Insulation R-Value Calculator
R-Value Is a Rate, So Thickness Is the Variable
R-value measures resistance to heat flow per inch of material, which means hitting a target R-value is entirely a question of how thick a layer you install — and that required thickness changes dramatically depending on which insulation material you choose. A material with double the R-value per inch needs half the depth to reach the same performance, a distinction that matters enormously in a shallow attic or a wall cavity with limited depth to work with.
The Formula
Where This Calculation Matters
- Fitting insulation into a fixed cavity — a standard 2x4 wall has roughly 3.5 inches of depth; knowing the required thickness for a target R-value shows immediately whether a given material can physically achieve it.
- Attic insulation upgrades — attics typically target much higher R-values than walls, and the required depth can be substantial with lower-density materials like loose-fill fiberglass.
- Comparing material costs at equal performance — a higher R-per-inch material costs more per inch but needs less of it; this formula is the first step in a true cost comparison at matched performance.
- Meeting energy code minimums — building codes specify minimum R-values by region and assembly type, and this calculator translates that requirement into a physical thickness for a chosen material.
Typical R-Value per Inch by Material
Approximate, commonly cited R-value-per-inch figures (actual values vary by product and density — check the specific manufacturer's rating):
| Material | R-value/inch (typical) | Thickness for R-38 |
|---|---|---|
| Fiberglass batt | ~3.2 | 11.9 in |
| Blown cellulose | ~3.6 | 10.6 in |
| Open-cell spray foam | ~3.7 | 10.3 in |
| Rigid XPS foam board | ~5.0 | 7.6 in |
| Closed-cell spray foam | ~6.2 | 6.1 in |
| Polyisocyanurate board | ~6.5 | 5.8 in |
For a standard 2x4 wall cavity targeting R-13, the same materials need roughly 2 to 4 inches of thickness — well within the cavity depth for higher R-per-inch materials, tighter for lower-density options.
How to Use This Calculator
- Enter your Target R-Value (from local building code or your energy goals).
- Enter the R-Value per Inch for the specific material you're considering, from its product label or spec sheet.
- Select Calculate to see the thickness of that material required to reach the target R-value.
Related Calculations
Insulation upgrades often go hand in hand with window improvements — see the Window Cost Calculator, or budget the full renovation with the Remodeling Cost Calculator.