Cellulose attic insulation is loose-fill insulation made from recycled paper, treated with borate for fire and pest resistance, and blown across the attic floor. GreenFiber’s SANCTUARY, a common retail example, is made with 85% recycled materials and rated 3.7 R per inch at a settled density of 1.5 lb per cubic foot, and reaching R-49 takes 68.5 of its 25 lb bags per 1,000 sq ft, about 1.7 lb on every square foot of ceiling. Every cellulose product sold in the United States must meet CPSC 16 CFR Part 1209 (the federal version of ASTM C739) for critical radiant flux and smoldering combustion. Its trade-offs are settling, weight, and a strong dislike of moisture.
What is cellulose attic insulation?
Cellulose attic insulation is ground-up recycled paper, mostly newsprint and cardboard, treated with borate chemicals and blown loose across the attic floor with a machine. It is a loose-fill product, not a batt. Recycled content is high: GreenFiber lists 85% for SANCTUARY and Menards lists 85% recycled paper fiber for INSULMAX, and the Cellulose Insulation Manufacturers Association (CIMA) describes cellulose as having one of the highest post-consumer contents of any insulation.
Two facts separate cellulose from other blown materials: the paper base and the fire-retardant treatment. The treatment is what lets shredded paper pass the 16 CFR 1209 tests: GreenFiber’s SANCTUARY technical data lists critical radiant flux of at least 0.12 W/cm², smoldering combustion weight loss of 15.0% or less, and a flame spread of 5 and smoke developed of 5. Those two facts explain almost every pro and con below.
What is the R-value of cellulose insulation?
Blown cellulose is commonly cited at R-3.2 to R-3.8 per inch, and GreenFiber rates SANCTUARY at 3.7 R per inch (at 4 inches thickness). Owens Corning’s AttiCat PINK loose-fill fiberglass reaches R-49 at 17 inches (OC Pub. 10011273-L, May 2026), which works out to about R-2.9 per inch. That density advantage means cellulose reaches a target R-value in fewer inches: 13.3 settled inches for R-49 versus 17 for AttiCat.
ENERGY STAR’s retrofit table, based on 2021 IECC Table R402.1.3, sets the attic levels most retrofits aim for: for an uninsulated attic, R-30 in climate zone 1, R-49 in zones 2 and 3, and R-60 in zones 4 through 8; where 3 to 4 inches already exist, add R-25, R-38 or R-49 respectively. The table below converts those targets into maker coverage-chart depths and weights.
| Target R-value | Cellulose installed / settled (GreenFiber SANCTUARY) | Cellulose bags per 1,000 sq ft (25 lb) | Cellulose weight, lb/sq ft | AttiCat fiberglass thickness | AttiCat weight, lb/sq ft | ENERGY STAR zones |
|---|---|---|---|---|---|---|
| R-30 | 8.9 in / 8.3 in | 36.5 | 0.91 | 10.75 in | 0.402 | Zone 1, uninsulated attic |
| R-38 | 11.2 in / 10.4 in | 50.1 | 1.25 | 13.5 in | 0.523 | Zones 2 to 3, adding to 3 to 4 in |
| R-49 | 14.3 in / 13.3 in | 68.5 | 1.71 | 17 in | 0.688 | Zones 2 to 3 uninsulated; zones 4 to 8 adding |
| R-60 | 17.3 in / 16.1 in | 83.5 | 2.09 | 20.5 in | 0.866 | Zones 4 to 8, uninsulated attic |
Cellulose weight is bags per 1,000 sq ft x 25 lb nominal bag weight; AttiCat weight is Owens Corning’s published minimum weight per square foot. Blow to the bag count, not just a depth line. CIMA notes that cellulose R-values “are based on settled thickness and bag count, not installed thickness”, and that settled thickness on the chart follows the Federal Trade Commission R-Value Rule (16 CFR Part 460). For a full cross-material reference, see our insulation R-value chart.
How many inches of cellulose do I need for R-49?
You need about 13.3 settled inches of GreenFiber SANCTUARY to reach R-49, blown to an initial installed thickness of 14.3 inches, using at least 68.5 bags per 1,000 sq ft (14.6 sq ft per bag), per GreenFiber’s coverage chart. Other brands publish their own charts, so read the bag you are buying: the FTC R-Value Rule makes the bag count and settled thickness the numbers that define R-49.
Does cellulose insulation settle?
Yes. Loose-fill cellulose loses some depth as the fibers compress under their own weight, and current coverage charts build that loss in: GreenFiber’s chart drops from 14.3 installed inches to 13.3 settled inches at R-49, about 7%, and from 17.3 to 16.1 at R-60. AttiCat fiberglass lists the same installed and settled thickness (17 inches at R-49). Dense-pack cellulose is installed at a minimum of 3.5 lb per cubic foot in walls and floors per the same GreenFiber sheet, which is a cavity technique, not an open-attic one.
Settling is the single most misunderstood point about cellulose. A CIMA reader reported 16 inches settling to about 12; CIMA’s answer was that the R-value follows the bag count, so the question is how many bags went in, not how tall the pile looks now. The failure mode is an installer who blows to a depth line with too few bags. Ask the crew to leave the empty bags, or a bag-count receipt, and divide by your attic square footage.
If your attic insulation has settled below the chart’s settled thickness, count backward: measure the attic area, read the current depth, and compare it to the brand’s settled-thickness column. Topping up to the next ENERGY STAR level is the usual fix.
Is cellulose insulation fireproof? The borate question
Cellulose is not fireproof, but borate-treated cellulose is fire-resistant and chars rather than flaming when it passes the federal test. During manufacturing the paper is treated with borate salts (borax and boric acid), which act as a flame retardant, a mold inhibitor, and an insect deterrent at once. GreenFiber markets SANCTUARY as Class 1/A with “permanent fire retardants”, and lists an ASTM E84 flame spread of 5, well inside the 0 to 25 Class A band. Fiberglass loose-fill sits in a different category: Owens Corning says AttiCat passes ASTM E136 and is “considered non-combustible insulation by the model building codes”.
Every batch sold in the U.S. must pass CPSC 16 CFR Part 1209, which tests critical radiant flux (minimum 0.12 W/cm²), smoldering combustion (maximum 15% weight loss) and corrosiveness. That regulation exists because cellulose sold before the late-1970s federal standard was a genuine fire risk. When you get quotes, name the product and ask for its data sheet with those three results, plus whether the fire retardant is borate-only; some formulations also use sulfate salts, and the corrosiveness result is the number that tells you how the product treats metal fasteners and wiring.
- Fire: must meet 16 CFR 1209 (0.12 W/cm² radiant flux, 15% smolder limit); SANCTUARY lists flame spread 5.
- Pests: Borate is toxic to insects, so it deters ants, cockroaches, and wood-boring bugs.
- Mold: Borate suppresses mold growth, but only while the material stays dry; GreenFiber lists fungi resistance as “Acceptable”.
How does cellulose handle moisture?
Moisture is cellulose’s biggest weakness. Because it is paper, it absorbs water readily, and saturated cellulose loses R-value, sags, mats down, and can grow mold once the borate protection is overwhelmed. At R-49 a 1,000 sq ft attic holds about 1,710 lb of cellulose, so a soaked section also adds real load to the ceiling drywall below. It dries out and recovers some performance after a minor, one-time wetting, but a roof leak or a chronic moisture source ruins a cellulose attic faster than it would ruin fiberglass.
The practical rule: do not blow cellulose over an active leak or into an attic with a moisture problem. Fix the roof and the ventilation first; IRC R806.2 sets the minimum vent area at 1/150 of the attic floor (1/300 with balanced high and low vents). Cellulose rewards good air sealing of the attic done before the insulation goes down, because the 2021 IECC (Table R402.4.1.1) says air-permeable insulation “shall not be used as a sealing material”; the air barrier at the ceiling has to be caulk, foam and gaskets, not the loose-fill on top.
Cellulose vs fiberglass insulation
Cellulose beats blown fiberglass on per-inch R-value, sound dampening, and recycled content, and it fills gaps around joists and wiring more tightly. Fiberglass beats cellulose on settling, moisture tolerance, and weight. At R-49, cellulose needs 13.3 settled inches at 1.71 lb/sq ft; AttiCat needs 17 inches at 0.688 lb/sq ft. Neither replaces an air barrier under the 2021 IECC.
| Factor | Cellulose (GreenFiber SANCTUARY) | Blown fiberglass (Owens Corning AttiCat) |
|---|---|---|
| R-value per inch | 3.7 (maker rating) | About 2.9 (R-49 at 17 in) |
| Settling at R-49 | 14.3 in installed to 13.3 in settled | 17 in installed and settled |
| Bags per 1,000 sq ft at R-49 | 68.5 (25 lb bags) | 25 (27.5 lb bags) |
| Weight on ceiling at R-49 | About 1.71 lb/sq ft | 0.688 lb/sq ft minimum |
| Moisture tolerance | Poor, absorbs water | Good, does not absorb |
| Fire | 16 CFR 1209; flame spread 5 | ASTM E136 non-combustible; flame spread 0 |
| Product standard | ASTM C739 / 16 CFR 1209 | ASTM C764 Type I |
| Recycled content | 85% (GreenFiber) | See maker’s current sheet |
One caution unique to cellulose: it is heavier. On old or undersized ceiling joists, an R-60 cellulose blow adds about 2.09 lb per square foot of dead load, against 0.866 for AttiCat at R-60. The IRC ceiling joist span tables (R802.5.1) size joists for a stated dead load, so if the framing is marginal, already sags, or carries old insulation you are leaving in place, have the joists checked against that table before adding about 2 lb per square foot. For the full material breakdown, see our guides to fiberglass attic insulation and the broader category of blown-in insulation, which covers both materials and the machine process.
Cellulose insulation pros and cons
Cellulose’s strengths are per-inch R-value (3.7 for SANCTUARY), high recycled content (85%), and sound control. Its weaknesses are settling (about 7% on GreenFiber’s chart), moisture sensitivity, weight (about 1.71 lb/sq ft at R-49), and dust during install.
- Pro, gap filling: Dense loose fibers pack around wiring and joists; GreenFiber says SANCTUARY forms “a dense barrier that reduces air infiltration”, but the IECC still requires a separate air barrier.
- Pro, recycled content: 85% recycled materials in SANCTUARY and 85% recycled paper fiber in INSULMAX.
- Pro, depth: R-49 at 13.3 settled inches versus 17 for AttiCat, useful where the attic floor meets a low roof edge at the eaves.
- Pro, no glass fibers: No glass fibers to itch, though the install is dusty.
- Con, settling: About 7% from installed to settled on GreenFiber’s chart, so blow by bag count.
- Con, moisture: Absorbs water, sags, and can mold if a leak is not fixed first.
- Con, weight: About 2.5 times AttiCat’s weight per square foot at R-49.
- Con, dust: Blowing generates fine paper dust; an N95 or better respirator and containment are needed.
How much does cellulose attic insulation cost?
Cellulose attic insulation costs about $3.75 to $7.77 per square foot professionally installed, per Homewyse’s May 2026 estimate for treated cellulose blown to R-45 with vent baffles at the soffits. A DIY blow is mostly bag cost: SANCTUARY 25 lb bags listed at $15.00 to $19.99 from building-supply dealers in October 2026, and big-box stores tie a free blower day to a minimum purchase (Home Depot lists 10 bags of cellulose for its GreenFiber blower; a Lowe’s SANCTUARY listing says 20 bags).
Price moves with attic size, target R-value, whether old insulation is removed first, and local labor. Bag count is the lever you can check: R-49 takes 68.5 bags per 1,000 sq ft and R-60 takes 83.5, so stepping from R-49 to R-60 adds 15 bags, about $225 to $300 at the listed prices. For a project-level number across materials, see our breakdown of how much it costs to insulate an attic.
| Scope | Cost | Source / math |
|---|---|---|
| SANCTUARY bag, 25 lb | $15.00 to $19.99 | Dealer listings, Oct 19, 2026 |
| DIY material, 1,000 sq ft to R-49 | $1,035 to $1,379 | 69 bags x $15.00 to $19.99 |
| DIY material, 1,000 sq ft to R-60 | $1,260 to $1,679 | 84 bags x $15.00 to $19.99 |
| Pro blown-in cellulose, R-45 with baffles | $3.75 to $7.77 / sq ft | Homewyse, May 2026 |
| Pro, 1,000 sq ft attic | $3,750 to $7,770 | 1,000 x Homewyse range |
DIY cellulose install: the short version
Blowing cellulose in an attic is a realistic DIY job for a dry, accessible attic, and it usually takes two people: one feeding the machine below and one aiming the hose above. For a 1,000 sq ft attic at R-49 that is 68.5 bags, or about 1,710 lb of material carried and fed by hand. Air-seal and install baffles first, because you cannot fix those once the attic floor is buried.
- Air-seal penetrations. Caulk and foam around wiring, plumbing, chimneys, and top plates before any insulation goes down; the IECC does not let air-permeable insulation count as the seal.
- Install rafter baffles. Keep soffit vents clear so the 1/150 (or 1/300) vent area under IRC R806.2 is not blocked at the eaves.
- Dam the access hatch and fixtures. Build cardboard or metal dams around the hatch and any non-IC-rated recessed lights; NEC 410.116(B) requires insulation to stay at least 3 inches from a non-IC fixture.
- Set depth markers. Staple rulers or marked stakes to joists at the installed thickness on your bag’s chart (14.3 in for R-49 with SANCTUARY).
- Blow by bag count. Divide the attic area by the chart’s square feet per bag (14.6 at R-49) and use at least that many bags.
- Rake level and check the markers. Even the surface and confirm depth across the whole attic.
Wear an N95 or better respirator and eye protection; the dust is heavy. If the attic is wet, too cramped to crawl safely, or has knob-and-tube wiring, hire a pro instead. For the full walkthrough across materials, see our guide to attic insulation installation.
When cellulose is the right choice (and when it is not)
Choose cellulose for a dry, well-ventilated attic where depth is limited, recycled content matters and the ceiling framing is sound. Skip it where moisture risk is high, the attic sees chronic leaks, or the joists are old and undersized, since its 1.71 lb/sq ft at R-49 and its water sensitivity turn from features into liabilities in those conditions.
| If your attic has | Choose | Why (published figure) |
|---|---|---|
| Dry deck, venting meets IRC R806.2, sound joists | Cellulose | R-49 at 13.3 settled in (SANCTUARY) |
| Low clearance at the eaves | Cellulose | About 3.7 in less depth than AttiCat at R-49 |
| Active or recent roof leak | Fix the roof first, then either material | Wet cellulose mats down and molds |
| Undersized or sagging ceiling joists | Loose-fill fiberglass | 0.688 vs about 1.71 lb/sq ft at R-49 |
| Non-IC recessed lights you will not replace | Either, with dams | NEC 410.116(B) 3 in clearance |
| Existing 3 to 4 in of dry insulation, zone 4 to 8 | Top up with either | ENERGY STAR: add R-49 |
Whatever you pick, fix the roof and ventilation before insulating. Insulation buries the deck, so a leak found afterward means tearing out wet material. Compare the field against the rest of the category in our overview of the best insulation for an attic.
Frequently asked questions
Is cellulose or fiberglass better for attic insulation?
Neither wins outright. Cellulose reaches R-49 in 13.3 settled inches (GreenFiber SANCTUARY) and uses 85% recycled material, so it suits dry attics with sound framing. Loose-fill fiberglass needs 17 inches for R-49 (Owens Corning AttiCat) but weighs 0.688 lb/sq ft against about 1.71, and it does not absorb water, so it suits attics with any water risk or weak joists.
How many inches of cellulose do I need for R-49?
GreenFiber’s SANCTUARY chart calls for 14.3 inches installed, settling to 13.3 inches, using at least 68.5 bags per 1,000 sq ft for R-49. Other brands publish their own charts, and the bag count, not the depth, is what defines R-49 under the FTC R-Value Rule.
How long does cellulose insulation last?
Dry, undisturbed cellulose keeps its labeled R-value because the label is already based on settled thickness and bag count. Its life is cut short by moisture: a roof leak, condensation, or high humidity can mat it down and force replacement. Keeping the attic dry and vented to IRC R806.2 is what determines longevity.
Can I blow cellulose over existing fiberglass?
Yes. You can blow cellulose directly over existing loose-fill or batt fiberglass to top up R-value, as long as the old insulation is dry and not moldy. ENERGY STAR’s table gives the add-on level where 3 to 4 inches already exist: R-25 in zone 1, R-38 in zones 2 and 3, R-49 in zones 4 to 8.
Is cellulose insulation safe?
Borate-treated cellulose sold in the U.S. must pass CPSC 16 CFR Part 1209 for critical radiant flux (at least 0.12 W/cm²), smoldering combustion (15% maximum weight loss) and corrosiveness. It contains no glass fibers. The main jobsite step is wearing an N95 or better respirator during the blow, because the paper dust is heavy while airborne.
Does cellulose insulation attract mold or pests?
Borate treatment deters both. Borate is toxic to insects and suppresses mold growth, and GreenFiber lists SANCTUARY’s fungi resistance as acceptable under the standard’s test. The protection fails when the material gets and stays wet, which overwhelms the borate and lets mold take hold.