R-value per inch by material
How many inches of insulation you need is just your target R divided by the material’s R per inch. Pick a material and this shows its labeled band and the depth it takes to hit the common targets — the whole table is below.
Calculator
Closed-cell spray foam delivers about R6.0–R7.0 per inch (midpoint 6.50), so R-49 takes roughly 7.5 inches. Typical published typicals — confirm on the data sheet.
Formula
inches = target_R ÷ R_per_inch
Higher R per inch means fewer inches for the same R. That is the whole reason foam goes in shallow cavities and loose-fill goes on open attic floors — depth is cheap on a floor, expensive in a wall.
Worked example
Closed-cell spray foam is labeled R-6.0 to R-7.0 per inch, a midpoint of 6.5. To reach R-49 you need 49 ÷ 6.5 = about 7.5 inches; for R-38, about 5.8 inches. Compare that with blown fiberglass at ~2.5 per inch, which needs roughly 19.6 inches for the same R-49 — same target, wildly different depth, which is exactly what this table is for.
How to read the bands
Every material shows a range, not a single number, and the range is honest — here is why and how to use it:
- Product and density vary. A high-density batt or a well-packed cellulose job sits at the top of the band; a compressed or gappy install sits below the bottom. Use the rated value on the bag or data sheet, not a blog average.
- Loose-fill settles. Blown fiberglass and cellulose lose a little depth over time, so their effective R/inch is lower than a fresh, fluffed pile — blow deep to the coverage chart, not just to a depth line.
- Polyiso is temperature-dependent. Its labeled R/inch is measured warm; in the cold it delivers less. Do not design a cold-climate exterior wall on the full number.
- R/inch is not the whole assembly. Air films and thermal bridging through framing change the finished R — total the real layers with the R-value calculator.
These are typical published planning values, not a certified design. For the sourced dataset see R-value per inch by material.
Reference table
| Material | Labeled R per inch | Inches for R-38 | Inches for R-49 |
|---|---|---|---|
| Fiberglass batt | R3.1–R3.4/in | 11.7" | 15.1" |
| Blown fiberglass | R2.2–R2.7/in | 15.5" | 20.0" |
| Cellulose | R3.2–R3.8/in | 10.9" | 14.0" |
| Mineral wool (Rockwool) | R3.0–R3.3/in | 12.1" | 15.6" |
| Open-cell spray foam | R3.5–R3.7/in | 10.6" | 13.6" |
| Closed-cell spray foam | R6.0–R7.0/in | 5.8" | 7.5" |
| Rigid EPS | R3.6–R4.2/in | 9.7" | 12.6" |
| Rigid XPS | R4.5–R5.0/in | 8.0" | 10.3" |
| Rigid polyiso | R5.6–R6.5/in | 6.3" | 8.1" |
Full dataset with sources at R-value per inch by material. Labeled published typicals — real R depends on install quality, thermal bridging and settling.
Frequently asked questions
What is the R-value per inch of spray foam?
Open-cell spray foam is labeled about R-3.5–3.7 per inch; closed-cell is about R-6.0–7.0 per inch. Closed-cell packs far more R into a shallow cavity, which is why it costs more per board-foot.
Which insulation has the highest R per inch?
Among common materials, closed-cell spray foam (~6.0–7.0) and rigid polyiso (~5.6–6.5) lead, followed by XPS (~4.5–5.0). Fiberglass, cellulose and mineral wool sit around 3.0–3.8 per inch.
How thick does insulation need to be for R-49?
Divide 49 by the material R per inch: roughly 7.5" of closed-cell foam, 14" of cellulose, or ~19.6" of blown fiberglass. Pick your material above to see the depth.
Are these R/inch numbers exact?
No — they are labeled planning typicals shown as a band. Real R depends on product, density, install quality, settling and temperature; use the rated value on your product’s data sheet.