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R-Value Calculator

An R-value calculator converts an insulation material and thickness into a total R-value, and works backwards to tell you the thickness needed to hit a target R. R-value is the standard measure of how well insulation resists heat flow — higher is warmer in winter and cooler in summer.

Your measurements

Result

Different materials give very different R per inch (rigid foam packs far more than batts), so this is how you compare options and check whether a cavity depth can reach the R-value your climate calls for.

How it is calculated

Total R-value = R per inch × thickness in inches. U-factor = 1 ÷ R. R per inch is material-specific (≈3.2 fiberglass batt, 3.5 cellulose, 5 XPS, 6-6.5 foam board/spray foam). Nominal R assumes no compression or gaps; thermal bridging and voids lower the effective whole-assembly R.

Steps

Pick the insulation material.
Enter the thickness you have or plan to install.
Read the total R-value (and U-factor).
Or enter a target R-value to see the thickness needed.
Compare materials when space is tight — foam gives more R per inch.

Worked example

6 inches of fiberglass batt (R-3.2 per inch) gives about R-19. To reach R-49, you would need about 15 inches of it.

Common questions

How do I calculate R-value?

Multiply the material's R per inch by its thickness in inches. 6 in of R-3.2 batt is about R-19.

What R-value do I need?

It depends on climate and where it goes — attics often target R-38 to R-60. Enter a target to see the thickness needed.

Which insulation has the highest R per inch?

Closed-cell spray foam (~6.5) and polyiso board (~6) top the list; batts are lower (~3.2) but cheaper per R.

Does compressing insulation change R-value?

Yes — squashing a batt into a thinner space lowers its R. Use the right thickness for the rated value.