Blown-In Insulation Calculator
Your attic
What you need
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How to use the blown-in insulation calculator
- Measure attic area: length × width of the footprint; ignore the roof slope.
- Measure current depth: push a ruler to the drywall in a few spots and average. Old settled insulation still counts toward R-value.
- Pick your target R: R-38 for the southern US, R-49 for most of the north, R-60 for the coldest zones.
- Choose material: cellulose insulates slightly less per inch but costs less per R; fiberglass is lighter and doesn't settle as much.
- Buy the bag count: most stores lend the blower free with a 10+ bag purchase.
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 roughly 17–19 inches of added depth: about 45–50 bags. Fiberglass covers more per bag but insulates slightly less per inch, so counts land close. 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): ~R-2.2 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.
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.
Standards and references
R-value targets in this calculator follow the 2021 IECC (International Energy Conservation Code) climate-zone map: R-49 attics in zones 4-8, R-38 in zones 2-3, R-30 in zone 1. Recommended install depths per bag are based on manufacturer coverage charts required by ASTM C739 (cellulose loose-fill) and ASTM C764 (fiberglass loose-fill), which mandate settled-density labeling. If your local jurisdiction has adopted a newer IECC, use the local minimum instead.
How the bag count is calculated
Existing R-value = current depth (in) × R per inchR-value still needed = target R − existing RAdded depth (in) = R needed ÷ R per inchBags = round up( area × R needed ÷ (R per inch × coverage per bag) )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 4–6, 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 (open-blow) runs about 2.2 per inch settled, demanding closer to 27 inches for the same R-value. The calculator uses settled depth, not installed depth - account for a 20–25% 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 weight per square foot. Owens Corning AttiCat, for example, lists 22.4 bags per 1,000 sq ft at R-38 but that figure assumes an installed depth of 12.25 inches settling to 10.25 inches. Ordering to installed depth without the compression offset leaves you 8–12% short - typically one to two bags per 500 sq ft. Always cross-reference the bag's coverage chart against the installed weight: the IRC requires a minimum of 1.0 lb per sq ft for cellulose at R-38 as a quality floor, independent of depth.
What this calculator does not cover
Bag count alone does not determine project cost. Rental blower machines run $75–$150/day; professional installation adds $0.50–$1.20 per sq ft in labor. The calculator ignores existing insulation depth - if you already have 4 inches of old fiberglass batts (≈R-13), you only need to top up to the code target, not start from zero. Air-sealing materials - typically 2–3 cans of foam at $8–$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.
Frequently Asked Questions
How many bags of insulation for 1,000 sq ft?
To R-49 from bare: ~55 bags of cellulose or ~40 of fiberglass. From existing R-10: roughly 35 and 28. Enter your depth above for the exact count.
What R-value should my attic have?
R-38 in the south, R-49 across most of the north, R-60 in the coldest zones. If you can see your joists, you're under R-19 and any top-up pays back fast.
Can I blow new insulation over old?
Yes: as long as the old material is dry and mold-free. New loose-fill over old batts (or vice versa) is standard practice; the R-values add.
Is blown-in insulation a DIY job?
Very much: two people, a rental blower (free with most 10+ bag purchases), and an afternoon. One feeds the hopper, one holds the hose. Wear a respirator either material.