Insulation Calculator
Calculate how much insulation you need for walls or ceilings.
Enter area and R-value to get batts, rolls, or blown-in estimates.
How Insulation R-Value Works
R-value measures thermal resistance — how well insulation resists the flow of heat. A higher R-value means better insulation. R-values are additive: combining multiple layers of insulation adds their R-values together.
Heat loss formula:
Heat loss (BTU/hr) = Area (sq ft) × Temperature difference (°F) ÷ Total R-value
Worked example:
- Attic area: 1,200 sq ft
- R-value of existing insulation: R-19
- Indoor temperature: 68°F, Outdoor: 10°F → ΔT = 58°F
Heat loss = 1,200 × 58 ÷ 19 = 3,663 BTU/hr
Adding R-19 more (total R-38):
Heat loss = 1,200 × 58 ÷ 38 = 1,832 BTU/hr — 50% reduction
DOE recommended R-values by climate zone:
| Climate Zone | Attic | Wall (cavity) | Floor |
|---|---|---|---|
| Zone 1 (Hot: Miami) | R-30 | R-13 | R-13 |
| Zone 3 (Mixed: Atlanta) | R-38 | R-13 | R-19 |
| Zone 5 (Cold: Chicago) | R-49 | R-20 | R-30 |
| Zone 7 (Very cold: Alaska) | R-60 | R-21 | R-38 |
R-value per inch by insulation type:
| Material | R per inch |
|---|---|
| Fiberglass batt | R-2.2 to R-2.7 |
| Cellulose (blown) | R-3.2 to R-3.8 |
| Open-cell spray foam | R-3.5 to R-3.8 |
| Closed-cell spray foam | R-6.0 to R-7.0 |
| Rigid foam (XPS) | R-5.0 |
| Rigid foam (polyiso) | R-6.5 to R-7.0 |
Thickness needed:
Inches needed = Target R-value ÷ R per inch
To achieve R-49 with blown cellulose (R-3.5/inch):
Thickness = 49 ÷ 3.5 = 14 inches
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
SuperGlobalCalculator is independently built and maintained. See how we build and verify our calculators.