A blow-in insulation calculator tells you how many bags of loose-fill you need to hit a target R-value, and the honest answer starts with the coverage chart printed on the bag you buy. Multiply your square footage by the bags-per-1,000-sq-ft figure for your R-value, then confirm you can reach the minimum installed thickness. For Owens Corning AttiCat fiberglass at R-49, that is 25 bags per 1,000 sq ft at 17 inches deep. Loose-fill cellulose needs far more bags for the same R-value, and the exact count changes by brand.
How do you calculate blown-in insulation?
You calculate blown-in insulation in two moves: convert your target R-value into a depth in inches, then convert your floor area into bags using the bag’s coverage rating. Skip either check and you can install the right number of bags too thin, or the right depth with too few bags, and miss the rated R-value.
- Measure the area. Multiply the attic length by width in feet to get square feet. For an irregular attic, add the rectangles.
- Pick your target R-value. Most attics land between R-38 and R-60 depending on climate zone.
- Find the depth. Divide the target R-value by the material R-per-inch. Cellulose runs about R-3.2 to R-3.8 per inch, loose-fill fiberglass about R-2.2 to R-2.9 per inch, per the U.S. Department of Energy.
- Find the bag count. Divide your area by the bag’s maximum net coverage at that R-value, or read the bags-per-1,000-sq-ft line straight off the chart.
- Check both minimums. The bag lists a minimum bag count and a minimum thickness. You must meet both.
If you want the general attic sizing job across every material, including batts and spray foam, use our attic insulation calculator. This page stays focused on loose-fill blow-in math for attics and walls.
How many bags of blown-in insulation do I need?
For a 1,000 sq ft area, blown-in fiberglass needs roughly 15 bags at R-30 and 25 bags at R-49, while loose-fill cellulose needs about twice as many because its bags cover less area. The table below reads the published Owens Corning AttiCat chart against a representative Greenfiber cellulose chart, both per 1,000 net square feet.
| Target R-value | AttiCat fiberglass bags / 1,000 sq ft | AttiCat min thickness | Cellulose bags / 1,000 sq ft (typical) | Cellulose depth |
|---|---|---|---|---|
| R-13 | 5.9 | 5.0 in | ~11 | ~3.75 in |
| R-19 | 9.0 | 7.0 in | ~16 | ~5.5 in |
| R-30 | 14.6 | 10.75 in | ~27 | ~8.75 in |
| R-38 | 19.0 | 13.5 in | ~35 | ~11 in |
| R-49 | 25.0 | 17.0 in | ~48 | ~14 in |
| R-60 | 31.5 | 20.5 in | ~60 | ~17 in |
Fiberglass numbers are exact from the Owens Corning AttiCat product data sheet (27.5 lb bag). Cellulose figures are representative and vary widely by product: lighter 19 lb cellulose bags can run 70 to 90 bags per 1,000 sq ft at R-49, roughly double a heavier product. That spread is the reason you read the specific bag, not a generic online tool.
Read the coverage chart on the bag, not a generic estimate
Every bag of loose-fill insulation sold in the United States carries a coverage chart by federal law, and it is the most accurate calculator you will find. The FTC R-Value Rule (16 CFR Part 460) requires each bag to print bags per 1,000 sq ft, maximum net coverage per bag, minimum installed thickness, and minimum weight per square foot for each R-value. Generic online calculators that use one fixed coverage constant ignore this and can be off by 20 to 40 percent.
The chart gives you two hard limits that both matter. The minimum bag count sets the material mass, and the minimum thickness sets the depth. Hit the bag count but rake the pile too thin and you fall short of the rated R-value. The Owens Corning data sheet states it plainly: failing to provide both the required bags and at least the minimum thickness results in lower R-value.
Loose-fill fiberglass and cellulose are not interchangeable on the chart. A cellulose bag and a fiberglass bag at the same R-value list different depths, different bag counts, and different coverage. Always match the chart to the exact product on the pallet.
Blown-in R-value per inch: cellulose vs fiberglass
Cellulose delivers more R-value per inch than loose-fill fiberglass, so it hits a target R-value in a shallower layer but with more bags by weight. Fiberglass is lighter and fluffier, so it needs a taller pile for the same R-value but fewer, larger bags. The table compares the two on the numbers that drive your count.
| Property | Loose-fill cellulose | Blown-in fiberglass |
|---|---|---|
| R-value per inch | R-3.2 to R-3.8 | R-2.2 to R-2.9 |
| Depth for R-49 | ~14 inches | ~17 inches |
| Settling | Settles about 15 to 20 percent | Settles minimally |
| Bags for R-49 / 1,000 sq ft | ~48 (varies by product) | 25 (AttiCat) |
| Typical bag weight | ~19 to 30 lb | ~27.5 lb |
For a deeper side by side on cost, moisture, and fire behavior, see our guide on cellulose vs fiberglass attic insulation. For per-inch numbers across every insulation type, the insulation R-value chart lays them out by material.
What R-value should you blow in?
Most U.S. attics target R-38 to R-60, with the exact number set by climate zone. The Department of Energy and ENERGY STAR recommend higher R-values in colder zones. Use the target below as your calculator input, then convert it to depth and bags with the charts above.
| Climate zone | Example regions | Recommended attic R-value |
|---|---|---|
| Zones 1 to 2 | South Florida, South Texas, Hawaii | R-30 to R-49 |
| Zone 3 | Southern U.S., coastal California | R-30 to R-49 |
| Zones 4 to 5 | Mid-Atlantic, Midwest, Pacific Northwest | R-38 to R-49 |
| Zones 6 to 8 | Northern U.S., mountain states, Alaska | R-49 to R-60 |
These are recommendations for the finished attic, not a code minimum for every jurisdiction, which can differ. Before you blow anything in, seal the gaps: read how to air seal the attic first so the insulation performs at its rated value.
Worked example: R-49 in a 1,000 sq ft attic
Take a 1,000 sq ft attic being brought to R-49 over bare joists. With Owens Corning AttiCat fiberglass, the chart calls for 25 bags installed to a minimum 17 inches. With loose-fill cellulose at roughly R-3.5 per inch, you reach R-49 at about 14 inches, and a typical product runs near 48 bags per 1,000 sq ft.
- Fiberglass (AttiCat): 25 bags, 17 in deep, about 39.9 sq ft of coverage per bag.
- Cellulose (typical): ~48 bags, 14 in deep, roughly 20 to 21 sq ft per bag.
- If joists already hold R-19: you only add the difference, about R-30, which drops the fiberglass count to roughly 15 bags.
Always round bags up. A partial bag left short of the chart minimum leaves you under the rated R-value across the whole attic.
Can you use a blow-in calculator for walls?
Yes, but wall cavities need dense-pack, not the loose-fill attic settings, and the material count roughly doubles per cubic foot. Attic loose-fill sits at about 1.0 to 1.5 lb per cubic foot, while a dense-packed wall cavity is packed to about 3.2 to 3.5 lb per cubic foot to stop settling and air movement inside the wall.
Because density is so different, an attic bags-per-1,000-sq-ft number will badly undercount a wall job. For walls, calculate by cavity volume in cubic feet, then multiply by the target density, then divide by the bag weight. Dense-pack is usually a job for a pro with the right blower pressure, since under-packed cavities settle and leave a cold gap at the top.
How much does blown-in insulation cost?
Blown-in insulation often runs about $0.60 to $1.80 per square foot installed, or roughly $0.30 to $0.60 per square foot in materials for a DIY attic. Cellulose bags commonly cost about $8 to $12 each, and expanding fiberglass bags such as AttiCat about $40 to $50 each, though the fiberglass bag covers far more area.
Many home centers rent the blower free with a minimum purchase, often around 10 bags of cellulose or 20 bags of AttiCat, which favors a DIY attic if you can meet the threshold. For the full breakdown of pro versus DIY pricing and where the labor goes, see our blown-in insulation cost and DIY vs pro guide.
Frequently asked questions
How many bags of blown-in insulation do I need for 1,000 square feet?
It depends on R-value and material. For R-49, Owens Corning AttiCat fiberglass needs 25 bags per 1,000 sq ft, while typical loose-fill cellulose needs about 48, and some lighter cellulose products need 70 to 90. For R-38, plan on about 19 fiberglass bags or roughly 35 cellulose bags. Always confirm against the chart on the exact bag you buy.
How much blown-in insulation do I need for R-49?
R-49 needs about 17 inches of blown-in fiberglass or about 14 inches of cellulose. In bags per 1,000 sq ft, that is 25 bags of Owens Corning AttiCat fiberglass or roughly 48 bags of a typical cellulose product. Install to at least the chart minimum thickness, not just the bag count, or you fall short of R-49.
How do I calculate blown-in insulation?
Divide your target R-value by the material R-per-inch to get depth, then divide your square footage by the bag’s maximum net coverage at that R-value to get bags. Cellulose is about R-3.2 to R-3.8 per inch, loose-fill fiberglass about R-2.2 to R-2.9. Meet both the minimum bag count and the minimum thickness on the bag chart.
Does blown-in cellulose settle?
Yes, loose-fill cellulose settles about 15 to 20 percent after installation, while blown-in fiberglass settles minimally. The coverage chart already accounts for this by listing a minimum initial installed thickness. If you install cellulose only to its final settled depth, it will drop below that and lose R-value, so install to the chart minimum.
Is cellulose or fiberglass better for blown-in attic insulation?
Cellulose gives more R-value per inch and better air resistance, so it performs well in a shallower layer, but it is heavier and settles. Blown-in fiberglass is lighter, does not settle much, and resists moisture, but needs a taller layer for the same R-value. The right pick depends on ceiling load, budget, and climate.
Can I use an attic calculator to figure out wall insulation?
No, walls need dense-pack at about 3.2 to 3.5 lb per cubic foot, roughly double the density of loose-fill attic insulation, so an attic bag count undercounts a wall job. Calculate walls by cavity volume times target density, then divide by bag weight. Dense-pack usually needs a pro blower to reach the pressure that prevents settling.
Reviewed by The Roofing Brief Team. Last reviewed July 2026.