From R-value gap to bag count to installed price — price the whole job in 60 seconds.
Quick answer
Blown-in attic insulation costs $1.50–$2.50 per square foot installed for fiberglass and $1.75–$2.75 for cellulose in 2026 — a typical 1,000 sq ft attic runs $1,500–$2,750. Your bag count comes from the R-value gap: divide the R-value you need to add by 2.5 (fiberglass) or 3.5 (cellulose) for inches, then bags = attic sq ft × inches ÷ ~48 (fiberglass) or ~40 (cellulose).
(Target R − Current R) ÷ R-per-inch = Inches → Bags → Cost
Inches needed = your R gap ÷ 2.5 (fiberglass) or 3.5 (cellulose). Bags = attic sq ft × inches ÷ coverage per bag (~48 fiberglass, ~40 cellulose), rounded up. Installed cost = sq ft × $1.50–$2.50 (fiberglass) or $1.75–$2.75 (cellulose).
Air sealing is separate
The estimate covers blown material + install labor. Budget $300–$800 extra for attic-floor air sealing — skipping it is the #1 blown-in regret.
1,000 sq ft attic, R-19 → R-49, cellulose:
30 R ÷ 3.5 = 8.6 inches
1,000 × 8.6 ÷ 40 = 215 bags
1,000 × $2.25 = $2,250
Range: $1,750 – $2,750 installed.
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Blown-in insulation is the highest-ROI project in home improvement — the DOE estimates proper attic insulation pays for itself in 2–4 years through lower heating and cooling bills. Yet most quotes hide the two numbers that matter: how many inches you're actually getting, and how many bags that means.
This calculator starts from your R-value gap and works forward: inches needed, bags required, and an installed price range for fiberglass or cellulose. Bring the bag count to the quote table and watch the conversation change.
| Material | R/inch | Installed $/sq ft | 1,000 sq ft attic |
|---|---|---|---|
| Blown fiberglass | R-2.5 | $1.50–$2.50 | $1,500–$2,500 |
| Blown cellulose | R-3.5 | $1.75–$2.75 | $1,750–$2,750 |
| Air sealing adder | — | — | +$300–$800 |
| Old insulation removal | — | $1–$2/sq ft | +$1,000–$2,000 |
2026 installed ranges. Sources: pickhvac.com, bhumicalculator.com, dwellia.ah-studio.com.
1,000 sq ft attic, current R-19, target R-49, blown cellulose:
215 bags of cellulose, ~$2,250 installed — the single cheapest way to cut that home's heating bill.
Enter your attic's square footage, its current R-value (check the depth: inches × R-per-inch of what's there), your target R-value (R-49 is the common retrofit goal), and the material. The calculator converts your R-value gap into inches, inches into bags, and bags into an installed price range.
Added inches = (target R − current R) ÷ R-per-inch (2.5 fiberglass, 3.5 cellulose). Bags = attic sq ft × inches ÷ bag coverage (~48 sq-ft·inches fiberglass, ~40 cellulose), rounded up. Installed cost = sq ft × material rate: $1.50–$2.50 fiberglass, $1.75–$2.75 cellulose (2026).
Depth is everything — going from R-19 to R-49 needs roughly twice the material of R-30 to R-49. Access matters too: low-pitch attics and scuttle-only entry slow the crew. And the quote-trap to watch: air sealing and baffles are often excluded from the per-square-foot price but essential to the result.
1,000 sq ft attic, R-19 to R-49, cellulose: 30 R ÷ 3.5 = 8.6 inches; 1,000 × 8.6 ÷ 40 = 215 bags; installed at $2.25/sq ft = $2,250, ranging $1,750–$2,750. Same job in fiberglass: 12 inches, 250 bags, $2,000 mid at $2.00/sq ft.
Cellulose packs tighter around wires and joists, hits target R with fewer inches, and is mostly recycled content — but it must stay dry, and it settles a few percent over the first years (pros overblow to compensate). Fiberglass is hydrophobic and holds depth for decades, but needs ~40% more inches for the same R-value, which means more bags and blower time. Cost per finished R is nearly a wash.
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