Attic Insulation Calculator

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, and 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:

Worked example: Attic: 1,200 ft², bare joists (R-0), target R-49, blown cellulose at R-3.5 per inch. Thickness needed: 49 / 3.5 = 14 inches Coverage falls as depth rises. A cellulose bag that covers about 32 ft² at R-30 covers only 32 × (30/49) = 19.6 ft² at R-49. Bags: 1,200 / 19.6 = 61.2, so 62 bags.

This is the step most people get wrong. They read “40 sq ft per bag” off the front of the bag, divide, and order half what they need. That 40 is quoted at a shallow R-value. Turn the bag over: the coverage table on the back gives a separate square-footage figure for every R-value, and at R-49 it will be somewhere near a third of the headline number. The chart printed on your actual brand beats this calculator, which uses a generic curve.

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.