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