How many bags of blown-in insulation do I need?
Loose-fill is sold by the bag, and each bag covers less area the deeper you fill. There are two clean ways to count bags — the coverage chart on the bag, and the board-feet method. Both are on the same page here.
Why bags aren't a flat number
A bag of loose-fill holds a fixed amount of material. Spread it thin (low R) and it covers a lot of floor; pile it deep (high R) and it covers less. So coverage per bag is quoted at a target R, and it drops as R rises. Every bag prints a coverage chart — that chart is your source of truth. Count bags in the blown-in bag calculator or, for an attic, the attic bag / depth calculator.
Method A: coverage-per-bag-at-R (off the printed chart)
bags = ceil(area ÷ coverage-at-R). Read the coverage number on the bag for your target R and divide. Example, a 1,000 ft² attic in blown fiberglass:
| Target R | Coverage per bag | Bags for 1,000 ft² |
|---|---|---|
| R-30 | ~47 ft²/bag | ceil(1000÷47) = 22 bags |
| R-38 | ~37 ft²/bag | ceil(1000÷37) = 28 bags |
| R-49 | ~28 ft²/bag | ceil(1000÷28) = 36 bags |
Same attic, more bags as the target climbs — because you're filling deeper. Full grid in the bags per 1,000 ft² table.
Method B: the board-feet (depth) method
If your bag lists a board-foot yield instead of a coverage chart, work through depth. First the depth: depth = target R ÷ R-per-inch. Then bags = ceil(area × depth ÷ bag yield in board-feet). For 1,000 ft² to R-49 in cellulose (R-3.5/in), a 450 board-foot bag: depth = 49 ÷ 3.5 = 14 in; bags = ceil(1000 × 14 ÷ 450) = ceil(31.1) = 32 bags. The two methods are independent — use whichever number your bag actually prints.
Both self-check to real numbers
1,000 ft² at R-38 by coverage = 28 bags. 1,000 ft² at R-49 by board-feet = 32 bags. Those aren't contradictions — they're different targets and different products. Match the method to the chart on your bag.
Blow it deep — settling is real
Loose-fill settles after install, especially fiberglass and un-stabilized cellulose. If you blow exactly to depth, you'll finish below target in a year. Blow a little over — most bags print a minimum installed thickness and a settled thickness; hit the installed number. Set your rake depth markers before you start.
What to check before you buy
- The coverage chart on the exact bag — brands differ.
- Whether the number is coverage-at-R or a board-foot yield (that decides your method).
- Minimum bag count for the machine to run right — and free blower rental usually needs a minimum purchase.
- Attic square footage measured, not eyeballed; add for dropped soffits and knee walls.
Machine, crew and the day itself
Bag count is half the plan; the blow is the other half. A rented blower needs a two-person crew — one feeding bags and breaking up clumps at the hopper, one on the hose in the attic — and a hopper run dry mid-pass leaves ridges. Most home-center blowers move roughly one bag every couple of minutes once you're rolling, so a 36-bag attic is a solid half-day with setup, not an hour. Buy or rent enough hose to reach the far eaves (add couplers for a long attic), wear a real respirator and eye protection, and stage the bags near the access so you're not hauling them up one at a time.
Depth markers and the settled-vs-installed trap
The single most common miss is finishing below target because the fill settled. Bags print two numbers: installed thickness (what you blow to, fluffed) and settled thickness (where it ends up). Blow to the installed number. Staple depth-marker cards (or mark the trusses) at your target height every few feet before you start, and rake to the cards — they're also what an inspector or appraiser looks for as proof of R. On the coverage chart, the minimum bags per 1,000 ft² and the minimum installed thickness both have to be met; hitting one but not the other means you spread the material too thin.
Dense-pack is a different animal
Blowing loose-fill onto an open attic floor is not the same as dense-packing a closed wall or slope cavity. Dense-pack targets a density (roughly 3.5 lb/ft³ for cellulose) so it won't settle inside the wall, which means far more material per cubic foot than an attic blow — count it by volume (area × depth × density ÷ bag weight), not by the attic coverage chart, or you'll come up badly short. When in doubt for a wall retrofit, price it against the wall insulation calculator and confirm the target density on the bag.
Quick numbers to leave with
- Method A: bags = ceil(area ÷ coverage-at-R). 1,000 ft² at R-38 ≈ 28 bags; at R-49 ≈ 36 bags.
- Method B: depth = target R ÷ R/inch, then bags = ceil(area × depth ÷ bag yield). 1,000 ft² to R-49 cellulose ≈ 32 bags.
- Blow to installed thickness, not settled — set depth-marker cards first.
- Dense-pack walls are counted by density (~3.5 lb/ft³), not the attic coverage chart.
- Read YOUR bag — coverage-at-R varies by brand and product.
Count both ways if you can, take the larger, and round up. A blower left idle mid-job because you ran two bags short is the most common DIY-day killer.