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.