Spray Foam R-Value Per Inch: Open Cell, Closed Cell and How They Stack Up

R-value is the number everyone asks about first, and for good reason. It is the standard way to compare how well insulation resists heat flow. With spray foam, the answer depends on which foam you mean, and the number on its own does not capture everything foam does.

The quick numbers

Exact values vary by product, so always check the manufacturer’s data sheet. For a broader table, ArmorThane keeps an insulation R-value chart covering most common materials.

What it means in a real wall

A standard 2×4 wall cavity is about 3.5 inches deep.

• Filled with open-cell foam, that is roughly R-13.
• With closed-cell, 2 to 3 inches can deliver roughly R-13 to R-20, and filling the cavity can go higher, though most contractors do not fill full depth with closed cell because of cost and heat during curing.

In a 2×6 wall at 5.5 inches, open-cell reaches roughly R-20, which meets common requirements in many climate zones.

Attics and roof decks

Attic R-value targets are much higher, often R-38 to R-60 depending on climate zone. Hitting those numbers with foam alone on a roof deck takes a lot of material, so many projects combine foam with other insulation, or follow code paths that credit foam’s air sealing. We cover the design choices in our post on unvented attics with spray foam.

Why R-value is not the whole story

Air sealing

R-value only measures conductive heat flow in a lab. It says nothing about air leaking through gaps. Fiberglass with gaps around wiring and outlets can underperform its label noticeably. Spray foam seals those gaps as it insulates, which is a big part of why foam-insulated homes often perform better than the R-value numbers alone would predict.

Moisture

Closed-cell foam also acts as a vapor retarder at enough thickness. Open-cell does not, which matters in cold climates and on roof decks.

Aging

Closed-cell foam’s R-value can settle slightly over time as blowing agents diffuse. Manufacturers often publish an aged or long-term value. Compare those numbers, not just the fresh ones.

Open or closed cell?

Use closed-cell where you need maximum R per inch, vapor control, or added rigidity: rim joists, basement walls, metal buildings, and tight spaces. Use open-cell for interior walls, sound control and many roof deck applications in milder climates. Our post on where to use open cell vs closed cell has the full breakdown, and the installers at Spray Foam Forum regularly debate real-world results.