Spray Foam Insulation R-Value | Per-Inch Rates by Type

Whether you’re insulating a 2×4 wall or topping off an attic to meet code, the spray foam insulation R-value you get depends on one choice first: open-cell or closed-cell. Open-cell foam delivers R-3.5 to R-3.8 per inch, while closed-cell foam ranges from R-5.6 to R-8.0 per inch, with most residential applications landing at R-6.0 to R-7.0. The table below shows what those numbers mean at real wall and roof thicknesses you’ll actually build.

Open-Cell vs Closed-Cell: What’s The R-Value Difference?

Open-cell spray foam consistently delivers R-3.5 to R-3.8 per inch regardless of how thick you spray it. Closed-cell foam delivers R-5.6 to R-8.0 per inch, with the exact number depending on density and formulation.

Open-cell is low-density foam that stays soft and flexible after curing. Its R-value holds steady at roughly R-3.6 per inch whether you spray it 2 inches deep or 11 inches deep. A standard 2×6 wall filled with open-cell hits roughly R-19 to R-21.

Closed-cell foam comes in medium-density (about 2 lb/ft³) and high-density (3 lb/ft³) versions. Medium-density closed-cell lands at R-6.0 to R-7.0 per inch — the most common range in residential work. High-density closed-cell reaches R-7.0 to R-8.0 per inch but costs more and is typically used in commercial or high-performance builds.

Calculating Spray Foam R-Value For Your Climate Zone

Divide your target R-value by the foam’s per-inch rating, and you get the thickness required. The formula works the same for any climate zone.

For a Zone 4A attic requiring R-60: closed-cell at R-7 per inch needs about 8.5 to 10 inches. Open-cell at R-3.6 per inch needs about 17 inches — more than double the depth. For a 2×6 wall targeting R-20: closed-cell needs roughly 3 inches, while open-cell needs about 5.5 inches, which fills the cavity completely.

Zone 5 roofs (parts of Ohio and Pennsylvania) require R-30 for new construction. Closed-cell at R-6.5 per inch reaches that in about 4.6 inches. Open-cell would need roughly 8.5 inches for the same target.

How Specific Products Compare On R-Value

Icynene’s product line shows how formulation changes the numbers. Their open-cell Classic and Classic Max deliver R-3.7 per inch — slightly above standard. The ProSeal closed-cell variant achieves R-21 in a single 3-inch pass, and the ProSeal LE can reach R-35 in a 5-inch pass. ArmorThane confirms closed-cell at R-6 to R-7 per inch across their standard formulations.

These product-level differences matter when you’re trying to hit a specific code requirement without overspraying. A builder needing R-21 in a 2×6 wall can use Icynene ProSeal at 3 inches and leave no cavity wasted. For homeowners taking on the job themselves, our roundup of the best closed cell spray foam kits for homeowners compares tested options by delivered R-value and coverage area.

Common Spray Foam R-Value Mistakes

The most frequent error is assuming open-cell can match closed-cell R-value without accounting for the thickness difference. Open-cell needs roughly double the depth to hit the same number — 17 inches versus 8.5 inches for R-60. That extra thickness eats into living space and adds framing cost.

Another mistake is treating all closed-cell foam as identical. Standard-density (2 lb/ft³) foam lands at R-6.0 to R-7.0 per inch, while high-density (3 lb/ft³) reaches R-7.0 to R-8.0. Specifying the wrong density leaves R-value on the table or wastes money on performance you don’t need. Johns Manville’s spray foam R-value breakdown explains how density drives the per-inch rating.

Using open-cell in foundations or rim joists is the third common error. Open-cell is permeable and cannot block moisture. Closed-cell acts as a Class II vapor barrier and is mandatory in below-grade and rim-joist applications.

When Does Closed-Cell Make More Sense Than Open-Cell?

Closed-cell wins wherever moisture protection, structural strength, or high R-value in a thin cavity matters. Open-cell wins where budget, sound dampening, or filling an irregular cavity is the priority.

Closed-cell adds structural strength to walls and roofs — open-cell is softer and flexible. Closed-cell maintains its R-value across all outdoor temperatures, unlike rigid polyiso boards that drop to R-4 to R-4.5 in cold weather. Open-cell costs roughly one-third of closed-cell, making it the budget-friendly choice for attics and interior walls where moisture isn’t a concern.

The R-Value Numbers That Matter For Your Build

For most residential projects, the practical range is narrow. Target R-60 for attics in Climate Zone 4, R-20 for walls, and use closed-cell whenever moisture is a factor. Open-cell works fine for interior walls and cathedral ceilings where a vapor barrier isn’t needed, but plan for the extra depth.

The three numbers to remember: R-3.6 per inch for open-cell, R-6.5 per inch for standard closed-cell, and the formula — target divided by per-inch rating equals the inches you need. That’s the whole calculation.