Cellulose Insulation Calculator: Bag Counts, Settling Factor & Borate vs Ammonium Sulfate

How Many Bags Do You Need? The Formula

The bag count calculation for blown cellulose insulation follows a straightforward formula:

The coverage constant varies slightly by manufacturer and product density. Always cross-check the calculated bag count against the coverage chart printed on the bag — the FTC R-Value Rule (16 CFR Part 460) requires manufacturers to publish settled-depth coverage tables for every cellulose product, and your specific product's chart takes precedence over any generic estimate.

Bag Count by Attic Size and R-Value

Counts assume no existing insulation, blown cellulose at 3.7 R/inch, settled depth. Cross-check against your bag label.

Source: DOE Insulation Guide (energy.gov) — R-per-inch = 3.7 for loose-fill cellulose. Depths rounded up. Coverage constant from ASTM C1050 standard test methods. Verify against bag label for your specific product.

Borate vs Ammonium Sulfate Fire Retardants: A Buyer's Guide

Cellulose insulation is roughly 20% fire retardant by weight. The chemistry of that retardant matters — particularly in humid climates or attics with exposed metal.

Most major manufacturers — including GreenFiber, Applegate, and National Fiber — have moved toward borate-primary blended formulations, though ammonium sulfate may still appear in the mix. The specific chemistry is printed on the bag label in the fire retardant ingredients section. In attics with exposed galvanized ductwork, aluminum electrical junction boxes, or any metal near the insulation surface, borate formulations carry lower corrosion risk over the long term.

Neither formulation is "unsafe" for residential use when the insulation remains dry. The performance difference becomes relevant if the attic develops a moisture problem — borate maintains its properties better when dry and wet cycles occur, while ammonium sulfate products can become more reactive. Both types meet the same fire rating requirements under ASTM C739 (standard specification for cellulose loose-fill thermal insulation).

Cellulose Settling: Installed Depth vs Settled Depth

Loose-blown cellulose settles up to 15–20% in thickness. This settling happens within 48–72 hours of installation — not gradually over years — as the fibers reach their natural resting density (typically 1.8–2.0 lbs/cu ft for attic applications).

Manufacturers account for this in the coverage charts on the bag label. The FTC R-Value Rule (16 CFR Part 460) requires that coverage charts use settled depth, not installed (freshly blown) depth, as the basis for R-value claims. This means the bag count from the label's chart already includes the settling allowance — you do not add a separate overfill margin on top.

Dense-pack cellulose (3.0–4.2 lbs/cu ft, used in wall cavities) settles far less than loose-blown attic cellulose — sometimes no measurable settling — because the compression during installation eliminates the voids that allow loose-fill to compress further.

Why Air Sealing Matters More for Cellulose

Cellulose is more permeable to airflow than closed-cell foam or rigid foam board, and slightly more permeable than fiberglass batts. In an unsealed attic floor, air can move through the cellulose layer by convection — warm air from below pushing upward through the material, which reduces the effective R-value by an estimated 10–15% in practice.

This is not a defect in cellulose — it is a material characteristic. The solution is air sealing the attic floor before blowing in insulation. DOE research shows air sealing alone reduces heating and cooling costs 15–30%, and when combined with cellulose insulation the two effects compound.

Air-seal these locations before blowing:

• Recessed light rough-ins (use IC-rated covers or non-IC covers with sealed drywall boxes)
• Top plates where interior walls meet the attic floor
• Wire and pipe penetrations through the top plate
• Pull-down stair rough-in (one of the largest air-leakage points in most attics)
• HVAC duct penetrations and gaps around ducts
• Dropped soffits over kitchen cabinets or built-ins

Use low-expansion spray foam for gaps up to 1 inch and two-part foam kits for larger openings. Install rigid rafter vent baffles before blowing — these maintain the soffit-to-ridge ventilation path required by IRC §R806 ↗ and prevent the insulation from blocking soffit vents.

Bag-count comparison — cellulose vs fiberglass loose-fill brands

Bag counts at a given R-value target vary across brands because each manufacturer formulates its product at a slightly different settled density. GreenFiber Cocoon and Applegate Stabilized are both cellulose loose-fill products; Owens Corning AttiCat is a fiberglass loose-fill product included here for direct comparison. Because fiberglass has a lower R-per-inch (2.5 vs 3.7), AttiCat requires fewer bags to reach the same R-value but requires roughly 50% more depth — a critical trade-off in shallow joist bays.

Sources: GreenFiber Cocoon install guide (settled-depth coverage chart); Applegate Stabilized install guide; Owens Corning AttiCat L77 coverage chart. All values per FTC R-Value Rule 16 CFR Part 460 settled-depth disclosure basis. Verify against the coverage chart on your specific bag before purchasing.

The difference between GreenFiber Cocoon (~33 bags at R-30) and Applegate Stabilized (~31 bags at R-30) reflects a slightly higher settled density in the Applegate product. AttiCat reaches R-30 in ~24 bags because fiberglass provides only 2.5 R/inch versus 3.7 R/inch for cellulose — but that requires ~20 inches versus ~13 inches for cellulose at R-49, which can matter in shallow joist bays.

Cellulose vs Blown Fiberglass: Cost per R-Value

Cellulose and blown fiberglass are the two most common attic insulation choices. Material cost comparisons often mislead by focusing on price per bag rather than price per unit of R-value.

The net cost-per-R is competitive between the two materials. Cellulose's higher R-per-inch means you use roughly 35% less depth to reach the same R-value, which matters in attics with shallow joist bays or where you want to minimize weight on the ceiling.

Cellulose's ~80–85% post-consumer recycled paper content (newsprint, cardboard) gives it a lower embodied carbon footprint than fiberglass. The borate fire retardant is mineral-derived rather than petrochemical. Both factors contribute to ENERGY STAR and LEED recognition for cellulose insulation.

Wet-Spray vs Dry-Blown Cellulose

There are two application methods for cellulose insulation, each suited to a different context:

This page covers dry-blown attic cellulose. If you are insulating new construction wall cavities with wet-spray cellulose, the coverage rates and bag counts differ — consult the specific wet-spray product's technical data sheet.

Install Safety: Dust & Respirator Requirements

Cellulose dust is fine, recycled-paper fiber — not toxic, not a recognized carcinogen — but it does cause respiratory irritation during installation. Standard safety requirements for blown cellulose attic work:

• N95 respirator — minimum. Required for all dry-blown attic work. Rated to filter ≥95% of airborne particles ≥0.3 microns. Change the respirator if it becomes damp.
• P100 half-face respirator — recommended for dense-pack wall work, where dust concentration is substantially higher. Provides ≥99.97% filtration.
• Indirect-vent safety goggles. Cellulose dust irritates eyes. Safety glasses without a sealed frame allow dust to enter from the sides.
• Disposable coveralls or old clothes that cover all skin. Cellulose fibers can cause skin and throat irritation. Discard or wash coveralls before returning to the living space.
• Walk-boards across joists. Never step on attic ceiling drywall — it will not support body weight.
• LED headlamp. You need both hands free for the blower hose.

The attic should be vacated by anyone not directly involved in the installation. Close HVAC returns in the living space to prevent dust from being pulled through the system. Plan to air out the attic for several hours after installation before re-entering.

What Does Cellulose Insulation Cost?

Cellulose insulation costs depend on whether you DIY or hire a contractor, and on the R-value target for your climate zone. The figures below reflect 2024–2026 retail pricing at major home improvement retailers and published contractor-installed ranges. All costs are for attic loose-fill cellulose; wall dense-pack rates differ.

DIY Material Cost at R-49 (1,000 sq ft attic)

At R-49, a 1,000 sq ft attic requires approximately 55 bags of cellulose (GreenFiber Cocoon, per coverage chart). Retail pricing at Home Depot and Lowe's (approximate as of 2026 — verify with your local store before purchasing): GreenFiber Cocoon $11–$14/bag, Applegate Stabilized $13–$16/bag, Owens Corning AttiCat (fiberglass) $12–$15/bag. At GreenFiber pricing, 55 bags runs roughly $600–$770 for the 1,000 sq ft example. Blown fiberglass (AttiCat) requires fewer bags at R-49 (~38 bags), making the 1,000 sq ft material cost roughly $456–$570 — lower but requiring 20 inches of depth versus 13 inches for cellulose.

Blower rental: Home Depot provides a free blower with purchase of 10 or more bags (deposit required, refunded on return). Lowe's requires 20 or more bags for the free rental. For a 1,000 sq ft attic at R-49, you will almost certainly clear the 10-bag threshold at either retailer, making the effective blower cost $0. For blower rental terms, throughput rates, and dense-pack vs loose-fill density specs, see our blown-in insulation calculator guide.

Cost summary: DIY vs contractor

Contractor-Installed Cost

Contractor-installed blown cellulose typically runs $1.20–$2.00 per sq ft for attic applications, which includes materials, labor, and equipment setup. For a 1,500 sq ft attic, that is roughly $1,800–$3,000 fully installed at R-49. Rates vary by region and contractor demand — Cold-climate regions (Climate Zones 5–7) often have higher contractor rates due to sustained demand and more complex vapor retarder requirements.

Energy Savings and Tax Credits

DOE research shows that adding attic insulation to meet or exceed DOE recommendations reduces heating and cooling costs by 15–30% annually. For a home spending $2,400/year on HVAC, that is a potential saving of $360–$720 per year — payback period of roughly 2–5 years on a DIY install.

Cellulose insulation may qualify for the IRS Energy Star 25C tax credit (30% of materials and labor, up to $1,200/year). See energystar.gov/about/federal_tax_credits for current eligibility and Form 5695. Consult a tax professional for your specific situation.

Climate Zone Considerations: Vapor Retarder Requirement

Whether your attic insulation requires a Class II vapor retarder depends on your IECC climate zone. The requirement varies by zone, and cellulose's relatively high vapor permeability compared to fiberglass or spray foam makes it a preferred choice in mixed-humid climates. Per IRC N1102.4 ↗ (Thermal Envelope Air Sealing) and IRC §R702.7 ↗ (Vapor Retarders), requirements differ by IECC zone as follows:

Climate Zones 1–3 (Hot and Mixed-Dry)

In Climate Zones 1, 2, and 3 — covering most of Florida, Texas, Arizona, California desert regions, and the Gulf Coast — no Class II vapor retarder is required per IRC for cellulose attic insulation. The dominant moisture risk in these zones is humidity entering from outside (not from conditioned space vapor drive), and a vapor retarder on the ceiling plane could trap moisture and cause condensation issues. Cellulose is appropriate in these zones without additional vapor retarder provisions, and fiberglass loose-fill is also acceptable. Confirm with your local jurisdiction; per IRC, some states in Zone 3 have adopted local amendments.

Climate Zones 4–7 (Mixed-Humid to Cold)

In Climate Zones 4 through 7 — covering the Mid-Atlantic, Midwest, Northeast, Mountain West, and most of the Northern tier — a Class II vapor retarder (perm rating 0.1–1.0) is required per IRC §R702.7 ↗ where the insulation is in direct contact with the ceiling plane in cold conditions. Cellulose's relatively high vapor permeability (it allows some vapor transmission while still retarding bulk moisture flow) makes it a better choice than closed-cell spray foam in mixed-humid zones like 4A and 5A, where walls and ceilings need to dry to the interior during summer and to the exterior during winter. Trapping vapor completely with an impermeable product in these zones can cause moisture accumulation.

Zone 4 R-value target: DOE recommends R-49 for uninsulated attics in Zone 4 (both Zone 4A mixed-humid — Mid-Atlantic, coastal New England — and Zone 4B mixed-dry — Intermountain West). Zone 5 through 8 attics target R-60. Zone 4A (humid) and Zone 4B (dry) share the same DOE R-value minimum but differ in vapor retarder strategy: 4A assemblies need to dry to both interior and exterior; 4B assemblies have lower absolute humidity and are more tolerant of less permeable products. For DOE climate-zone R-value targets (Zone 4 R-49, Zone 5-8 R-60) and Energy Star 25C tax credit details, see our how much attic insulation guide. Consult your local building department for the applicable IRC edition and any state or local amendments.

Climate Zone 8 (Subarctic)

In Climate Zone 8 (Alaska and extreme northern regions), vapor control is critical due to extreme cold and interior moisture loads. A Class II or Class I (0.1 perm or less) vapor retarder may be required depending on the assembly. Per IRC, this zone requires careful attention to both the air barrier and vapor retarder provisions — consult a qualified contractor familiar with subarctic building assemblies before specifying any loose-fill insulation.

Jurisdiction note: Vapor retarder requirements vary by climate zone and by the specific code edition adopted by your state or municipality. This content reflects the 2021 IECC and 2021 IRC. Depending on your jurisdiction, an earlier edition (2018, 2015) or a state-specific energy code may apply. Confirm all vapor retarder and insulation requirements with your local building authority before installation.

Frequently Asked Questions