Open-cell vs closed-cell spray foam: which should I use?

Use closed-cell spray foam anywhere moisture, exterior contact or structural rigidity matters; use open-cell for interior walls and vented attic slopes where budget and sound damping rule the decision.

The choice between open-cell and closed-cell comes down to five variables: R-value per inch, density, vapor permeance, structural contribution and cost per R-unit. Closed-cell is denser, more rigid and vapor-retarding. Open-cell is lighter, vapor-open and better at absorbing sound. In Climate Zone 6 and 7, the wrong choice in an unvented roof deck can trap moisture and void a warranty.

• Closed-cell: R-6.5 to R-7 per inch, ~2 lb/cu ft, Class II vapor retarder at 1.5"+. Adds racking strength. Best for rim joists, crawlspaces, unvented roof decks and below-grade walls.
• Open-cell: R-3.6 to R-3.8 per inch, ~0.5 lb/cu ft, vapor-open, better sound damping. Best for interior partitions, attic floors and sound walls.
• Cost gap: closed-cell runs roughly 2.2x open-cell per R-unit installed.
• Climate Zone 6 rule: an unvented roof deck needs closed-cell (or a code-compliant hybrid with rigid foam) to keep the condensing surface warm.
• Vapor strategy: closed-cell ≥1.5" needs no separate barrier; open-cell in Zone 6/7 needs vapor-retarder paint or a smart membrane on the warm side.

The trade-off is cost and rigidity versus breathability and sound control. Closed-cell is overkill and unnecessarily expensive for simple interior walls where open-cell performs well. Open-cell is risky anywhere bulk water or exterior vapor can reach it.

Match the product to the assembly: closed-cell for the building envelope below the roof sheathing, open-cell for interior partitions and attic floors, and never use poly on both sides of any assembly.