calcs.homes
calcs.homes
Enter your attic area and current and target R-value to get blow-in insulation depth, bags needed, and cost. Uses the US/Imperial R-value scale common on North American insulation bags.
Estimate only — bag coverage varies by brand; check the coverage chart printed on your bag.
We subtract your current R-value from the target to get the R-value gap, then divide by the material's R-value per inch — 2.5 for loose-fill fiberglass, 3.7 for cellulose, 3.0 for rockwool — to find the depth needed. Multiplying attic area by that depth (in feet) gives total cubic feet, which we divide by the material's bag yield (30 cu ft for fiberglass, 25 for cellulose, 24 for rockwool) and round up to a whole bag.
R-value per inch differs by material because each traps heat differently at the same thickness, which is why reaching the same target R-value takes noticeably less cellulose than fiberglass. Bag yield is based on the settled, not fluffed, coverage manufacturers print on the label, since loose-fill insulation compresses over time after installation — using settled yield keeps your attic at the target R-value years later, not just on install day, when it's already begun to compact.
Most US climate zones recommend R-38 to R-60 for attics — check your local energy code or the Department of Energy's zone map for a specific target.
It depends on your attic's area, the R-value gap you're filling, and the material's coverage per bag — this calculator combines all three. Always cross-check against the coverage chart on your specific bag.