Code and window ratings often use U-factors because they can be added up by area. R-values add up in layers: R-13 cavity insulation plus R-5 sheathing gives R-18 at the insulated part of the wall.
How the R-value calculator works
RValueCalc multiplies the depth of the existing insulation by its typical R-value per inch, subtracts that from your target and divides the rest by the new material's R-value per inch. The range uses the lowest and highest per-inch values in the chart. U = 1 ÷ R, RSI = R × 0.1761 and heat flow = area × temperature difference ÷ R.
R-value per inch
| Material | R per inch | Typical | Inches for R-30 | Inches for R-49 |
|---|---|---|---|---|
| Fiberglass batt | 2.9–3.8 | 3.2 | 9.4 | 15.3 |
| Loose-fill fiberglass | 2.2–2.7 | 2.5 | 12 | 19.6 |
| Loose-fill cellulose | 3.2–3.8 | 3.5 | 8.6 | 14 |
| Mineral wool batt | 3–4.3 | 4 | 7.5 | 12.3 |
| Loose-fill mineral wool | 2.8–3.3 | 3 | 10 | 16.3 |
| Open-cell spray foam | 3.5–3.7 | 3.6 | 8.3 | 13.6 |
| Closed-cell spray foam | 6–7 | 6.5 | 4.6 | 7.5 |
| EPS foam board | 3.6–4.4 | 4 | 7.5 | 12.3 |
| XPS foam board | 4.6–5 | 5 | 6 | 9.8 |
| Polyiso foam board | 5.6–6.5 | 6 | 5 | 8.2 |
ENERGY STAR recommended R-values (existing wood-framed homes)
| Zone | Attic, uninsulated | Attic with 3–4″ already | Floor |
|---|---|---|---|
| Zone 1 | R-30 | add R-25 | R-13 |
| Zone 2 | R-49 | add R-38 | R-13 |
| Zone 3 | R-49 | add R-38 | R-19 |
| Zones 4A and 4B | R-60 | add R-49 | R-19 |
| Zones 4C, 5 and 6 | R-60 | add R-49 | R-30 |
| Zones 7 and 8 | R-60 | add R-49 | R-38 |
Per-inch values: US Department of Energy and Building America Solution Center (PNNL) typical ranges; mineral wool batts vary most (older batts ≈ R-3.1, current stone-wool batts up to ≈ R-4.3). Recommended levels: ENERGY STAR, based on 2021 IECC Table R402.1.3. Results are estimates: product labels give the rated R-value for a stated thickness, and local codes set the legal minimums.
Frequently asked questions
How do I convert R-value to U-value?
Divide 1 by the R-value: U = 1/R. R-38 is U-0.026.
What is RSI?
The metric R-value, in m²·K/W. RSI = R × 0.1761; R = RSI × 5.678.
Is a lower U-value better?
Yes. Lower U means less heat flow, just as higher R means more resistance.
Is RValueCalc free?
Yes, with no sign-up or ads.
Are my inputs saved?
Yes, in this browser only.