How many bags of cellulose insulation do I need? Enter your area and R-value target to get exact bag counts, blown depth, weight, and cost for dry blow, dense-pack, or wet-spray applications.
10% for settling and coverage gaps
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Last updated March 31, 2026 by our expert review team
Attics
Most common DIY method
Walls
Professional application
New construction
Adheres to surfaces
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 March 2026
How we verify our calculatorsPlan for 15 to 20% settling in loose-fill attics. If you need R-38 (10.3 inches), install to 12 to 12.5 inches so settled thickness still meets your target. See our BTU calculator to check how cellulose insulation lowers your heating load.
Dense-pack cellulose (3.5 lbs/cu ft) in wall cavities delivers R-3.8/inch and virtually eliminates air infiltration. Requires a professional with a 2-inch fill tube.
Cellulose is treated with 15 to 20% borate compounds for Class I fire resistance, mold inhibition, and pest deterrence. Learn more from the Cellulose Insulation Manufacturers Association.
For moisture-prone areas, choose borate-only treated cellulose over ammonium sulfate formulations, which can release sulfur odors and corrode metal fasteners.
Rent a cellulose blower free from Home Depot or Lowe's with 20+ bags. Use the 3-inch hose for attics. The DOE insulation guide covers application best practices. Our blown-in insulation calculator compares cellulose, fiberglass, and mineral wool side by side.
Wear an N95 respirator, sealed safety goggles, and long sleeves during installation. Cellulose dust irritates airways and borate fibers can irritate skin.
Our cellulose insulation calculator first determines the additional R-value needed by subtracting your existing insulation from your target (e.g., R-38 target minus R-11 existing = R-27 to add). It then divides by cellulose's R-3.7 per inch to get the required depth, in this example 7.3 inches. Bag counts are calculated using the industry-standard coverage rate of approximately 1 bag per 40 sq ft at 3.5 inches (R-13), then scaled for your specific thickness and adjusted for application method.
Dense-pack applications use a 0.85 coverage factor because the higher density (3.5 lbs/cu ft vs. 1.5 lbs/cu ft for loose-fill) requires more material per cubic foot. Wet-spray applications use a 0.9 factor plus a 1.5x cost multiplier for the adhesive and specialized equipment. For closed-cell foam alternatives, see our spray foam calculator. Your waste factor accounts for settling, voids around framing, and coverage inconsistencies. 10% is standard for open attics, while 15 to 20% is recommended for complex spaces with many obstructions.
Formula
Thickness = (Target R - Existing R) ÷ 3.7
Bags = Area × Thickness × 0.007 (adjusted for method)
Is cellulose safe and fire-resistant?
Yes. Borate treatment gives it a Class I fire rating. It chars and self-extinguishes rather than melting like fiberglass.
How much does cellulose settle?
Loose-fill settles 15 to 20% in the first 1 to 2 years. Over-install by 15 to 20% to hit your target. Dense-pack settles less than 5%.
Dense-pack vs. loose-fill?
Loose-fill is for open attics at 1.5 lbs/cu ft ($0.80 to $1.20/sq ft). Dense-pack fills wall cavities at 3.5 lbs/cu ft ($1.50 to $2.50/sq ft).
Can I blow cellulose over fiberglass?
Yes. Cellulose fills gaps that reduce batt performance by 20 to 30%. Use our attic insulation calculator to determine the right depth when layering over existing insulation.
How many bags do I need?
At R-38, expect 25 to 30 bags per 1,000 sq ft. Each 25 lb bag covers about 40 sq ft at 3.5 inches (R-13).
Is cellulose better than fiberglass?
Cellulose offers R-3.7/inch vs. R-2.5 for blown fiberglass, fills around obstacles better, and is 85% recycled. Compare all types with our insulation calculator.
Does cellulose absorb moisture?
It can absorb up to 30% of its weight and release it. Sustained exposure from leaks will degrade performance. Proper ventilation is essential.
Which brands are best?
GreenFiber ($11 to $14/bag at Home Depot) and Nu-Wool ($14 to $17/bag, borate-only formula) are the most widely available.
Not Accounting for the 15 to 20% Settling Factor
If you install to exact target depth, you will end up 2 to 3 inches short. For R-38, blow to 12+ inches instead of the calculated 10.3 inches.
Using Loose-Fill in Wall Cavities Instead of Dense-Pack
Loose-fill at 1.5 lbs/cu ft settles dramatically in walls, leaving gaps. Wall applications require dense-pack at 3.5 lbs/cu ft with a professional blower.
Skipping Ventilation Baffles in Attic Applications
Cellulose near soffits can block intake ventilation. Install rigid baffles at every rafter bay before blowing, maintaining a 1-inch air channel.
Choosing Ammonium Sulfate Formulations for Humid Climates
Ammonium sulfate can release sulfur odors and corrode metal fasteners. In humid climates or near ductwork, use borate-only formulations.
Running the Blower Machine Too Fast
Feed bags at 1 per 45 to 60 seconds. Use the 3-inch hose for attics, break up clumps before feeding, and keep the hose moving.
Neglecting to Measure Settled Depth After 6 Months
Check depth at multiple points using ruler stakes. If settling exceeds 20%, add a top-up layer. Install depth markers during the initial blow.
1,100 sq ft | R-0 to R-49 | GreenFiber cellulose
Tip: Blow to 16 inches initial depth so settling still meets your R-49 target.
1,200 sq ft exterior walls | Nu-Wool borate-only | Professional
Tip: Use borate-only formulation near ductwork to avoid sulfur odor issues.
1,500 sq ft | R-19 to R-60 | GreenFiber cellulose (Zone 6)
Tip: Install depth markers every 8 feet and check settling after 6 months.
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.