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, meaning how well insulation resists the flow of heat. A higher R-value means better insulation. R-values are additive, so layering insulation adds their R-values together.
Heat loss formula:
Heat loss in BTU (British Thermal Units) per hour = 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
Notice how the second R-19 buys you far less than the first. Going from nothing to R-19 cuts the loss enormously; doubling R-19 to R-38 only halves what is left. That diminishing return is why attics get deep insulation and walls do not, and why chasing R-60 in a mild climate rarely pays back.
Batts are sized to fit standard framing, so buy for your cavity depth rather than for a target R-value alone. An R-30 batt crushed into a 2x6 wall loses much of the rating you paid for. Air sealing first, insulation second: a small gap around a recessed light or a top plate will undo more performance than a couple of R-points ever gain back.
DOE (Department of Energy) 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-3.0 to R-3.8 |
| Fiberglass (blown, loose-fill) | 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, Extruded Polystyrene) | 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.
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