Blown-In Insulation Calculator
How many bags of blown-in insulation do I need? Calculate cellulose, fiberglass, or mineral wool for attics, walls, and crawl spaces. Get bag count, thickness, and cost estimates.
Area Dimensions
Length (ft)
Width (ft)
Target R-Value
R-38 is common for attics in zones 4-5
Material Type
Application Area
Ready to calculate
Enter your area dimensions
Last updated June 8, 2026 by our expert review team
Blown-In Material Comparison
Cellulose (Blown)
R-3.7/inch
$12/bag (25 lbs)
Fiberglass (Blown)
R-2.5/inch
$30/bag (30 lbs)
Mineral Wool (Blown)
R-3.3/inch
$35/bag (30 lbs)
Expert Contributors
Hawkin
Certified Cost & Estimating Professional
AACE-certified estimator working with 20+ insulation companies including the two largest franchises in America.
See full profileEhsan Ghazanfari
Licensed Structural Engineer
FISE-certified structural engineer with 11+ years designing bridges, retaining walls, and foundations. MSc from Aalto University.
See full profileUpdated June 2026
How we verify our calculators
R-Value per Inch by Material
R/inchHigher R-value per inch means less thickness needed to reach the same insulation level.
Pro Tips
Cellulose is 75% recycled newsprint and the most cost-effective blown-in option. It also provides better air sealing than fiberglass because it packs tightly into gaps and irregular spaces.
Rent a blower machine from Home Depot or Lowe's for free with a purchase of 20 or more bags. This saves $50 to $150 compared to renting separately.
For a side-by-side comparison of batt and blown insulation methods, use our attic insulation calculator to see which approach fits your attic best.
Seal all air leaks (plumbing, wiring, recessed lights) before blowing. Air sealing alone saves 10 to 20% on energy bills and prevents warm air from bypassing your new insulation.
For a cellulose-specific deep dive with settling calculations and density recommendations, try our cellulose insulation calculator.
Wall cavities need dense-pack technique (3.5 lbs/cu ft) to prevent settling. Standard attic blow is 1.5 lbs/cu ft. Using attic density in walls leads to voids within a few years.
How the Calculator Works
The calculator subtracts your existing insulation R-value from your target to find the additional R-value needed. It then divides that by the material's R-value per inch to estimate installed depth.
Bag count is area times the material coverage rate, depth, application multiplier, and waste factor. Attic loose-fill uses the base rate. Wall dense-pack uses more material because it is packed tighter to limit settling.
The DIY cost reflects bags only. The professional estimate uses a higher square-foot rate for wall dense-pack than for open attic blowing because walls require drilling, patching, and density control.
Formula
Additional R = Target R - Existing R
Thickness = Additional R / R per inch
Bags = Area x Coverage Rate x Thickness x Application Factor x (1 + Waste%)
Quick Reference
- Cellulose R/inch
- R-3.7
- Fiberglass R/inch
- R-2.5
- Mineral wool R/inch
- R-3.3
- Cellulose settling
- 10-20%
- Dense-pack density
- 3.5 lbs/cu ft
- Free blower rental
- 20+ bags
Blown-In Insulation FAQ
How many bags for a 1,000 sq ft attic?
35 to 45 bags of cellulose to reach R-38 from a bare ceiling. That is about 10.3 inches of blown cellulose at standard attic density (1.5 lbs/cu ft).
Cellulose vs fiberglass blown-in?
Cellulose is cheaper per bag, provides better air sealing, and is made from 75% recycled paper. Fiberglass will not absorb moisture but costs roughly 2x more per bag and needs greater thickness for the same R-value.
How thick should blown-in insulation be?
R-38 needs about 10 inches of cellulose or 15 inches of fiberglass. R-49 needs 13 inches of cellulose or 20 inches of fiberglass. Always check your climate zone target.
Can I blow insulation over existing?
Yes, as long as the existing insulation is dry and mold-free. Do not compress old batts. Blow new material right on top to reach your target R-value.
How much does blown-in insulation cost?
DIY cellulose runs $0.50 to $1.00 per square foot. Professional installation costs $1.50 to $2.50 per square foot. Most big box stores offer free blower rental with a purchase of 20 or more bags.
Does blown-in insulation settle?
Cellulose settles 10 to 20% in the first year. Over-blow by 15% to compensate. Fiberglass settles less, typically 5 to 10%, making it more stable long-term.
Can I blow insulation into walls?
Yes, using dense-pack technique through 2-inch holes drilled in each stud bay. This requires professional equipment and experience to achieve the 3.5 lbs/cu ft density needed to prevent settling.
What R-value do I need?
R-38 for climate zones 2-3, R-49 for zones 4-5, and R-60 for zones 6-8 per the 2021 IECC energy code. Walls typically need R-13 to R-21 depending on zone.
Blown-In Insulation Estimates for Common Projects
Small Attic
800 sq ft, R-38, cellulose, no existingA weekend project for two people. Rent the free blower and buy bags in bulk for best value.
Average Attic (Top-Off)
1,000 sq ft, R-49, cellulose, existing R-19Adding over existing batts is the highest ROI insulation upgrade. Do not compress the old layer.
Large Attic
1,500 sq ft, R-60, fiberglass, no existingAt this volume, get a professional quote. Installed fiberglass may cost less than DIY at this scale.
Wall Cavity Retrofit
1,200 sq ft walls, R-13, cellulose, noneWall cavities require dense-pack equipment and patching. Budget for professional dense-pack installation.
Blown-In Insulation Mistakes That Cost You Money
Not air sealing before insulating
Insulation slows heat transfer but does not stop air leaks. Caulk and foam every penetration (wiring, plumbing, recessed lights) before blowing. Air leaks account for 25 to 40% of energy loss.
Compressing existing batts with blown-in
Laying heavy blown-in directly on old fiberglass batts crushes them, reducing the R-value of both layers. Blow loosely on top and let the material settle naturally.
Blocking soffit vents
Install polystyrene rafter baffles in every bay before blowing insulation. Blocked vents cause moisture buildup, mold, and ice dams in cold climates.
Not accounting for settling
Cellulose settles 10 to 20% in the first year. Over-blow by at least 15% so you still hit your target R-value after settling. Fiberglass settles less (5 to 10%).
Blowing too thin
Half the recommended thickness means half the R-value and roughly half the energy savings. Measure depth with a ruler in multiple spots to verify uniform coverage.
Important Disclaimer
These estimates are for planning purposes only. Actual costs vary by location, material availability, and project complexity. Always get at least 3 local quotes. This calculator does not replace professional advice.