How much R-value does each inch of spray foam give?
About R-3.7 per inch for open cell and up to about R-6.5 per inch for closed cell. Multiply by the inches to get the R-value, or divide the R-value you need by those numbers to get the inches. Exact figures vary a little by product, so check the one on your quote.
| Thickness | Open cell | Closed cell |
|---|---|---|
| 1 inch | R-3.7 | R-6.5 |
| 2 inches | About R-7 | About R-13 |
| 3 inches | About R-11 | About R-19.5 |
| 3½ inches (a full 2x4 wall) | About R-13 | About R-23 |
| 4 inches | About R-15 | About R-26 |
| 5½ inches (a full 2x6 wall) | About R-20 | About R-36 |
| 6 inches | About R-22 | About R-39 |
| 8 inches | About R-30 | About R-52 |
| 10 inches | About R-37 | About R-65 |
How thick should spray foam be in a wall?
Open cell usually fills the whole stud space: about R-13 in a 2x4 wall and about R-20 in a 2x6 wall. Closed cell is usually sprayed around 3 inches rather than filling the wall, because it is hard to trim. That gives about R-19.5 — close to a full 2x6 of open cell in about half the depth.
What your wall legally needs depends on your climate zone and which version of the energy code your state uses, and many states are on older versions. As one example, the 2021 national model code raised a climate zone 4 wood-frame wall from R-20 to R-30 in the cavity, or R-20 plus R-5 of continuous insulation outside, among other options. Ask your installer what your local code requires before comparing quotes.
How many inches does an attic need?
More than anywhere else in the house. Reaching R-49 takes about 13 inches of open cell or about 7½ inches of closed cell under the roof deck. Our spray foam attic insulation guide covers the levels for each climate zone, and when the roof deck is worth it compared with the attic floor.
How thick should spray foam be on a rim joist?
Under 3¼ inches if you want to leave it exposed. The rim joist — the band of wood that sits on top of the foundation wall — is one of the few places the building code lets spray foam stay uncovered, without drywall over it, as long as it is thinner than that. Building Science Corporation rates both open and closed cell as acceptable there, but notes open cell may need extra moisture control in climate zone 6 and colder.
How thick for basement and crawl space walls?
Closed cell is the usual choice below ground, because it holds up to water (see our open vs closed cell guide). ENERGY STAR’s recommendations for these walls work out to roughly:
| Climate zone | Recommended | Closed cell needed |
|---|---|---|
| Zone 3 | R-5 | About 1 inch |
| Zones 4A and 4B | R-10 | About 1½ inches |
| Zones 4C and 5 through 8 | R-15 | About 2½ inches |
Outside the rim joist, foam in living space is generally covered with drywall or a similar fire-rated layer. Ask your installer what your space needs.
What’s the least spray foam that still does the job?
- To seal air leaks: about 1 to 1½ inches of closed cell, or 3 to 5½ inches of open cell.
- To block moisture: about 1½ inches of closed cell. One foam maker’s test data shows its closed cell at 1.25 perms at one inch and 0.83 at an inch and a half — under the 1.0 perm the building code calls a Class II vapor retarder. Open cell never gets there at any thickness; in climate zones 5 through 8 and marine zone 4 it needs a separate vapor retarder, such as a vapor-retarder paint on the drywall.
- Per pass: at least half an inch, so the foam has enough material to cure properly.
Why is thick spray foam applied in layers?
Spray foam heats up as it cures. The Spray Polyurethane Foam Alliance says the inside of the foam should stay at or below about 180°F, and warns that foam sprayed too thick in one pass can scorch or even catch fire. So installers spray it in passes, and let each one cool for about 15 to 30 minutes before the next. The maximum thickness per pass depends on the product.
So a six-inch closed-cell job taking several passes is normal. An installer who says they can do it all at once is worth asking about.
How do you make sure you get the thickness you paid for?
Get it in writing. A good quote states the inches, the foam type and the R-value for each area. It does go wrong: one homeowner on Green Building Advisor paid for 4½ inches of closed cell and found 2 to 2½. Ask the crew to check depth as they go, look at it yourself before any drywall goes up, and if an area measures short, raise it before the final payment.
Do most spray foam contractors offer closed cell?
Almost all of them. Of the 290 contractors in the FoamMatch directory whose listing says which foam they spray, 280 spray closed cell and 199 spray both — only 10 offer open cell alone. So you can usually get both priced from one company. Ask for the thickness and R-value of each, area by area, and compare them side by side.