Blown-In Insulation Calculator
Calculate how many bags of blown-in attic insulation you need at R-30 coverage, based on your attic's square footage, plus total cost.
How this calculator works
Blown-in attic insulation is estimated at about 40 square feet of coverage per bag at an R-30 target, a common attic insulation level. This calculator divides your attic’s floor area by that coverage rate.
Formula: Bags = Attic area (sq ft) ÷ 40 sq ft per bag (at R-30).
Worked example
A 30 ft x 20 ft attic floor (600 sq ft):
- Area: 30 x 20 = 600 sq ft
- Bags: 600 ÷ 40 = 15.00 bags
- At $14/bag, that’s 15.00 × 14 = $210.00.
Coverage reference
| Attic area | Bags needed (40 sq ft/bag at R-30) |
|---|---|
| 150 sq ft | 3.75 bags |
| 600 sq ft | 15.00 bags |
| 2,400 sq ft | 60.00 bags |
How to measure your attic
Measure the attic floor’s length and width where insulation will actually be blown, and check the bag’s printed coverage chart for your target R-value before ordering, since coverage per bag changes with both R-value and product type.
Frequently asked questions
How many bags of blown-in insulation do I need for my attic?
This calculator uses roughly 40 sq ft of coverage per bag at R-30, a common target R-value for attics in moderate climates. Divide your attic's floor area by that coverage rate to get an estimated bag count.
Why does coverage change with R-value?
Bag coverage is inversely tied to R-value — a higher target R-value (like R-49 for cold climates) means blowing the material in deeper, which covers less square footage per bag. Always check the coverage chart printed on your specific bag, since it lists coverage at multiple R-values for that exact product.
Does insulation type change the bag count?
Yes. Cellulose, fiberglass, and mineral wool blown-in products all have different densities and R-value-per-inch ratings, so their bag coverage charts differ even at the same target R-value. This calculator's 40 sq ft/bag figure is a general R-30 estimate — swap in your product's actual chart value for a precise order.
How do I measure my attic for blown-in insulation?
Measure the attic floor's length and width at the level where insulation will be blown, not the roof's footprint. Irregular attics with dormers or hips should be broken into rectangular sections and added together.
Should I remove old insulation before blowing in new material?
Not usually — blown-in insulation is typically added on top of existing insulation to boost R-value, as long as the old material is dry and not contaminated with pests or mold. Only remove existing insulation if it's damaged, wet, or you're addressing an air-sealing issue underneath it first.
Estimates only. Verify quantities with your supplier before purchasing.