Insulation R-Value Calculator
Find the total R-value from material thickness and R per inch, or the inches of insulation required to hit a target R-value for your climate.
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.
Check whether your insulation hits the mark. Enter the material's R per inch and thickness to get the total R-value, and see how many inches you need to reach your climate's target.
Formula
R-value measures resistance to heat flow and adds up across thickness:
Total R = R per inch × Thickness (in)
To hit a target R-value, solve for thickness:
Inches needed = Target R ÷ R per inch
Shortfall = max( Target R − Total R , 0 )
How it works
R-value is the standard measure of an insulation's resistance to conductive heat flow: the higher the R-value, the slower heat moves through the material. Because resistance is additive, the R-value of a uniform layer is simply its rated R per inch multiplied by its installed thickness. This tool reports that total and, working the other direction, the thickness required to reach a target assembly R-value.
Target R-values depend on climate zone and the part of the building. The U.S. Department of Energy recommends roughly R-30 to R-60 for attics, R-13 to R-21 for 2x4 and 2x6 walls, and R-25 to R-30 for floors, with the higher end applying to colder northern zones. Enter the recommendation for your zone as the target.
The calculation assumes a single homogeneous material installed at its rated density without gaps or compression. Real assemblies lose effective R-value through thermal bridging at studs and through compressed or voided batts, so treat the computed thickness as a clean-cavity minimum. Air sealing matters as much as R-value: even thick insulation underperforms if air leaks bypass it.
Worked example
10 inches of fiberglass (R-3.2/in) toward an R-49 attic
- Total R = 3.2 × 10 in = R-32.
- Inches needed for R-49 = 49 ÷ 3.2 = 15.31 in.
- Shortfall = 49 − 32 = R-17 below target.
Total R-32.00 | Need 15.31 in | Shortfall R-17.00
Approximate R-value per inch by insulation material
| Material | R per inch | Common use |
|---|---|---|
| Fiberglass batt | 3.1 – 3.4 | R-13 in 2x4, R-19 in 2x6 |
| Blown cellulose | 3.2 – 3.8 | Attic loose-fill |
| Open-cell spray foam | 3.5 – 3.6 | Air-sealing, interior walls |
| Closed-cell spray foam | 6.0 – 7.0 | Highest R per inch; vapor barrier |
| Rigid EPS foam board | 3.6 – 4.2 | Sheathing, foundations |
| Rigid XPS foam board | 5.0 | Below-grade, moisture-resistant |
| Polyiso foam board | 5.6 – 6.5 | Roof, sheathing |
Key terms
- R-value
- A measure of an insulating material's resistance to conductive heat flow. Higher R-values insulate better; values add across layers.
- R per inch
- The R-value contributed by each inch of a material's thickness. Fiberglass is about 3.2, closed-cell spray foam about 6.5.
- Thermal bridging
- Heat that bypasses insulation by conducting through framing members like studs and joists, lowering an assembly's effective R-value.
- Climate zone
- A geographic energy-code region that sets recommended insulation levels; colder zones call for higher R-values.
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.