Calculate how many bags of blown-in insulation you need from attic area, R-value, and material, plus depth, cost, and added ceiling weight.
Blown-In Insulation Formula
The calculator works from the settled depth required to hit your target R-value:
D = (Rt - Re) / RPI
Bags = (A * D/12) / V
Di = D / (1 - S)
W = p * D/12
Variables:
- D is the settled depth of new insulation needed (inches)
- Rt is the target R-value and Re is the R-value of any existing insulation (Re = existing depth * its R per inch)
- RPI is the R-value per inch of the new material, settled (about 3.5 for blown cellulose, 2.5 for blown fiberglass)
- A is the attic area (square feet) and V is the coverage of one bag (cubic feet), so Bags is rounded up to whole bags
- Di is the initial installed depth before settling, and S is the settling allowance as a decimal (cellulose settles roughly 15-20%, blown fiberglass 2-5%)
- W is the added ceiling load in pounds per square foot, where p is the settled density of the material (about 1.6 lb/cu ft for cellulose, 0.55 for blown fiberglass)
The default mode applies all four formulas: enter your attic area, material, and target R-value and it returns the settled depth, the deeper initial blow depth, the bag count, the material cost, and the weight the insulation adds to your ceiling. Check the top-up option to subtract the R-value of insulation already in place. The second mode returns only the depth needed for a target R-value, and the third mode works backwards, converting a measured depth of existing insulation into its approximate R-value and the extra depth needed to reach R-38, R-49, or R-60.
Depth, Bags, and Code Targets by R-Value
The table below shows the settled depth, typical initial blow depth, and approximate bags per 1,000 square feet for the two common blown-in materials. Bag counts assume roughly 16 cu ft of settled coverage per 25 lb cellulose bag and 24 cu ft per blown fiberglass bag; always confirm against the chart printed on your bag. Under the 2021 IECC, Zone 1 attics require R-30, Zones 2-3 require R-49, and Zones 4-8 require R-60; many states still enforce the older R-38/R-49 targets.
| Target | Cellulose settled / initial depth | Fiberglass settled / initial depth | Cellulose bags per 1,000 sq ft | Fiberglass bags per 1,000 sq ft |
|---|---|---|---|---|
| R-30 | 8.6 in / 10.5 in | 12.0 in / 12.5 in | ~45 | ~42 |
| R-38 | 10.9 in / 13.2 in | 15.2 in / 15.8 in | ~57 | ~53 |
| R-49 | 14.0 in / 17.1 in | 19.6 in / 20.4 in | ~72 | ~68 |
| R-60 | 17.1 in / 20.9 in | 24.0 in / 25.0 in | ~89 | ~83 |
Blown-in insulation also adds real weight to the drywall below it, something most bag-count estimates ignore. Cellulose is roughly three times denser than blown fiberglass, and at R-60 a cellulose layer approaches the load limit commonly cited for 1/2 in drywall on joists spaced 24 in on center (about 1.3 lb/sq ft, versus about 2.2 lb/sq ft for 1/2 in drywall on 16 in centers or 5/8 in drywall). The calculator reports this load for your exact inputs.
| Target | Cellulose ceiling load (lb/sq ft) | Fiberglass ceiling load (lb/sq ft) | OK for 1/2 in drywall, 24 in centers? |
|---|---|---|---|
| R-30 | 1.14 | 0.55 | Yes (both) |
| R-38 | 1.45 | 0.70 | Fiberglass yes; cellulose borderline |
| R-49 | 1.87 | 0.90 | Fiberglass yes; cellulose over limit |
| R-60 | 2.29 | 1.10 | Fiberglass yes; cellulose needs 16 in centers or 5/8 in board |
Example Problems
Example 1: A 1,200 sq ft attic with bare joists needs R-49 using blown cellulose. The settled depth is 49 / 3.5 = 14.0 inches, and with an 18% settling allowance the initial blow depth is 14.0 / 0.82 = 17.1 inches. The volume is 1,200 * 14/12 = 1,400 cu ft, so at 16 cu ft per bag you need 1,400 / 16 = 87.5, rounded up to 88 bags. At $14 per bag the material cost is about $1,232, and the settled layer adds 1.6 * 14/12 = 1.87 lb/sq ft to the ceiling.
Example 2: A 1,000 sq ft attic already has 4 inches of blown fiberglass (4 * 2.5 = R-10) and the owner wants R-38 using cellulose on top. The R-value to add is 38 – 10 = 28, so the settled depth is 28 / 3.5 = 8.0 inches, blown to about 8.0 / 0.82 = 9.8 inches initially. The volume is 1,000 * 8/12 = 667 cu ft, which is 667 / 16 = 41.7, rounded up to 42 bags. That order also clears the free blower rental minimums at both major home centers.
Blown-In Insulation FAQ
Can you blow cellulose over existing fiberglass? Yes. Blown cellulose can go directly on top of existing blown or batt insulation as long as the old material is dry and mold-free. Use the top-up option in the calculator to credit the existing layer’s R-value and only buy bags for the difference. Remove and replace the old insulation instead if it has been wet, is heavily contaminated by rodents, or is compressed and matted flat.
Why is the initial depth deeper than the settled depth? Loose-fill insulation compacts under its own weight for months after installation. Cellulose loses roughly 15-20% of its height and blown fiberglass 2-5%, and the R-value on the bag is rated at settled depth. If you blow only to the settled depth, the finished attic ends up below the target R-value, so installers overfill by the settling allowance and mark the initial height on depth rulers stapled to the rafters.
Do you need to air-seal before blowing insulation? Yes, and it is the most commonly skipped step. Loose fill slows heat conduction but does not stop air leaking through top-plate gaps, recessed lights, and pipe penetrations, and the Department of Energy estimates air-sealing adds 15-30% in heating and cooling savings beyond the insulation itself. Seal those openings with canned foam and covers, install soffit baffles so the new material cannot block ventilation, and then blow the insulation.
