R-value is thickness times R per inch
Every insulation material has an R-value per inch. Multiply it by how many inches you install and you get the assembly's R-value. To hit a target instead, divide: inches = target R ÷ R per inch. Higher R means slower heat flow and lower bills.
Same thickness, very different R
Two inches of material is not two inches of R-value. Rigid polyiso packs nearly twice the resistance of fiberglass in the same space, which is why tight cavities use the denser boards.
The same 2-inch thickness delivers R-12 in polyiso but only about R-6.4 in fiberglass batt.
R-value per inch by material
| Material | R per inch | Inches for R-30 |
|---|---|---|
| Fiberglass batt | ~3.2 | ~9.5″ |
| Blown cellulose | ~3.7 | ~8″ |
| Mineral wool | ~3.5 | ~8.5″ |
| EPS rigid board | ~4.0 | ~7.5″ |
| XPS rigid board | ~5.0 | ~6″ |
| Closed-cell spray / polyiso | ~6.0 | ~5″ |
Values are US imperial R (°F·ft²·h/BTU). Layers add up: R-13 batt plus R-6 board is R-19 total.
What eats into the rated R
- Compression. A batt squeezed into a shallow cavity loses R-value fast; stuffing R-19 into an R-13 space does not give you R-19.
- Moisture. Wet fiberglass or mineral wool can lose half its R-value, so a vapor barrier matters in cold climates.
- Thermal bridging. Wood studs conduct around the insulation, so the real wall performs below the batt's rated R. Continuous rigid board over the studs helps.
Common questions
How do I find R-value from thickness?
Multiply the thickness in inches by the material R-value per inch. Fiberglass batt is about R-3.2 per inch, so a 6-inch batt gives roughly R-19. Rigid polyiso is near R-6 per inch, so 6 inches reaches about R-36.
How thick does insulation need to be for R-30?
It depends on the material. About 9.5 inches of fiberglass batt, 8 inches of cellulose, or 5 inches of polyiso rigid board all reach roughly R-30. Denser materials hit the target in less space.
What R-value do I need in my attic?
US energy codes call for roughly R-30 to R-60 in attics depending on climate zone, R-13 to R-21 in walls, and R-13 to R-30 in floors. Check your local code for the exact zone requirement.
