R-38 Attic Insulation with Fiberglass Batts
At 3.2 R per inch, you need about 12 inches of fiberglass batt insulation to reach an R-38 target value.
How to use this tool
- Look up the R per inch for your insulation material (see the table below).
- Enter the installed thickness in inches.
- Enter your target R-value from local energy-code recommendations.
- Read the total R-value, the inches needed, and any shortfall.
- Air-seal before insulating so the rated R-value performs.
Find out if your fiberglass batt thickness is enough to achieve an R-38 attic insulation rating.
Frequently asked questions
- How do I calculate total R-value?
- Multiply the material's R per inch by its installed thickness. For layered assemblies, add the R-values of each layer together. Studs and joists create thermal bridges that lower the effective whole-wall R-value.
- What R-value do I need for my attic?
- The U.S. Department of Energy recommends roughly R-30 to R-60 for attics depending on climate zone, with R-49 a common target in moderate-to-cold regions. Enter your local recommendation as the target.
- Which insulation has the highest R-value per inch?
- Closed-cell spray foam leads at about R-6 to R-7 per inch, followed by polyiso board at R-5.6 to R-6.5. Fiberglass batts are about R-3.1 to R-3.4 per inch.