Blown-in insulation prices by the bag, not the square foot: this calculator converts your target depth into bags using each material's real coverage rate, then prices those bags at $25–$70 depending on material, plus a flat $0.50–$1.00 per sq ft for the blower crew. A 400 sq ft attic going from bare deck to R-30 with cellulose runs $850–$1,440 installed — $2.13–$3.60 per sq ft.
How this calculator counts bags, not cubic feet
Insulation contractors order in bags, so that's what this calculator computes. It first converts your R-value gap into inches of depth using each material's R-per-inch rating — 2.5 for fiberglass, 3.7 for cellulose, 3.3 for rockwool — then multiplies square footage by inches by a coverage-rate constant (bags per sq ft-inch) to get a bag count, rounded up.
Why this page also shows a cost-per-square-foot figure
This calculator's results include a cost-per-square-foot line alongside the whole-job total specifically so the output can be checked against this page's published $1–$4.50 per sq ft installed range. A whole-job dollar figure on a large or unusual attic can look reasonable in isolation while actually sitting well outside that band — dividing back down to a per-square-foot number is the fastest way to catch it, and it's the same check worth running on any contractor bid quoted as a single lump sum.
Bag coverage is the constant that determines almost everything downstream of it — get it wrong and both the bag count and the total can be off by a large multiple even though every other input is correct, which is exactly why this calculator checks its own output in a unit a bid can be compared against instead of reporting only a total that has nothing to be measured against.
Cellulose, fiberglass, and rockwool at the same target
Loose-fill materials pack to very different densities, so "bags needed" varies by more than the R-per-inch numbers alone suggest. Holding the same 400 sq ft attic and R-30 target constant across all three materials this calculator offers:
| Material | R per inch | Inches needed | Bags needed | Material cost |
|---|---|---|---|---|
| Cellulose | 3.7 | 9 | 26 | $650–$1,040 |
| Fiberglass | 2.5 | 12 | 35 | $1,225–$1,925 |
| Rockwool | 3.3 | 10 | 31 | $1,395–$2,170 |
Computed from this calculator's own rPerInch, bagsPerSqftInch, and bagCost constants. Labor ($200–$400 on this footprint) is flat per sq ft and does not vary by material.
Cellulose needs both the fewest inches and the fewest bags to reach the same R-value here, and it's the cheapest per bag too — which is why it's this calculator's default material for attic work. Fiberglass is lighter and more moisture-resistant, which is the tradeoff for costing roughly twice as much to reach the same target.
Savings, house age, and what the two percentages on this topic actually mean
This calculator's own annual-savings estimate scales with a house-age multiplier — 1.4× for pre-1980 homes, 1.2× for 1980–2000, 1.0× for 2000–2015, 0.7× for post-2015 — on the theory that older, less-insulated homes have more to gain from the same R-value increase. That multiplier is this calculator's own internal estimate, separate from any published percentage.
For general context on what insulation is worth at the household level, ENERGY STAR states: "EPA estimates that homeowners can save an average of 15% on heating and cooling costs (or an average of 11% on total energy costs) by air sealing their homes and adding insulation in attics, floors over crawl spaces and basements." That figure combines air sealing with insulation — it is not this calculator's own per-job savings number, and the two shouldn't be read as the same claim.
What this estimate excludes, and reading a quote against it
This price covers material and blower-crew labor only. It does not include baffle installation to keep soffit vents from being buried, air sealing of top plates and penetrations before the blower starts, or removal of any existing insulation that's damaged or contaminated — that's a different job, priced on the attic insulation replacement calculator.
Because bag count is the number every quote should be built on, ask for it directly: a bid that states only a total price and a target R-value hasn't shown its work, and there's no way to check it against the $1–$4.50 per sq ft range without the bag count and coverage rate behind it.
Methodology
Inches, bag counts, material cost, and labor are computed from this calculator's own rPerInch, bagsPerSqftInch, bagCost, and laborPerSqft constants, cross-checked against the page's own published $1–$4.50 per sq ft band. The EPA/ENERGY STAR savings figure is quoted verbatim and kept separate from this calculator's own house-age savings multiplier.
Sources
- ENERGY STAR — Why Seal and Insulate? — accessed 2026-09-05