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: Greenfiber SANCTUARY lists 68.5 bags per 1,000 sq ft at R-49, and the exact count changes by brand.
By The Roofing Brief Team. Last reviewed: October 2026.
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, such as 27.4 sq ft per SANCTUARY bag at R-30 on Greenfiber’s product page. 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. ENERGY STAR’s targets for an uninsulated attic run from R-30 in climate zone 1 to R-60 in zones 4 to 8.
- Find the depth. Divide the target R-value by the material R-per-inch. Cellulose runs about R-3.1 to R-3.8 per inch and loose-fill fiberglass about R-2.2 to R-2.9 per inch, per HomeGuide’s 2026 blown-in insulation cost guide.
- 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 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. The math here covers loose-fill only: bags per 1,000 sq ft for attics and pounds per cubic foot for walls. To choose between loose fill and batts first, see our blown-in insulation vs batts comparison.
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 more than twice as many because its bags cover less area. The table below reads the published Owens Corning AttiCat chart against the Greenfiber SANCTUARY coverage chart (25 lb bag, loose-fill attic application), both per 1,000 net square feet.
| Target R-value | AttiCat fiberglass bags / 1,000 sq ft | AttiCat min thickness | SANCTUARY cellulose bags / 1,000 sq ft | SANCTUARY initial depth |
|---|---|---|---|---|
| R-13 | 5.9 | 5.0 in | 13.8 | 3.9 in |
| R-19 | 9.0 | 7.0 in | 20.5 | 5.7 in |
| R-30 | 14.6 | 10.75 in | 36.5 | 8.9 in |
| R-38 | 19.0 | 13.5 in | 50.1 | 11.2 in |
| R-49 | 25.0 | 17.0 in | 68.5 | 14.3 in |
| R-60 | 31.5 | 20.5 in | 83.5 | 17.3 in |
Fiberglass numbers are exact from the Owens Corning AttiCat product data sheet (27.5 lb bag). Cellulose figures are from Greenfiber’s published SANCTUARY chart for a 25 lb bag; other cellulose products list different bag weights and coverage. That spread is the reason you read the specific bag, not a generic online tool. For the full AttiCat coverage chart and machine specs, see our AttiCat review.
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 460.12) requires the chart to show, for R-13, R-19, R-22, R-30, R-38 and R-49, the minimum settled thickness, initial installed thickness, maximum net coverage area, bags per 1,000 sq ft and minimum weight per square foot. Generic online calculators that use one fixed coverage constant skip these product-specific numbers.
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. At R-30, AttiCat lists 14.6 bags at 10.75 inches while SANCTUARY lists 36.5 bags at 8.9 inches, so 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. Worked from the two charts at R-49, AttiCat sits at about 0.49 lb per cubic foot (25 bags of 27.5 lb over 17 inches) and SANCTUARY at about 1.44 lb per cubic foot (68.5 bags of 25 lb over 14.3 inches).
| Property | Loose-fill cellulose | Blown-in fiberglass |
|---|---|---|
| R-value per inch | R-3.1 to R-3.8 (HomeGuide); 3.7 settled at R-49 (SANCTUARY) | R-2.2 to R-2.9 (HomeGuide); 2.9 at R-49 (AttiCat) |
| Installed density at R-49 | About 1.44 lb/cu ft (computed from chart) | About 0.49 lb/cu ft (computed from chart) |
| Depth for R-49 | 14.3 inches initial, 13.3 settled (SANCTUARY) | 17 inches (AttiCat) |
| Settling | Settles; SANCTUARY’s chart allows about 7% at R-49 | Settles minimally |
| Bags for R-49 / 1,000 sq ft | 68.5 (SANCTUARY; varies by product) | 25 (AttiCat) |
| Bag weight | 25 lb (SANCTUARY; varies by brand) | 27.5 lb (AttiCat) |
For a deeper side by side on cost, moisture, and fire behavior, including HomeGuide’s $1.20 to $2.80 installed cost per sq ft for cellulose against $1.00 to $2.80 for fiberglass, 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?
ENERGY STAR recommends R-30 to R-60 for an uninsulated attic, depending on climate zone, and a smaller top-up where 3 to 4 inches already sit on the attic floor. Use the target below as your calculator input, then convert it to depth and bags with the charts above. Once the bags are blown, an attic insulation ruler shows whether you reached that depth.
| Climate zone | Example regions | Uninsulated attic | Adding to 3 to 4 inches of existing insulation |
|---|---|---|---|
| Zone 1 | South Florida, Hawaii | R-30 | R-25 |
| Zones 2 to 3 | Gulf Coast, Southern U.S., coastal California | R-49 | R-38 |
| Zones 4 to 8 | Mid-Atlantic, Midwest, Pacific Northwest, northern U.S., mountain states, Alaska | R-60 | R-49 |
Source: ENERGY STAR recommended home insulation R-values, which ENERGY STAR bases on the 2021 IECC Table R402.1.3. These are retrofit recommendations, not a code minimum for every jurisdiction. Before you blow anything in, seal the gaps: read how to air seal the attic first so the insulation performs at its rated value.
| If your attic has | Blow in to | AttiCat bags / 1,000 sq ft | SANCTUARY bags / 1,000 sq ft |
|---|---|---|---|
| Bare joists in zone 1 | R-30 | 14.6 | 36.5 |
| Bare joists in zones 2 to 3 | R-49 | 25.0 | 68.5 |
| Bare joists in zones 4 to 8 | R-60 | 31.5 | 83.5 |
| Existing R-19 batts, target R-49 (add about R-30) | R-30 layer on top | 14.6 | 36.5 |
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 Greenfiber SANCTUARY cellulose (R-3.7 per inch), the chart calls for 68.5 bags per 1,000 sq ft and 14.3 inches at initial install, settling to 13.3 inches.
- Fiberglass (AttiCat): 25 bags, 17 in deep, about 39.9 sq ft of coverage per bag.
- Cellulose (SANCTUARY): 68.5 bags (buy 69), 14.3 in deep at install, 14.6 sq ft of coverage 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. The Building America Solution Center, run by Pacific Northwest National Laboratory for the U.S. Department of Energy, recommends dense-packing cellulose to 3.5 to 4.5 lb per cubic foot and fiberglass to 1.8 to 2.6 lb per cubic foot. That compares with about 1.44 lb per cubic foot for SANCTUARY loose-fill at R-49 in an attic, so the wall needs two to three times the material per cubic foot.
Because density is so different, an attic bags per 1,000 sq ft number will badly undercount a wall job. Calculate walls by cavity volume: wall area times cavity depth, less the 1.5 inches of every 16 inches taken by studs, times the target density, divided by the 25 lb bag weight.
| If the wall is | Cavity volume per 1,000 sq ft of wall | Cellulose at 3.5 to 4.5 lb/cu ft | 25 lb bags (rounded up) |
|---|---|---|---|
| 2×4 studs at 16 in on center (3.5 in cavity) | 264 cu ft | 925 to 1,189 lb | 38 to 48 |
| 2×6 studs at 16 in on center (5.5 in cavity) | 415 cu ft | 1,454 to 1,869 lb | 59 to 75 |
Dense-pack is usually a job for a pro with the right blower pressure: HomeGuide prices dense-pack cellulose at $1.60 to $4.20 per square foot installed, and an under-packed cavity below 3.5 lb per cubic foot can settle and leave a cold gap at the top.
How much does blown-in insulation cost?
Blown-in insulation costs $1.00 to $2.80 per square foot installed, and blown-in attic jobs $0.90 to $2.40 per square foot or $900 to $3,600 on average, per HomeGuide’s 2026 guide; DIY runs $0.35 to $1.50 per square foot. October 2026 listings put Owens Corning AttiCat at $68.27 per 27.5 lb bag at Lowe’s and Greenfiber SANCTUARY at $17.97 per 25 lb bag at Home Depot, with resellers from $15.00 to $19.99.
| R-49 over 1,000 sq ft (DIY materials) | Bags | Price per bag (Oct 2026) | Material total | Source |
|---|---|---|---|---|
| Owens Corning AttiCat fiberglass | 25 | $68.27 | $1,707 | AttiCat chart; Lowe’s listing |
| Greenfiber SANCTUARY cellulose | 69 (68.5 rounded up) | $17.97 | $1,240 | SANCTUARY chart; Home Depot listing |
| Pro install, any loose-fill | 24 to 26 (HomeGuide) | Included | $1,100 to $1,900 | HomeGuide 2026, R-49 per 1,000 sq ft |
HomeGuide puts blower rental at $100 to $200 per day and notes that some home improvement stores lend the machine free with 10 to 20 or more bags, so a 25 bag AttiCat job or a 69 bag SANCTUARY job usually clears that threshold; confirm the store’s current minimum before you buy. For the full breakdown of pro versus DIY pricing and where the labor goes, including HomeGuide’s $0.70 to $1.70 per square foot labor rate, 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 Greenfiber SANCTUARY cellulose (25 lb bags) needs 68.5. For R-38, plan on about 19 AttiCat bags or about 50 SANCTUARY 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 AttiCat blown-in fiberglass or about 14.3 inches of Greenfiber SANCTUARY cellulose at initial install. In bags per 1,000 sq ft, that is 25 bags of AttiCat or 68.5 bags of SANCTUARY. 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-3.1 to R-3.8 per inch, loose-fill fiberglass about R-2.2 to R-2.9 (HomeGuide). Meet both the minimum bag count and the minimum thickness on the bag chart.
Does blown-in cellulose settle?
Yes, loose-fill cellulose settles after installation; Greenfiber’s SANCTUARY chart lists R-49 at 14.3 inches initial and 13.3 inches settled. 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, so R-49 takes 14.3 inches of SANCTUARY against 17 inches of AttiCat, but it is heavier (about 1.44 lb per cubic foot against 0.49) and settles to 13.3 inches. Blown-in fiberglass is lighter and settles minimally but needs a taller layer. On October 2026 bag prices, SANCTUARY materials for R-49 over 1,000 sq ft cost about $1,240 against $1,707 for AttiCat.
Can I use an attic calculator to figure out wall insulation?
No, walls need dense-pack: the DOE Building America Solution Center recommends 3.5 to 4.5 lb per cubic foot for cellulose, two to three times loose-fill attic density, 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.