What Blown-In Insulation Is Actually Made Of
Start at the source, because every row in the comparison table traces back to it. Cellulose is shredded post-consumer newsprint — the bales that become “newspaper insulation” — ground to fluff and dosed with roughly 15% boric acid and ammonium sulfate. The borates do triple duty: fire retardant (Class C on the ASTM E84 tunnel test), insecticide, and mold inhibitor. The fibers are flat, dense paper strips, which is why cellulose mats well, seals gaps well, and settles noticeably.
Fiberglass is silica sand and recycled glass melted above 2500°F and forced through spinnerets into fibers thinner than human hair. The finished product is mostly trapped air — glass itself barely conducts heat and never absorbs water, but the loose 0.7 PCF form used in rental-blower attics traps that air less securely than denser products, which is exactly where its lower settled R-per-inch and its moderate air-sealing score both come from.
Rockwool (mineral wool) starts as volcanic basalt and blast-furnace slag, melted at about 2900°F and spun into coarse, rigid fibers. The material is literally stone: it cannot burn (Class A, non-combustible, good to 2000°F+), cannot feed mold, and its fibers are too stiff to compact — hence under 2% settling. The price of stone-grade stability is stone-grade logistics: fewer plants, heavier bags, and a 60–100% premium installed.
One hose, three possible fibers — the material decides the numbers, not the machinePhoto: UnsplashSide-by-Side: Eleven Attributes Across Three Fibers
All values are settled, attic-floor application values — the same assumptions every calculator on this site uses. Read the R-per-inch row against the installed-cost row: fiberglass’s low price per bag buys the least R per inch, so its bag count is the highest, while rockwool’s blower row is the one that quietly disqualifies it from most DIY shopping lists.
| Attribute | Loose Cellulose (GreenFiber / GoldBond) | Loose Fiberglass (Owens Corning AttiCat) | Loose Mineral Wool (Rockwool / Roxul) |
|---|---|---|---|
| Source Material | Recycled newspaper + boric acid fire retardant | Silica sand + recycled glass + resin binder | Volcanic rock or steel slag + inorganic binder |
| Settled R-Value Per Inch | 3.2–3.8 (avg 3.50) | 2.2–3.2 (avg 2.50) | 3.0–3.5 (avg 3.25) |
| Settling (Year 1) | 10–15% — most aggressive | 5–10% | <3% |
| Class Rating | C — boric acid treated | C — resin binder | A — non-combustible inorganic |
| Moisture Absorption @90% RH | ~8% weight gain — boric acid migration risk | ~0% — glass doesn't absorb | ~5% — binder only |
| Dust / Fibers Risk | Mild — N95 recommended | Moderate — P100 + eye protection REQUIRED | Moderate — eye protection |
| R-Value Loss if Wet | ~50% — must stay dry | Zero | Moderate |
| DIY Blower Requirement | Standard 3.0 HP @ 60 PSI works | Standard blower works | Needs 100 PSI minimum |
| Installed Cost / Sq Ft | $1.00–$2.00 installed | $0.90–$1.80 installed | $1.50–$2.80 installed |
| Best Climate Zones | All Zones | Zones 1–6 | All Zones |
| Caveats | Boric acid mildly toxic; don't let kids/pets play with new cellulose | Fiberglass fibers embed + itch; always full PPE | More expensive; limited big-box availability |
💡 Translate to inches: The R-per-inch row feeds the full R-value chart — 14.0” of cellulose at R-49 vs 19.6” of fiberglass is often the real tiebreaker in low-pitch attics.
⚠️ Cost per R, not cost per bag: Fiberglass looks cheapest until the depth target doubles the bag count. Compare the installed-cost row at equal R — cellulose usually wins the 1200 sq ft R-49 wallet fight despite the pricier pallet.
💡 Price your attic: Enter your square footage and zone in the main attic insulation calculator — material cost, bags, and labor scale together, and the material reference panel mirrors this table’s R-per-inch values.
Fire Ratings Deep Dive — ASTM E84, Borates, and What “Class C” Really Means
Every insulation product sold in the US carries a fire rating from ASTM E84 — the ten-minute tunnel test that measures surface flame spread and smoke development against a calibrated baseline. Class A means flame spread under 25 (inherently non-combustible territory); Class B is 26–75; Class C is 76–200, still code-acceptable for attic floors in most jurisdictions. Cellulose earns its Class C rating chemically — the borate load chars the paper instead of letting it flash — while fiberglass earns it physically, because glass fibers have nothing to burn. Rockwool is in a different category entirely: Class A non-combustible, stable past 2000°F.
⚠️ The label is the product: Buy only bags printed with the ASTM E84 listing and a recognized evaluation report (ICC-ES). Untreated or under-treated bulk paper fiber is a genuine fire load, and the bargain “equivalent” product without the printed rating has no verified place in your attic. The rating applies to the tested formulation — substitutions void it.
Ratings also assume clearances that installation practice provides: keep loose-fill off recessed light housings without IC covers, maintain the code-required gap at chimneys and flues, and shield any incandescent-dimmer or appliance junction boxes buried by deep fill. A Class C blanket six inches deep around an overheated can light is a preventable classic — the fiber rating never excuses the clearance rule.
Rockwool Near Chimneys, Stoves, and Flues — the IRC R314.2 Verification
Where a heat source enters the attic — masonry chimney, metal flue, wood-stove chimney pipe — code requires thermal barriers and clearances that Class C fibers alone do not provide. Rockwool’s Class A, 2000°F+ tolerance is the reason it is the default specify within 3 feet of chimneys, wood stoves, and HVAC duct boots, and why insulation contractors quote a “mineral wool collar” line item on chimney-heavy roofs. Verify your local amendment cites IRC R314.2 (or the equivalent) before the crew arrives; jurisdictions differ on whether the requirement is the material’s rating, the shield assembly, or both.
❌ Do not assume “fire-safe enough”: Cellulose and fiberglass are rated for the attic floor, not for direct proximity to a hot flue. If your chimney run crosses the insulation plane, the collar of rockwool — or the code-listed clearance shield — is a separate, mandatory scope. Photograph the assembly before covering it; inspectors ask.
⚠️ Rockwool Class A verification: Some budget rockwool uses organic binders that degrade the Class A rating — they WILL burn. For installations within 3 feet of chimneys, wood stoves, or HVAC duct boots, explicit Class A non-combustible certification is mandatory per IRC R314.2. Verify the bag label, not the brand reputation.
For wildfire-interface homes the same logic scales up: rockwool at the eaves and Class A under-roof assemblies measurably change ember-ignition outcomes. The homepage’s climate-zone panel flags the western dry zones where this specification is worth the premium.
Settling and Moisture — the Two Behaviors That Decide the Real-World Winner
Settling is the quiet tax on loose-fill. Cellulose gives back 12.5% of its blown depth in year one, fiberglass about 7.5%, rockwool under 2% — and R-value loss tracks depth loss almost linearly. This is why every serious installer blows loose-fill at ×1.15 of the settled target (16.1” for an R-49 cellulose job) and why our R-value chart bakes the overage into its install column. If you suspect an older attic short-settled, measure and run the settling corrector before buying bags.
Moisture is the other decider, and the fibers split cleanly. Cellulose absorbs and holds water — a roof leak turns R-3.5 fluff into dense, mold-friendly mush that must be removed. Fiberglass and rockwool are hydrophobic: water passes through, and once the source is fixed the fibers recover nearly all their R. In hot-humid climates with condensation cycles, or under roofs with any leak history, that difference can outweigh every thermal advantage cellulose offers.
Dry + vented attic
Cellulose’s air-sealing and cost-per-R advantages dominate. This is the profile where it wins most comparisons.
Humid or leak-prone
Fiberglass’s hydrophobic fibers and rockwool’s stone stability de-risk the assembly; pay the R-per-inch price knowingly.
Near heat sources
Rockwool’s Class A rating is not a luxury — it is the specification that satisfies the flue-clearance scope.
Bonus Row Set: Dense-Pack vs Loose-Fill Cellulose by Application
The third comparison hides inside cellulose itself: the same fiber behaves differently at wall density versus attic-floor density. This is the wall-vs-attic decision in eight rows — and the reason “blown-in” is not one product. The full deep-dive lives in our dense-pack cellulose guide.
| Attribute | Loose-Fill Cellulose (Attic Standard) | Dense-Pack Cellulose (Wall/Rafter Retrofit) |
|---|---|---|
| Density | 0.5–0.7 lb/cu ft | 3.0–3.5 lb/cu ft — 6× denser |
| Settled R-Value Per Inch | 3.2–3.8 (avg 3.50) | 3.5–3.9 (avg 3.70) |
| Settling | 12.5% after 1 yr | <3% after 1 yr |
| Air Sealing | Moderate | Complete — compresses into cavities like hand-packed |
| Application Method | Blown from above through attic hatch | High-pressure nozzle inserted through 1" drill holes in wall/rafter cavities |
| Ideal Use Case | Attic floor coverage — cheapest DIY | Wall cavity insulation in existing homes; rafter bays |
| DIY Feasibility | Yes — standard blower | NO — $400 rental nozzle + 2-person team + hole patching expertise |
| Cost Premium | N/A | +$0.50–$1.00/sq ft installed over loose-fill |
🔧 The dense-pack wall process, end to end: Drill 1” holes at 24” intervals at the TOP of each wall bay → blow dense cellulose until the machine loads to target pressure → patch the holes. Done right, every cavity fills to roughly 3.5 lb/cu ft with no voids.
⚠️ The DIY wall trap: The “DIY Feasibility” row is where wall projects die — the $400 rental nozzle is only half the battle; you also need a 2-person team (one on the hose, one on the machine) and hole-patching expertise. The DIY risks are concrete: missed cavities, voids causing cold spots, and visible patching on interior walls. Walls are a hire-it-out scope.
Frequently Asked Questions
What is blown-in insulation made of?
It depends on which of the three fibers you buy. Blown-in cellulose is roughly 85% recycled newspaper — the 'newspaper insulation' nickname is literal — with 15% boric acid and ammonium sulfate fire retardants. Blown fiberglass is melted sand and recycled glass spun into thin fibers, mostly air by volume. Blown rockwool (mineral wool) is volcanic basalt rock and blast-furnace slag melted at about 2900°F and spun into fibers, which is why it neither burns nor feeds mold.
Is cellulose or fiberglass better for attic insulation?
For attics, cellulose wins on thermal performance per inch (R-3.5 settled vs R-2.2–2.5 for standard loose fiberglass), on air sealing (it conforms and mats around obstructions), and on cost per R-point. Fiberglass wins on moisture tolerance, price at the pallet, and on staying put — it settles less. In a dry, well-ventilated attic in a cold climate, cellulose is usually the better buy; in hot-humid climates where condensation cycles matter, fiberglass's hydrophobic fibers are a legitimate pick.
Is rockwool worth the extra cost?
Sometimes — it depends entirely on what you are paying for. Rockwool costs roughly 80–100% more than cellulose installed and delivers slightly less R per inch (3.25 vs 3.50 settled), so as pure thermal purchase it is not efficient. What the premium buys is a Class A non-combustible rating (necessary near chimneys and flues and valuable in wildfire zones), under 2% settling, superb sound damping, and 50+ year lifespan. If any of those four matter, the premium is rational; if only R-value matters, cellulose is the smarter spend.
What is the fire rating of blown-in insulation?
Loose-fill cellulose and fiberglass both achieve Class C (flame spread under 200, smoke developed under 450) on the ASTM E84 test — cellulose through its borate treatment, fiberglass because glass simply does not burn. Rockwool achieves Class A non-combustible (flame spread under 25), which is why it is specified near heat sources. These ratings apply to the tested product as installed; contaminated or improperly treated cellulose can fail, which is why the bag label matters.
Is cellulose insulation a fire hazard?
New code-compliant cellulose is not — the 15% borate load makes it smolder-resistant: exposed to flame it chars rather than ignites, and its flame spread is under the Class C threshold. The hazard stories come from three specific cases: pre-1978 product with inadequate treatment, cellulose installed against unshielded heat sources like recessed lights and flues without clearance, and moisture-damaged cellulose whose retardants have migrated. Buy ASTM-labeled bags, keep clearances to heat sources, and keep the attic dry.
What is ASTM E84?
It is the standard fire test — the 'Steiner tunnel' — that measures how fast flame spreads across the surface of a building material and how much smoke it generates. Materials get a flame spread index and smoke developed index; Class A is 0–25 flame spread, Class B is 26–75, Class C is 76–200. Insulation sold in the US must carry this rating, and it is exactly why the borate-treated cellulose label reads 'Class C, ASTM E84' rather than 'non-combustible.'
Can blown-in insulation get wet?
Each fiber fails differently. Cellulose absorbs water readily and, once soaked past roughly 20% moisture content, compacts into a dense mass that can drywall-load ceiling boards and grow mold — wet cellulose must be removed, not dried in place. Fiberglass fibers are hydrophobic: the water runs through and, once the source is fixed and the cavity dries, R-value recovers almost fully. Rockwool behaves like fiberglass and adds some inherent mold resistance. In all three cases the leak source matters more than the fiber.
What is GreenFiber insulation?
GreenFiber is the largest US manufacturer of blown-in cellulose — their Sanctuary and Cocoon product lines are what many rental blowers and pro crews actually install. It is made in regional plants from 85% recycled paper with borate treatment, carries ASTM E84 Class C listing, and performs to the same settled R-3.5 per inch used throughout our charts. If a quote mentions 'GreenFiber,' you are buying standard cellulose economics, not a premium material.
Why does cellulose settle more than fiberglass?
Because the fibers behave differently under load. Cellulose fibers are flat, dense paper strips that interlock loosely when blown; gravity and vibration work them into a tighter mat over the first year, costing about 12.5% of installed depth. Glass fibers are springy rods that resist compaction, so loose fiberglass settles only about 5–10% — and rockwool's coarse, rigid fibers barely move at all (under 3%). This is precisely why our R-value chart applies a ×1.15 blowing overage to loose-fill and none to rockwool or dense-pack.
Which blown-in insulation is best for soundproofing?
Rockwool leads — its dense, open fiber structure absorbs mid and high frequencies better than anything else blown or batt, which is why it is the standard in studio walls and party walls. Dense-pack cellulose is the best DIY-adjacent blown option for sound and noticeably beats loose-fill in walls because its 3.0–3.5 lb/cu ft density kills the cavity resonance that loose fill cannot. Loose fiberglass is the weakest of the three acoustically.
How much does blown-in insulation cost per square foot?
Installed, plan on roughly $1.00–$2.00 per sq ft for loose cellulose, $0.90–$1.80 for loose fiberglass, and $1.50–$2.80 for rockwool at a like-for-like attic target. Bag math makes it concrete: our coverage chart prices a 1200 sq ft attic at R-49 at about $1,558 in GreenFiber cellulose (80 bags) versus $1,885 in AttiCat fiberglass (86 bags) — fiberglass's cheaper per-bag price is offset by needing more bags at its lower R-per-inch. Verify current per-bag prices at purchase; retailers reprice seasonally.
Can I mix blown-in materials in one attic?
Yes, with one rule: the layers must be compatible on moisture, and the bottom layer's condition decides. Blowing fiberglass over old cellulose is accepted practice — the fiberglass acts as a capillary break over a fiber that absorbs. Blowing cellulose over fiberglass is equally fine. What is not fine is blowing any loose-fill over insulation that is already wet, compacted, or moldy; the new layer buries the problem. Fix, dry, or remove the old material first, then add on top.
How much rockwool do I need to insulate my attic?
Work from the bag label: a 25 lb bag of loose rockwool covers about 20 sq ft settled at R-30 — the practical DIY ceiling, because loose rockwool needs a 100 PSI blower that rental counters rarely stock. A 1200 sq ft attic at R-30 therefore takes roughly 60 bags, about $2,099 in material at typical big-box pricing, and that quote usually lands with a professional crew. For R-38 and above, blown rockwool is effectively pro-only — compare every brand's bag counts on our coverage chart before committing.