walls hitting R-22 require closed-cell because open-cell can't get there in 3.5"
4. flat roofs and unvented attics — humid Ontario summers need vapour control on the warm side, and closed-cell delivers it
5. any assembly exposed to potential bulk water (rim joist of a basement, cathedral ceilings, exposed roof underside).

The higher cost is offset by better long-term durability and the elimination of separate vapour barrier products.

Cost differences widen as you scale up. A typical 1,000 sq ft basement wall (8' high × 125' long perimeter) insulated to R-20 minimum with closed-cell at 3" runs $4,500–$8,000; the same area with open-cell can't even hit R-20 in cavity space without going to 5.5" thickness, which doesn't fit a standard 2x4 stud wall.

OBC SB-12 sets minimum R-values per building element in Climate Zone 6: R-22 for above-grade walls, R-20 for basement walls, R-60 for attics. Both foams meet the spec when sized correctly. The Canada Greener Homes Grant rebates apply equally to both products provided installed R-value targets are met.

For air leakage control under SB-12 prescriptive paths, both are recognized air barriers — but only closed-cell is a vapour barrier on its own. Mixing the two in a single assembly (closed-cell on the underside, open-cell as a topup) is increasingly common in cathedral ceilings and high-performance walls. We recommend specifying that approach with a building science consultant or experienced designer to avoid trapping moisture between layers.