Attic Insulation Calculator
Calculate insulation bags needed to reach a target R-value from attic area in sq ft.
Covers cellulose, fiberglass, and rockwool for R-30, R-38, R-49, and R-60.
Attic insulation is measured by R-value — the resistance to heat flow through a material. Higher R-value = better insulation = lower heating and cooling bills.
R-value formula:
R-value = Thermal Resistance (°F·ft²·h/BTU) per inch × Thickness in inches
Coverage area formula:
Bags Required = Attic Area (ft²) / Coverage per Bag
Coverage per bag varies by target R-value — manufacturers print a coverage table on every bag.
Department of Energy recommended R-values by US climate zone:
- Zone 1–2 (Hot: Florida, Hawaii): R-30 to R-49
- Zone 3–4 (Mixed: Virginia, Tennessee): R-38 to R-60
- Zone 5–6 (Cold: Minnesota, Colorado): R-49 to R-60
- Zone 7–8 (Very cold: Alaska, northern Canada): R-49 to R-60+
Common insulation types and R-value per inch:
| Type | R-value per inch |
|---|---|
| Blown fiberglass | 2.2–2.7 |
| Blown cellulose | 3.2–3.8 |
| Batt fiberglass (unfaced) | 2.9–3.8 |
| Spray foam (open-cell) | 3.5–3.9 |
| Spray foam (closed-cell) | 6.0–7.0 |
Worked example: Attic: 1,200 ft², target R-49, using blown cellulose (R-3.5/inch). Thickness needed: 49 / 3.5 = 14 inches If one bag covers 40 ft² at that depth: 1,200 / 40 = 30 bags
Existing insulation: Measure existing depth first. Subtract its R-value from your target before calculating how much to add. Most older homes have R-11 to R-19 in the attic — far below modern recommendations.
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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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