Open-Cell vs. Closed-Cell Spray Foam: Where Each Belongs in Your Home
Quick answer
Use closed-cell where you need a vapor retarder and maximum R-per-inch — rooflines, rim joists, metal buildings, crawl walls. Use open-cell where you want sound control and whole-cavity air sealing at lower cost — interior walls, sound walls, and vented attic floors with a separate vapor strategy.
The two foams look alike but do different jobs. Matching the foam to the assembly is the whole game.
Open-cell in one paragraph
Open-cell foam is soft, nominal 0.5 lb/cu ft density, ≈R-3.7/inch, and Class III vapor-permeable (>10 perms). It is excellent at air sealing and sound damping and costs less per board foot.
Closed-cell in one paragraph
Closed-cell foam is rigid, nominal 2.0 lb/cu ft density, ≈R-6.7/inch, and a Class II vapor retarder (≤1.0 perm at 1.5″). It adds structural rigidity (+200-300% racking shear strength) and is the right call wherever moisture and condensation are risks.
Where each belongs
A quick map for a Tennessee Valley home:
| Assembly | Open-cell | Closed-cell |
|---|---|---|
| Roofline / conditioned attic | Sometimes | Yes (vapor) |
| Rim joist | No | Yes |
| Metal building roof deck | No | Yes |
| Interior sound wall | Yes | Rarely |
| Vented attic floor | Yes (with vapor plan) | Sometimes |
Frequently asked questions
- Can I mix both in one house?
- All the time — closed-cell at the roofline and rim joist, open-cell or batt in the walls is a common, cost-smart hybrid.
- Is open-cell ever wrong?
- Yes — against a steel roof deck or in a damp crawl where a vapor retarder is needed, open-cell is the wrong tool.