Insulation R-Value per Inch: Thickness Needed and How Much Heat You Save
October 21, 2026
R-value measures resistance to heat flow, and higher is better. Total R-value is the R-value per inch times thickness. Fiberglass batts offer roughly R-3.1 to R-3.4 per inch, so reaching R-38 takes about 12 inches. Heat loss through a surface is area × temperature difference ÷ R, so doubling the R-value halves the loss.
Skip the math and get your answer instantly:
Open the Insulation Calculator →R-Value per Inch by Material
Values vary by product, density, and installation quality, so check the manufacturer's rating. These ranges are typical.
| Material | Approx. R per inch | Inches for R-38 |
|---|---|---|
| Fiberglass batts | 3.1 to 3.4 | 11 to 12 |
| Blown cellulose | 3.2 to 3.8 | 10 to 12 |
| Open-cell spray foam | 3.5 to 3.7 | 10 to 11 |
| Closed-cell spray foam | 6.0 to 7.0 | 5.5 to 6.5 |
| Rigid foam (XPS/polyiso) | 5.0 to 6.5 | 6 to 7.5 |
How Much Heat You Save
The rate of heat flow in BTU per hour is Q = A × ΔT ÷ R. For 1,000 square feet of ceiling with a 40°F temperature difference, R-19 loses 1,000 × 40 ÷ 19 ≈ 2,105 BTU per hour. R-38 loses 1,053 BTU per hour, half as much.
Returns diminish as R-value rises. Going from R-0 to R-19 cuts most of the loss, while going from R-38 to R-60 saves less in absolute terms. The best level depends on your climate, energy prices, and cost.
Where Installation Matters
Insulation only performs at its rated R-value when installed without gaps, compression, or voids. Compressing a batt reduces its thickness and therefore its R-value, and gaps around pipes and wiring create thermal bypasses.
Air sealing comes first. Sealing leaks around penetrations, attic hatches, and top plates reduces convective heat loss that insulation alone cannot address. Recommended R-values depend on climate zone and component: attics generally call for higher values than walls or floors, so check the current guidance for your region.
Frequently Asked Questions
What is the R-value per inch of fiberglass insulation?
About 3.1 to 3.4 for batts.
How many inches of insulation do I need for R-38?
About 12 inches of fiberglass or cellulose, or about 6 inches of closed-cell spray foam.
Does doubling the R-value halve energy loss?
Yes, for heat conduction through that surface. Air leaks and other surfaces are not affected.
Does compressing insulation lower its R-value?
Yes. Compressed insulation has less thickness, so it provides less resistance to heat flow.