Open-Cell vs. Closed-Cell Spray Foam: Which Should You Choose?

The core difference: what's happening at the cell level

Both types start as the same liquid chemical spray that expands on contact — the difference is what happens to the individual cells as it cures. In open-cell foam, the cells rupture during expansion, leaving behind a soft, sponge-like matrix of interconnected air pockets. In closed-cell foam, the cells stay intact and sealed, each one filled with a low-conductivity gas rather than plain air. That single structural difference is what drives every other distinction between them — density, R-value, moisture behavior, and rigidity.

R-value and cost side by side

Closed-cell delivers roughly double the R-value per inch, which matters most when cavity depth is limited — a thin wall assembly can hit its target R-value with closed-cell where open-cell physically couldn't fit enough material. But that performance comes at a real cost premium, which is why closed-cell isn't automatically the "better" choice everywhere.

The real decision factor: moisture

This is where the choice usually makes itself. Open-cell foam is vapor-permeable — water vapor passes through it, and the material itself can absorb roughly 4–6% of its volume in water if it gets wet, which degrades its insulating performance and creates mold risk. It's explicitly not recommended for exterior applications or anywhere it could contact water directly.

Closed-cell behaves the opposite way. At 1.5 to 2 inches thick, it reaches a Class II vapor retarder rating, blocking bulk liquid water and resisting mold growth even in consistently damp conditions. This is exactly why closed-cell is the standard choice for basements and crawl spaces — it doesn't just insulate, it doubles as the vapor barrier those spaces need. Our crawl space encapsulation cost calculator and basement waterproofing cost calculator both cover projects where this distinction matters directly.

Structural strength: closed-cell does double duty

Open-cell foam contributes essentially zero structural strength to a wall assembly — its compressive strength is under 1 psi. Closed-cell is different: its rigid, dense structure adds real racking resistance to a wall or roof assembly, which is exactly why building codes in coastal high-wind zones often specify it, and why it's used in metal buildings to add rigidity to the panels themselves without extra framing. If you're insulating a pole barn, shop, or metal building, this structural contribution is a genuine, code-relevant reason to choose closed-cell beyond just the moisture argument — our metal building cost estimator and pole barn cost estimator are both worth checking if that's your project.

Open-cell's other advantage: sound dampening

The same soft, porous structure that makes open-cell vulnerable to moisture makes it noticeably better at absorbing airborne sound — commonly cited at roughly twice the sound resistance of closed-cell in normal frequency ranges. If you're insulating interior walls around a media room, home office, or bedroom specifically for noise control, open-cell has a real, measurable edge that closed-cell doesn't match.

When building code requires closed-cell specifically

In a handful of situations, this isn't really a preference decision — code requires closed-cell outright:

• Flood zones — FEMA requirements call for moisture-resistant materials that open-cell doesn't qualify as.
• Below-grade walls — the IRC requires a vapor barrier in this application (R406.2), which closed-cell provides inherently.
• Coastal high-wind zones — structural racking resistance is a code factor here, not just a nice-to-have.
• Extreme cold climates (Zone 7+) — where installing a separate interior vapor barrier is impractical, closed-cell's built-in vapor resistance does that job.

A quick decision guide by application

Is either type unsafe to have in your home?

Both open-cell and closed-cell spray foam are typically Class 1 fire rated, meaning they self-extinguish once a flame source is removed rather than actively fueling a fire. Modern formulations from major manufacturers have minimized off-gassing significantly compared to older products, though a faint odor is still common with closed-cell's chemical blowing agent during the actual spraying and initial cure. That said, code still requires an ignition or thermal barrier over exposed spray foam in living spaces regardless of type — that's a code requirement, not an optional upgrade, and it's included as a line item in our spray foam insulation cost calculator.

What does spray foam actually cost?

Spray foam is priced by the board foot — one square foot sprayed one inch thick — which is why the same square footage can cost wildly different amounts depending on target thickness and foam type. Open-cell typically runs $0.40–$0.75 per board foot; closed-cell runs $1.00–$1.65 per board foot, reflecting that 40–50% cost premium discussed above. Our spray foam insulation cost calculator uses this same board-foot method to give you an instant estimate for either type based on your specific area, thickness, and application.

Frequently asked questions