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.