Resource Guide
Not all spray foam is the same product. Open-cell and closed-cell foam behave differently, cost differently, and belong in different parts of a building.
Open-cell foam has cells that are not fully closed, which makes it soft, spongy, and lower-density, roughly 0.5 lb per cubic foot. Closed-cell foam has fully closed, gas-filled cells packed at a much higher density, roughly 2 lb per cubic foot, which makes it rigid and much stronger.
Open-cell foam typically insulates at around R-3.5 to R-4 per inch. Closed-cell foam insulates at roughly R-6 to R-7 per inch, nearly double, which is why closed-cell is often chosen when cavity depth is limited.
Closed-cell foam is largely impermeable to water and acts as its own vapor retarder, which is why it is the right choice for below-grade applications like crawl spaces and basement walls. Open-cell foam is vapor-permeable and can absorb moisture, so it is generally kept away from below-grade assemblies.
Open-cell foams softer, more porous structure makes it notably more effective than closed-cell at dampening airborne sound, which is why it is frequently chosen for interior partition walls, home theaters, and offices.
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