Quantity · Blown-in insulation
Blown In Insulation Calculator
Size open-attic blown fiberglass or cellulose from area, existing R, and target R. Bags come from settled bag charts — not batt packs or spray-foam board feet. For climate-zone attic R tips across all product types, use the general insulation calculator.
Blown-in details
Heavier fiber; fewer bags than fiberglass at the same R on many charts. ~25 lb bags · R ≈ 3.2–3.8/in.
Bags to buy
41bags
Add R-30 · Blown cellulose
- Settled depth
- 8.3 in
- Coverage
- 26 sq ft/bag
- Bags / 1,000 sq ft
- 40.4
- Rough installed
- $1,200–$2,300
25 lb bag chart · ~26 sq ft/bag at R-30 settled
- R to blow = target − existing = 30
- Look up settled inches and sq ft/bag on the product chart
- Bags = ceil(area ÷ coverage × (1 + 5%))
Price data updated . National US ranges from published supplier and contractor averages. Not a bid. Local quotes vary.
How blown-in bag counts are calculated
You only buy the R gap. The calculator interpolates manufacturer-style open-attic charts for settled inches and square feet per bag, then applies overblow. Dense-pack walls and closed cavities need higher density charts — not this tool.
r_needed = max(0, r_target − r_existing) settled_in = chart_inches(r_needed) // open attic, settled sq_ft_per_bag = chart_coverage(r_needed) bags = ceil(area ÷ sq_ft_per_bag × (1 + overblow %))
Worked example. A 1,000 sq ft attic with R-19 today targeting R-49 needs R-30 more. On the cellulose chart that is about 8.3 in settled and ~26 sq ft per 25 lb bag → roughly 38–39 bags before overblow, or about 41 bags at 5% waste. Fiberglass for the same R-30 gap is closer to 10.5 in and ~24 bags after waste — more depth, fewer bags on many fiberglass charts.
Open-attic bags per 1,000 sq ft by settled R
Planning bag counts before overblow for a full blown layer at the listed R (existing = 0). Top-ups use the R gap instead of the full target. Always confirm the chart printed on your bags and blower settings.
| Settled R | FG inches | FG bags | Cellulose in | Cellulose bags |
|---|---|---|---|---|
| R-19 | 7.0" | 14 | 5.5" | 24 |
| R-30 | 10.5" | 22 | 8.3" | 38 |
| R-38 | 13.0" | 29 | 10.4" | 48 |
| R-49 | 17.0" | 37 | 13.3" | 63 |
| R-60 | 20.5" | 45 | 16.3" | 77 |
- FG ≈ 32 lb blown fiberglass bags; cellulose ≈ 25 lb bags — common US DIY sizes.
- Bag counts = 1,000 ÷ sq ft per bag, rounded to the nearest bag. Add ~5% overblow for irregular bays.
- Not for dense-pack walls, floors, or spray foam.
Sq ft per bag at common attic R targets
Coverage falls as R (and settled depth) rises. Interpolate between rows in the calculator when your R gap is between chart points.
| Settled R | Fiberglass sq ft/bag | Cellulose sq ft/bag |
|---|---|---|
| R-19 | 70 | 41 |
| R-30 | 45 | 26 |
| R-38 | 35 | 21 |
| R-49 | 27 | 16 |
| R-60 | 22 | 13 |
Blown In Insulation calculator FAQ
- How many bags of blown-in insulation do I need?
- Find the R you still need (target minus existing), look up square feet per bag on the fiberglass or cellulose chart, then divide attic area by that coverage and add a little overblow. This calculator does that math and rounds up to whole bags.
- Blown fiberglass vs blown cellulose — which uses fewer bags?
- On many open-attic charts, fiberglass covers more square feet per bag at the same R, so you buy fewer bags — but you need more settled inches. Cellulose is denser and thinner for the same R. Compare both products in the calculator.
- Does this work for wall dense-pack?
- No. These charts are open-attic settled coverage. For dense-pack cellulose walls (volume × lb/ft³), use the cellulose insulation calculator.
- Can I blow over existing insulation?
- Usually yes in an attic — enter your current R as existing and blow only the gap to target. Do not bury knob-and-tube, and keep clearances at recessed lights and flues per code.
- Is the dollar range a contractor bid?
- No — it is a rough installed band from national price rows for blown fiberglass or cellulose. For a full attic job with air sealing, baffles, and removal, use the attic insulation cost calculator. Bag counts are the primary output here.
- Where do I get climate-zone attic R tips?
- Use the general insulation calculator — it maps US state to a DOE-style attic R tip and also covers batts and spray foam. This page stays focused on blown bag charts.
Related tools
Cellulose calculator (attic + dense-pack) · Attic insulation calculator (depth → R) · Attic insulation cost calculator · All insulation calculators