Blown-In Insulation Calculator: Bags, Depth and Cost

How many bags of blown-in insulation do I need?

A 1,200 sq ft attic going from 4 inches of old insulation to R-49 with cellulose needs about 9.6 inches of added depth: 71 bags. The same job in blown fiberglass needs about 15.6 inches but only 24 bags, because a fiberglass bag is lighter and fluffier and covers far more area. This calculator credits what you already have, converts the R-gap to inches for your material, and rounds up to whole bags.

Cellulose vs. blown fiberglass

Cellulose (recycled paper, borate-treated): about R-3.6 per inch, denser, better air-sealing, cheaper per R, settles ~15% over time (installers overblow to compensate). Fiberglass: ~R-2.5 per inch, lighter on ceilings, no settling to speak of, slightly pricier per R. Both are DIY-friendly with a rental blower; either beats the bare joists you have now by a country mile.

Before you blow: the 90-minute prep that doubles the payoff

Air-seal first: foam the top-plate gaps, wire penetrations, and around flues (with fire-rated sealant); insulation slows heat but doesn't stop airflow, and attic bypasses are where the heating bill actually leaks. Build dams around recessed lights not rated IC, and baffle the soffit vents so the new depth doesn't choke ventilation.

What the Bags Actually Cost

Take the worked example from above: 1,200 sq ft going from 4 inches of old insulation to R-49 cellulose needs 71 bags. At a typical $12 to $15 per bag, materials run $852 to $1,065. Add a blower rental at $75 to $150 per day if you do not own one, though Home Depot and Lowe's commonly waive the rental fee when you buy 20 or more bags, which this job clears easily. Total DIY spend usually lands between $927 and $1,215 for a job this size.

Cross-Check Against the Bag Label

Calculators round differently than manufacturers. Before you buy, check the coverage chart printed on the bag itself.

For blown fiberglass, the Owens Corning AttiCat chart lists about 25 bags per 1,000 sq ft at R-49 (about 40 sq ft per bag). Charts are for new insulation over bare joists. Sources: Greenfiber INS541LD coverage chart, checked September 2026.

These figures vary by bag lot and settled depth, so if the label and this calculator disagree, trust the label.

Walls Are a Different Job

This calculator sizes loose fill attic blowing, not wall dense pack. Dense packed cellulose in a standard 2x4 wall cavity is installed at roughly 3.5 lb per cubic foot, close to 1 bag per 15 to 20 sq ft of wall, and needs the blower set to a higher pressure than an open attic run. Do not use the attic bag count for a wall job.

Payback on the Upgrade

For the same 1,200 sq ft attic, going from about R-11 to R-49 in a colder climate zone costs roughly $700 to $900 DIY and typically saves $150 to $250 a year in heating, a 3 to 6 year payback. Air sealing the attic floor before you blow in insulation is what makes that payback realistic: a well sealed attic holds its R-value, a leaky one loses much of it to airflow no matter how many bags you add.

Prep Shopping List

• Rafter baffles for airflow: about 50 pieces, $60 to $100
• Spray foam and caulk for air sealing top plates and penetrations: $40 to $80
• Cardboard depth rulers: usually free with bag bundles

Budget under $200 for prep before the first bag goes in. For the cost side of a full attic conversion, see the loft attic conversion cost calculator, and check ceiling joist capacity with the joist span calculator before adding weight. More builder tools are in construction.

Related tools

Converting the attic instead of insulating it? Run the loft conversion cost calculator and check whether your ceiling joists can carry a floor. Pricing insulation jobs for clients: job costing calculator.

How the bag count is calculated

R-value requirements by climate zone

The 2021 IRC (Table N1102.1.2) mandates attic insulation minimums that range from R-30 in Zone 2 to R-60 in Zone 7 and 8. Most US homeowners fall in Zones 4 to 6, where the floor target is R-49 to R-60. Blown cellulose delivers approximately 3.7 per inch; reaching R-60 requires roughly 16.2 inches of settled depth. Blown fiberglass runs about R-2.5 per inch, so it needs about 24 inches for the same R-value. The calculator uses settled depth, not installed depth, so account for a 20 to 25% compression factor when setting depth ruler stakes before blowing.

Settled depth vs. installed depth: the measurement error that wastes bags

Manufacturers publish coverage charts at a specific settled depth and a minimum number of bags per 1,000 sq ft. You need to hit both numbers to get the stated R-value: blowing to the right depth with too few bags leaves the insulation too loose, and it settles below the target. Always check the chart printed on your bag and use the bag count, not just a depth ruler.

What this calculator does not cover

Bag count alone does not determine project cost. Rental blower machines run $75 to $150/day; professional installation adds $0.50 to $1.20 per sq ft in labor. The calculator credits the insulation you already have, but it assumes the old layer is the same material you pick. Old fiberglass batts are about R-3.1 to R-3.4 per inch, so check the label if you can find one. Air-sealing materials, typically 2 to 3 cans of foam at $8 to $12 each per 1,000 sq ft, are outside scope. For full project budget planning, pair this tool with the job costing calculator to roll labor, materials, and equipment into a single number.