Blown-in insulation calculator: how many bags of blown-in insulation do I need?
22 bags
1,000 sq ft at R-38 · 21.1 bags per 1,000 sq ft, 47 sq ft a bag at most · blow 16.2 in, settles to 16.1 in · for a 40 × 25 ft attic floor at R-38
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3/8 or 10' 6".
22 bags
1,000 sq ft at R-38 · 21.1 bags per 1,000 sq ft, 47 sq ft a bag at most · blow 16.2 in, settles to 16.1 in
Source: Johns Manville Attic Protector Blow-in Loose-fill Fiber Glass Insulation, R-Value Fact Sheet, coverage chart, read October 16, 2026. Data sheet (PDF)
A 1,000 sq ft attic insulated to R-38 takes 22 bags of Johns Manville Attic Protector blown-in fiber glass, since its chart asks for at least 21.1 bags per 1,000 sq ft and no more than 47 sq ft per bag, blown 16.2 inches deep.
A worked example
An attic floor 31 ft 6 in by 22 ft 9 in brought to R-49 needs 21 bags. That is 716.6 square feet. The chart asks for 28.4 bags per 1,000 sq ft, 20.35 bags, and no more than 35 sq ft per bag, 20.48 bags; the larger rules, so 21 bags, blown at least 20.3 inches deep.
How many bags of blown-in insulation for R-38?
On the Johns Manville Attic Protector chart, R-38 takes at least 21.1 bags per 1,000 square feet, each covering no more than 47 square feet, blown at least 16.2 inches deep. A 1,500 sq ft attic is 31.7 bags by the first rule and 31.9 by the second, so 32 bags. Another product prints other numbers on its bag, and the calculator takes those in place of these.
What R-value should attic insulation be?
ENERGY STAR, working from the 2021 International Energy Conservation Code, recommends R30 for an uninsulated attic in climate zone 1, R49 in zones 2 and 3 and R60 in zones 4 to 8. Over 3 to 4 inches that are already there, it is R25, R38 and R49. Pick the R-value for your zone and the calculator gives the bags and the depth to blow.
What else it is called
Blown-in insulation is also called loose-fill or blow-in insulation, and blowing wool for fiber glass. The insulation calculator counts batts and rolls instead, by the square feet in a package.
Coverage chart
| R-value | Blow at least | After settling | Bags per 1,000 sq ft | Sq ft per bag, max | Bags for 1,500 sq ft |
|---|---|---|---|---|---|
| R-11 | 5.2 in | 5.1 in | 5.4 | 186 | 9 |
| R-13 | 6.1 in | 6.0 in | 6.4 | 155 | 10 |
| R-19 | 8.7 in | 8.5 in | 9.7 | 103 | 15 |
| R-22 | 9.9 in | 9.8 in | 11.4 | 88 | 18 |
| R-26 | 11.5 in | 11.4 in | 13.7 | 73 | 21 |
| R-30 | 13.1 in | 13.0 in | 16.1 | 62 | 25 |
| R-38 | 16.2 in | 16.1 in | 21.1 | 47 | 32 |
| R-44 | 18.5 in | 18.3 in | 25.0 | 40 | 38 |
| R-49 | 20.3 in | 20.2 in | 28.4 | 35 | 43 |
| R-60 | 24.3 in | 24.1 in | 36.2 | 28 | 55 |
Johns Manville Attic Protector blow-in fiber glass, R-Value Fact Sheet of December 2013. Every loose-fill product prints its own chart on the bag, with its own numbers.
Where the numbers come from
- Coverage chart: Johns Manville, Attic Protector Blow-in Loose-fill Fiber Glass Insulation, R-Value Fact Sheet (December 2013). For each R-value from R-11 to R-60 it gives the minimum installed thickness, the thickness after long-term settling, the minimum bags per 1,000 sq ft and the maximum net coverage per bag; R-38 is at least 16.2 in, 16.1 in settled, 21.1 bags per 1,000 sq ft and 47 sq ft per bag at most. The calculator meets both bag rules and keeps the larger count. Read October 16, 2026. Johns Manville fact sheet (PDF)
- R-value by climate zone: ENERGY STAR, Recommended Home Insulation R-Values, based on the 2021 IECC Table R402.1.3. For an uninsulated attic floor, R30 in zone 1, R49 in zones 2 and 3 and R60 in zones 4 to 8; with 3 to 4 inches already there, R25, R38 and R49. Read October 16, 2026. ENERGY STAR
- Area is geometry: attic length × width, the floor between the outside walls. Bags are the area ÷ 1,000 × bags per 1,000 sq ft, rounded up; with the Johns Manville chart, the area ÷ the coverage per bag too, keeping the larger.
Each loose-fill product, fiber glass or cellulose and each bag weight, has its own chart printed on the bag, and the numbers differ from one to another: type the bags per 1,000 sq ft from yours and the count follows it. Blow to the installed thickness, not the settled one; the settled figure is what the attic keeps after the insulation sinks over the years.
By Javier Miralles · Last updated . How this site is made.
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