Insulation Calculator

Measure attic floor length × width for square footage. Choose target R-value — R-38 needs about 12 inches of blown fiberglass. Divide area by coverage per bag (roughly 28 sq ft per bag at R-38 for blown fiberglass). Our calculator handles depth, bags, and cost automatically.

Most US homes should have R-38 to R-49 in the attic per DOE climate zone guidelines. Northern states (zones 5–7) benefit from R-49 to R-60. Southern attics (zones 1–3) often target R-30 to R-38. Check your zone at energy.gov.

R-38 blown fiberglass is approximately 12 inches deep. R-30 is 9 inches, R-49 is 15 inches, and R-60 is 18 inches. Depth varies slightly by product — always verify the label on your specific insulation bags.

Cellulose has higher R-value per inch, better air sealing at proper density, and uses recycled content. Fiberglass is lighter, non-settling when installed correctly, and does not absorb moisture as readily. Both perform well in attics when installed to manufacturer depth.

Yes — home improvement stores rent insulation blowers for $50–$100 per day. Two people are ideal: one feeds the hopper, one works the hose in the attic. Wear N95 respirator, goggles, long sleeves, and watch for recessed lights — use IC-rated covers or dams.

Professional blown attic insulation costs $1.20–$2.00 per sq ft for R-38 to R-49 in 2026. A 1,000 sq ft attic runs $1,200–$2,000 installed. DIY material cost is roughly half. Our calculator uses $1.20–$1.40/sq ft material at R-38 depending on type.

Usually no — add new blown insulation on top of existing fiberglass if it is dry and not moldy. Remove wet, moldy, or pest-contaminated insulation first. Do not install new material over vermiculite insulation from pre-1990 homes without asbestos testing.

Unfaced fiberglass batts can be laid between attic joists or cross-layered for higher R-value, but blown insulation fills gaps around joists and obstructions better. Batts are best for walls and floors where cavities are open during construction.