Open-Cell vs. Closed-Cell Spray Foam Insulation

What Makes These Two Foams Different

Spray polyurethane foam (SPF) is created when two liquid chemicals — an isocyanate and a polyol resin — are mixed at the nozzle and expand rapidly on contact with a surface. What determines whether that foam becomes open-cell or closed-cell is how its cellular structure forms as it cures.

Open-cell foam has a soft, spongy texture because the tiny gas bubbles (cells) that form during expansion rupture and remain open. This gives the material a low density and allows air — and eventually moisture vapor — to pass through it.

Closed-cell foam cures with intact, sealed cells packed tightly together. This rigid structure traps an insulating gas inside each cell, which is what drives its dramatically higher R-value. It also means water vapor cannot easily penetrate the material.

Understanding this structural difference is the key to every practical decision that follows — R-value, moisture behavior, and appropriate applications all trace back to whether those cells are open or closed. If you're uncertain how R-value fits into your broader insulation plan, common insulation misconceptions are worth reviewing before you decide.

R-Value and Thermal Performance

R-value measures a material's resistance to heat flow — the higher the number, the better the insulating performance. This is where the two foams diverge most sharply.

Open-cell foam typically achieves around R-3.5 to R-4 per inch. Closed-cell foam reaches R-6 to R-7 per inch, roughly double. In a standard 2×4 wall cavity (about 3.5 inches deep), open-cell foam yields approximately R-13, while closed-cell foam can deliver R-21 to R-24 in the same space.

This difference matters most when cavity depth is limited or when local building codes demand a minimum R-value that open-cell foam cannot reach without exceeding available depth. Signs your insulation is underperforming can help you gauge whether your current setup is falling short of what your climate demands.

Moisture Control: A Critical Difference

Moisture management is where choosing the wrong foam type causes real, lasting damage. Open-cell foam has a high vapor permeability — it allows moisture vapor to move through it. In dry interior applications, this isn't a problem. But in crawl spaces, basement walls, or any area exposed to groundwater or exterior humidity, open-cell foam can absorb and hold moisture, creating conditions favorable for mold growth and wood rot.

Closed-cell foam, by contrast, functions as a Class II vapor retarder (typically 0.1 to 1.0 perms at typical thicknesses). This means it significantly slows vapor movement, making it the appropriate choice wherever moisture is a routine concern.

For attics, the right choice depends on how the space is configured. A vented attic typically uses insulation at the floor level, where either foam type might be appropriate depending on the design. An unvented (conditioned) attic — where insulation is applied to the roof deck — may specifically require closed-cell foam or a combination approach to manage condensation risk. How insulation and ventilation interact in the attic explains these dynamics in detail.

Cost, Installation, and Practical Considerations

Open-cell spray foam generally costs less per board foot than closed-cell foam, sometimes significantly so. However, because closed-cell foam achieves higher R-values in less thickness, the total installed cost per R-value unit can be more competitive than the raw material price suggests. Get quotes that compare cost per R-value achieved, not just cost per square foot.

Both types require professional installation. The chemicals involved are hazardous before curing, require precise mixing ratios, and demand proper personal protective equipment and ventilation. This is not a DIY project, and most local building codes require permits for significant insulation work — check your municipality's requirements before any project begins.

Open-cell foam also expands dramatically (up to 100 times its liquid volume), which means installers must carefully control application thickness. Closed-cell foam expands less aggressively, making it easier to apply in precise, thin lifts. Very thick applications of closed-cell foam must be done in multiple passes to prevent overheating during the exothermic curing reaction.

Finally, consider that closed-cell foam adds measurable structural rigidity to wall assemblies — a secondary benefit in some framing applications — while open-cell foam remains flexible and can accommodate minor structural movement without cracking.