Open-cell vs closed-cell foam: which one to spray

Open-cell foam cures at roughly half a pound per cubic foot and gives about R-3.6 to R-3.9 per inch. It stays vapor permeable, so an assembly can dry, and it costs less per board foot. Closed-cell cures around two pounds per cubic foot, delivers roughly R-6 to R-7 per inch, acts as its own vapor retarder and adds rigidity to the assembly.

The practical rule: closed-cell anywhere moisture is a risk or space is tight — rim joists, below grade, unvented roof decks, crawl spaces. Open-cell where you want the assembly to dry and you have depth to fill, such as interior walls and vented attic floors.

Cost per R-value, not just cost per board foot

Closed-cell costs more per board foot installed, but it also delivers roughly double the R-value per inch. In a cavity where depth is limited, that difference often makes closed-cell the more efficient choice per dollar of R-value actually achieved, even though the sprayed cost looks higher on paper. In a deep cavity with no space constraint, open-cell can be the more economical way to hit a target R-value.

Moisture and vapor control

Closed-cell’s low permeability means it acts as its own vapor retarder at roughly two inches, which simplifies the assembly in climates or applications where a separate vapor control layer would otherwise be needed. Open-cell’s permeability is an advantage where a wall or roof assembly needs to dry to the interior or exterior — but it also means the assembly still needs its own vapor control strategy, and open-cell alone won’t stop bulk water or humid air from moving through.

Sound and structure

Open-cell’s softer, more flexible cell structure absorbs airborne sound noticeably better than closed-cell, which is one reason it’s often the pick for interior partition walls even in buildings that use closed-cell elsewhere. Closed-cell’s rigidity, on the other hand, adds real racking strength to a wall assembly — a factor worth considering on structures where that stiffness is doing double duty.

Fire code and ignition barriers

Both foam types require an approved thermal or ignition barrier over exposed spray foam in occupied space, per local code — foam is not left exposed in a living area or habitable attic regardless of which type is used. Requirements vary by jurisdiction and application (attic, crawl space, garage), so check the local code and the specific product’s listing before you spray, not after.

Mixing both in one assembly

Plenty of jobs use both: closed-cell at the rim joist and below-grade walls where moisture and space are tight, open-cell through the rest of the wall or roof cavity where depth allows it. Choosing per-cavity based on its actual exposure and constraints, rather than picking one foam for the whole job, is usually the more cost-effective approach.