Cellulose attic insulation is loose-fill insulation made from about 80 to 85 percent recycled paper, treated with borate for fire and pest resistance, and blown across the attic floor. It delivers R-3.2 to R-3.8 per inch, costs roughly $0.60 to $2.30 per square foot installed, and air-seals gaps better than fiberglass. Its trade-offs are settling of about 15 to 20 percent and a strong dislike of moisture. Every product sold in the United States must meet the federal cellulose standard, ASTM C739 and CPSC 16 CFR Part 1209. This guide covers the numbers, the standards, the borate fire question, settling, moisture, and how cellulose compares to fiberglass.
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 typically runs 80 to 85 percent, the highest of any common insulation, which is why it carries the strongest environmental claim in the category.
Two facts separate cellulose from other blown materials: the paper base and the borate treatment. Borate is what makes a pile of shredded newspaper fire-resistant and unappealing to insects and mold. Those two facts explain almost every pro and con below. According to the Cellulose Insulation Manufacturers Association (CIMA), borates are used in roughly 99 percent of cellulose insulation sold in North America.
What is the R-value of cellulose insulation?
Blown cellulose delivers R-3.2 to R-3.8 per inch, higher than the R-2.2 to R-2.7 per inch of loose-fill fiberglass. That density advantage means you reach a target R-value in fewer inches, though the real-attic difference is modest once you account for settling. Cellulose also holds its rating in cold weather, because its density resists the convective looping that can cut loose fiberglass performance on the coldest nights.
The U.S. Department of Energy and the 2021 International Energy Conservation Code (IECC Table N1102.1.3) set the target attic R-values most retrofits aim for: R-30 in climate zone 1, R-49 in zones 2 and 3, and R-60 in zones 4 through 8. The table below converts those targets into settled cellulose depth and compares it to loose-fill fiberglass.
| Target R-value | Cellulose (settled inches) | Loose-fill fiberglass (inches) | DOE / IECC 2021 climate zones |
|---|---|---|---|
| R-30 | 8 to 9 in | 11 to 13 in | Zone 1 (hot, far South) |
| R-38 | 10.5 to 11 in | 14 to 16 in | Older code minimum, mild zones |
| R-49 | 13.5 to 14.5 in | 18 to 20 in | Zones 2 to 3 (South, mid US) |
| R-60 | 16.5 to 18 in | 22 to 24 in | Zones 4 to 8 (North, cold) |
Blow to the settled depth, not the fluffed depth on install day. A crew that stops at the R-49 mark before settling leaves you short of R-49 within two years. Check the coverage chart printed on every bag, which the ASTM C739 label rule requires to list both installed and settled thickness for a given R-value and bag count. 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 13.5 to 14.5 settled inches of loose-fill cellulose to reach R-49, the DOE-recommended level for most of the central and southern United States. Because cellulose settles 15 to 20 percent, the installed depth on the day of the blow should be closer to 16 to 17 inches so the material still measures R-49 after it compresses. Always follow the bag’s coverage chart for the exact installed depth and bag count.
Does cellulose insulation settle?
Yes. Loose-fill cellulose settles roughly 15 to 20 percent in the first few years as the paper fibers compress under their own weight. Less depth means less R-value, so installers over-blow the material up front to hit the target after settling stabilizes. Dense-pack cellulose, packed to about 3.5 pounds per cubic foot, settles far less, roughly 2 to 5 percent, but dense-pack is a wall and cavity technique, not an open-attic one.
Settling is the single most misunderstood point about cellulose. It is not a defect; it is built into the coverage math the ASTM C739 label already accounts for. The failure mode is an installer who blows to the target depth on day one and ignores settling, which leaves the attic under-insulated later. The fix: confirm the crew blows to the manufacturer’s settled-density coverage rate, not just a depth line.
Is cellulose insulation fireproof? The borate question
Cellulose is not fireproof, but borate-treated cellulose is fire-resistant and chars instead of igniting when exposed to flame. 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. Treated cellulose meets the Class A (Class 1) fire rating, the same top rating as fiberglass.
Every batch sold in the U.S. must pass the federal cellulose standard, ASTM C739 and CPSC 16 CFR Part 1209, which test critical radiant flux (flame spread), smoldering combustion, corrosiveness, and odor. That regulation exists because early 1970s cellulose was a genuine fire risk before the standard was written. Insist on borate-only, sulfate-free product. Older ammonium-sulfate formulations can smell, corrode metal fasteners and wiring when damp, and lose fire retardancy over time. Borate-only cellulose does not corrode metal, does not off-gas an odor, and stays effective. This is a real spec to name when you get quotes.
- Fire: Class A (Class 1) rated; chars rather than flames when borate-treated to ASTM C739.
- 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.
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. 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. Cellulose belongs in a dry, well-ventilated attic, and it rewards good air sealing of the attic done before the insulation goes down, because air sealing is what keeps warm, humid indoor air out of the cold attic in the first place.
Cellulose vs fiberglass insulation
Cellulose beats blown fiberglass on air sealing, per-inch R-value, sound dampening, and recycled content. Fiberglass beats cellulose on settling, moisture tolerance, and weight. For most dry, well-ventilated attics, cellulose is the stronger air-sealing performer; for attics with any moisture risk or weak ceiling framing, fiberglass is the safer pick. Both carry a Class A fire rating and both are governed by ASTM standards, so the choice comes down to your attic’s condition, not safety.
| Factor | Cellulose | Blown fiberglass |
|---|---|---|
| R-value per inch | R-3.2 to R-3.8 | R-2.2 to R-2.7 |
| Settling | 15 to 20 percent | Under 2 percent |
| Air sealing | Excellent, fills gaps | Fair, lets air move |
| Moisture tolerance | Poor, absorbs water | Good, does not absorb |
| Weight on ceiling | Heavier (1.5 to 3 lb/cu ft) | Lighter (0.5 to 1 lb/cu ft) |
| Fire rating | Class A / Class 1 (borate, ASTM C739) | Class A, non-combustible (ASTM C764) |
| Recycled content | 80 to 85 percent | 40 to 60 percent |
| Skin/lung irritation | Dusty, low irritation | Itchy fibers |
One caution unique to cellulose: it is heavier. On old or undersized ceiling joists, a deep cellulose blow adds real dead load, roughly 1.5 to 3 pounds per cubic foot against 0.5 to 1 for fiberglass. If the framing is marginal, that difference matters. 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 air sealing, high recycled content, sound control, and per-inch R-value. Its weaknesses are settling, moisture sensitivity, weight, and dust during install. The list below sorts the honest trade-offs so you can match the material to your attic rather than to a sales pitch.
- Pro, air sealing: Dense loose fibers hug penetrations and fill gaps fiberglass leaves open, cutting drafts.
- Pro, recycled content: 80 to 85 percent recycled paper, the highest embodied-recycled share of common insulations.
- Pro, sound: Its density dampens airborne noise better than loose fiberglass.
- Pro, no itch: No glass fibers, so no skin irritation, though the install is dusty.
- Con, settling: Loses 15 to 20 percent of loose-fill depth, so it must be over-blown.
- Con, moisture: Absorbs water, sags, and can mold if a leak is not fixed first.
- Con, weight: Heavier than fiberglass, a concern on weak ceiling joists.
- Con, dust: Blowing generates fine paper dust; a respirator and containment are needed.
How much does cellulose attic insulation cost?
Cellulose attic insulation costs about $0.60 to $2.30 per square foot installed for loose-fill, or roughly $600 to $2,300 to bring a 1,000 square foot attic up to R-49. A DIY blow runs $0.30 to $0.50 per square foot in material if you rent the blower, which many home centers lend free with a bag purchase. Dense-pack, used in walls rather than open attics, runs higher, around $1.50 to $3.00 per square foot. Figures vary by region, attic access, and 2026 material pricing.
Price moves with attic size, target R-value, whether old insulation is removed first, and local labor. Cellulose material is often a touch cheaper per bag than fiberglass, but because it is denser you use more pounds to hit the same R-value, so the installed cost lands in a similar range. For a project-level number across materials, see our breakdown of how much it costs to insulate an attic.
| Scope | Cost range | Notes |
|---|---|---|
| DIY material (loose-fill) | $0.30 to $0.50 / sq ft | Blower often free with bag purchase |
| Pro loose-fill installed | $0.60 to $2.30 / sq ft | Attic floor, top-up or new |
| 1,000 sq ft attic to R-49 | $600 to $2,300 | Depends on removal and access |
| Dense-pack (walls) | $1.50 to $3.00 / sq ft | Not an open-attic method |
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. The steps below are the sequence pros follow. 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.
- Install rafter baffles. Keep soffit vents clear so airflow 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, which the IRC requires kept clear of insulation.
- Set depth markers. Staple rulers or marked stakes to joists at your settled-depth target.
- Blow to settled depth plus over-blow. Add the extra inches the coverage chart calls for so settling still leaves you at target.
- Rake level and check the markers. Even the surface and confirm depth across the whole attic.
Wear a rated respirator and eye protection; the dust is heavy. If the attic is wet, cramped, 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 air sealing and recycled content matter 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 weight and water sensitivity turn from features into liabilities in those conditions.
- Good fit: dry attic, sound framing, drafty older home, buyer who values recycled content and quiet.
- Poor fit: attic with active or past leaks, high indoor humidity, marginal ceiling joists, or no plan to fix ventilation first.
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 air-seals better, has a higher R-value per inch, and uses more recycled content, so it suits dry, drafty attics with sound framing. Fiberglass settles less and tolerates moisture better, so it suits attics with any water risk or weak joists. Both carry a Class A fire rating, so match the material to your attic’s condition rather than picking a universal winner.
How many inches of cellulose do I need for R-49?
You need about 13.5 to 14.5 settled inches of loose-fill cellulose for R-49, the DOE-recommended attic level across most of the central and southern United States. Because cellulose settles 15 to 20 percent, blow closer to 16 to 17 inches on install day so it still measures R-49 after it compresses. Follow the bag’s ASTM C739 coverage chart for the exact installed depth and bag count.
How long does cellulose insulation last?
Cellulose lasts 20 to 30 years or more if it stays dry, and settling stabilizes within the first few years. It does not degrade with age on its own. Its lifespan is cut short only by moisture: a roof leak, condensation, or high humidity can mat it down and force replacement well before its natural life ends. Keeping the attic dry 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, and the two materials coexist fine as long as the old insulation is dry and not compressed or moldy. Remove and replace the existing layer only if it is wet, contaminated, or pest-damaged. Otherwise, adding cellulose on top is a common and cost-effective upgrade.
Is cellulose insulation safe?
Borate-treated cellulose is considered safe once installed. The borate is low-toxicity to humans at the levels used, and cellulose contains no glass fibers, so there is no skin or lung irritation from the settled material. Every U.S. product must pass CPSC 16 CFR Part 1209 for flame and corrosion. The main jobsite step is wearing a rated respirator during the blow, because the paper dust is heavy while airborne.
Does cellulose insulation attract mold or pests?
Borate treatment actively deters both. Borate is toxic to insects and suppresses mold growth, so treated cellulose resists pests and mildew as long as it stays dry. The protection fails only when the material gets and stays wet, which overwhelms the borate and lets mold take hold. A dry attic keeps the treatment working for the life of the insulation.
Reviewed by The Roofing Brief Team. Last reviewed August 2026.