Insulation Thickness Calculator
Find out how many inches of insulation you need to hit your target R-value. Compare all materials to find the most space-efficient option.
Calculate Required Thickness
Enter your target R-value to see how thick each insulation material needs to be.
The total R-value you need to achieve
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Understanding Insulation Thickness
The Formula: R-Value = Thickness × R-per-Inch
Every insulation material has an R-value per inch rating. To achieve a target R-value, divide by the material's R-per-inch:
Thickness (inches) = Target R-Value ÷ R-per-inch
Example: R-38 ÷ 3.5 per inch (fiberglass) = 10.9 inches
R-Value Per Inch by Material
| Material | R-per-Inch | Inches for R-38 |
|---|---|---|
| Closed-Cell Spray Foam | R-6.0 to R-7.0 | 5.4" – 6.3" |
| Polyiso Foam Board | R-5.6 to R-6.5 | 5.8" – 6.8" |
| XPS Foam Board | R-4.5 to R-5.0 | 7.6" – 8.4" |
| Mineral Wool Batts | R-3.8 to R-4.3 | 8.8" – 10.0" |
| EPS Foam Board | R-3.6 to R-4.4 | 8.6" – 10.6" |
| Open-Cell Spray Foam | R-3.5 to R-3.8 | 10.0" – 10.9" |
| Blown-In Cellulose | R-3.2 to R-3.8 | 10.0" – 11.9" |
| Fiberglass Batts | R-3.0 to R-3.7 | 10.3" – 12.7" |
| Blown-In Fiberglass | R-2.2 to R-2.7 | 14.1" – 17.3" |
Why Thickness Varies
Each material has a range of R-values per inch depending on density, installation quality, and manufacturer. Higher-density fiberglass batts have higher R-per-inch than standard batts. Spray foam R-value depends on formulation and age.
Space-Constrained Applications
When space is limited — like 2×4 walls or shallow rafters — choose materials with higher R-per-inch values. Closed-cell spray foam (R-6.5/inch) delivers nearly double the R-value of fiberglass (R-3.5/inch) in the same cavity depth.
Hybrid Approaches
Many projects combine materials. Example: 2" of closed-cell spray foam (R-13) + 3.5" of mineral wool (R-15) = R-28 in a 2×6 cavity with excellent air sealing and moisture control. This "flash and batt" approach balances performance and cost.
Related Resources
R-Value Calculator
Find required R-values for your climate zone.
Cost Calculator
Estimate material and installation costs.
R-Value Chart
Complete reference for all insulation types.
R-Value Per Inch Guide
Detailed comparison by material type.
Climate Zone Map
Find your IECC climate zone.
Insulation Quiz
Get personalized recommendations.
Frequently Asked Questions
Can I compress insulation to fit in a smaller space?
No! Compressing insulation significantly reduces its R-value. For example, an R-19 batt compressed into a 3.5" cavity only delivers about R-13. If space is limited, use a thinner product rated for that cavity or choose a higher R-per-inch material like spray foam.
Does blown-in insulation settle over time?
Yes, cellulose settles about 20% after installation. Manufacturers account for this in their coverage charts — the installed depth includes extra to compensate. Blown fiberglass settles less (1-3%). Check the settled R-value, not initial.
Why does spray foam have a range of R-values?
Spray foam R-value varies by formulation, age, and temperature. Closed-cell foam typically rates R-6 to R-7 per inch fresh, declining slightly over years as blowing agent escapes. Polyiso foam board has the most significant cold-weather derating — it performs lower at temperatures below 40°F.
What if my target R-value exceeds my cavity depth?
Three options: (1) Use a higher R-per-inch material like closed-cell spray foam, (2) Add continuous insulation on the outside of the framing, or (3) Use a hybrid approach combining cavity insulation with exterior rigid foam. For attics, simply add more blown-in insulation to the desired depth.