Short answer: R-30 fiberglass batt insulation is about 9.4 inches thick, using about R-3.2 per inch. Blown cellulose reaches R-30 in about 8.1 inches, and closed-cell spray foam in about 4.6 inches.
| Target R-value | Fiberglass batts | Blown fiberglass | Blown cellulose | Closed-cell spray foam |
|---|---|---|---|---|
| R-13 | 4.1 | 5.2 | 3.5 | 2 |
| R-19 | 5.9 | 7.6 | 5.1 | 2.9 |
| R-30 | 9.4 | 12 | 8.1 | 4.6 |
| R-38 | 11.9 | 15.2 | 10.3 | 5.8 |
| R-49 | 15.3 | 19.6 | 13.2 | 7.5 |
How the thickness is worked out
Divide the R-value you want by the R-value per inch of the material. Fiberglass batts give about 3.2 per inch, blown cellulose about 3.7, and closed-cell spray foam about 6.5. Thickness then follows directly.
These are typical figures. Real products differ by brand and density, so a batt sold as R-13 may be 3.5 inches thick because it packs more R-value per inch. Always use the label on the product.
Choosing an R-value
The R-value you need depends on your climate and which part of the house you are insulating. Attics usually call for the highest values, and walls are limited by the depth of the framing. Check your local building code or energy office for the recommendation.
Worked example
To reach R-38 with fiberglass batts, divide 38 by 3.2 to get 11.9 inches. An attic of 1,000 sq ft with 10% extra needs about 1,088 cubic feet, or 28 packages that each cover 40 sq ft.
Use the Insulation CalculatorEnter your own measurements and get an exact amount.
Frequently asked questions
How thick is R-19 insulation?
About 5.9 inches for fiberglass batts at R-3.2 per inch. Many R-19 batts are sold at 6.25 inches.
Can I compress batts to fit a thinner space?
You can, but it lowers the R-value. Use a higher R-value per inch product instead, such as spray foam or foam board.
What is R-value?
It measures resistance to heat flow. A higher number means better insulation.
Related calculators
Figures are typical planning values. Material sizes, densities and local requirements vary, so confirm with your supplier before ordering.
Last updated October 11, 2026.