Cellulose or fiberglass? The honest blown-in answer.
We don't sell either material. Both insulate well when installed right; cellulose is denser and slows air movement, blown fiberglass is lighter and often a touch cheaper. Here is where each one actually wins.
Install quality beats material choice
Most of the ranking content on this question is written by someone selling one of the two. We aren't.
Both materials do the job blown into an attic. Cellulose packs denser, carries more R-value per inch, and slows air moving through it. Blown fiberglass is lighter, shrugs off moisture, and often comes in slightly cheaper installed. A careful fiberglass job outperforms a sloppy cellulose job every time, and vice versa.
What matters more than the bag: air sealing the attic floor first (see why sequencing matters), an even full depth with no thin spots, baffles at the eaves, and a quote that states the settled R-value. Get those four and either material will perform for decades.
Cost per square foot, side by side
Published national ranges from contractor-marketplace pricing. Sources disagree slightly per material, so both spreads are shown.
| Material | Installed range | Source spread |
|---|---|---|
| Blown-in cellulose | $0.60–$2.30 / sq ft | HomeGuide; TLS Energy Savers puts it at $0.80–$2.50 |
| Blown-in fiberglass | $0.50–$2.00 / sq ft | HomeAdvisor; Angi puts blown-in attic work overall at $0.90–$2.40 |
For a typical 1,000 sq ft attic that works out to roughly $800–$2,500 in blown-in cellulose (HomeGuide), with blown-in attic projects generally landing at $900–$3,600 (Angi). The overlap is the honest takeaway: the material difference is dimes per square foot, while access, prep, and removal of old insulation move the price by dollars. Full breakdown in the attic insulation cost guide and the insulation cost overview, or run your own number.
Per inch, cellulose packs more
Figures from the DOE/ORNL Insulation Fact Sheet. Both materials settle; good installers blow extra depth to compensate.
| Material | R-value per inch | Depth to reach R-49 |
|---|---|---|
| Loose-fill cellulose | R-3.2–3.8 | roughly 13–15 in |
| Loose-fill fiberglass | R-2.2–2.7 | roughly 18–22 in |
Fiberglass needs several more inches to hit the same target, which only matters if your attic is short on clearance at the eaves. On settling: all loose fill compacts over time, cellulose more than fiberglass. The industry answer is to blow past the target depth so the settled thickness still hits spec. Make sure the quote states settled R-value, not just installed depth on day one.
The four differences that actually matter
Air movement
Cellulose's density slows air washing through the insulation, a real advantage in windy, cold climates and leaky older homes. Loose fiberglass is more air-permeable, so wind at the eaves erodes its performance more. Neither replaces actual air sealing.
Moisture behavior
Cellulose absorbs and slowly releases moisture, which buffers small swings but means it needs a dry attic with working ventilation. Fiberglass doesn't absorb water, so it tolerates humid attics with marginal ventilation better. A roof leak ruins both.
Fire treatment and recycled content
Cellulose is mostly recycled paper treated with borates for fire and pest resistance. Fiberglass is spun glass, naturally non-combustible, typically with a lower recycled share. Both meet code when installed to spec; more on the material in the cellulose guide.
Dust during install
Cellulose install day is dusty; crews should seal the work area and wear respirators. Fiberglass sheds irritating fibers instead. Either way this is a one-day nuisance handled by a professional crew, not a reason to pick a material.
Where fiberglass is the right call
-
Topping up existing fiberglass. Adding more of the same over a clean, dry layer is the cheapest R-value you can buy, and there's no performance reason to switch materials for a top-up.
-
Humid attics with marginal ventilation. If the attic runs damp and the ventilation fix isn't happening first, fiberglass's indifference to moisture makes it the lower-risk choice.
-
Tight budgets. The published ranges start lower ($0.50 vs $0.60 per sq ft installed), and in some markets the gap per quote is real. If the choice is fiberglass now or cellulose someday, take the insulation now.
Where cellulose is the right call
-
Cold climates. More R per inch and better resistance to air washing through it mean cellulose holds its rated performance better exactly when the temperature difference is biggest.
-
Leaky older homes. Density helps, but seal the attic floor first either way: air sealing comes before insulation in every sensible retrofit order.
-
Dense-pack wall retrofits. This is cellulose's home turf: packed into closed wall cavities through drilled holes, where fiberglass loose fill can't match the density. Published national pricing runs $1.20–$4.20 per sq ft of wall area installed, with about $2 as the typical middle. More on why cellulose owns this niche.
Rebates pay for R-value, not material
Every efficiency program we track rebates insulation by the R-value achieved or the square footage treated, not by which material the installer blows in. So the material question never changes your rebate; it only changes the quote the rebate applies to.
Where states have launched the federal HEAR program, income-eligible households can get up to $1,600 toward insulation, air sealing, and ventilation combined. That allocation is not the same as availability: many states have not launched, and several that have are income-restricted or paused. The Rebate Matcher checks your state's actual status, including the state and utility programs that often cover a larger share than the federal money.
Cellulose vs fiberglass questions
Is cellulose better than fiberglass in the attic?
Neither wins everywhere. Cellulose carries more R-value per inch (R-3.2 to 3.8 vs R-2.2 to 2.7 for loose-fill fiberglass, per the DOE/ORNL Insulation Fact Sheet) and its density slows air moving through the material, which matters in cold, windy climates and leaky older homes. Blown fiberglass is lighter, often slightly cheaper installed, and does not absorb moisture, which makes it the safer call in humid attics with marginal ventilation. In a dry attic in a mild climate, install quality matters more than the material.
What is the R-value per inch of cellulose insulation?
Loose-fill cellulose runs R-3.2 to R-3.8 per inch according to the DOE/ORNL Insulation Fact Sheet. Loose-fill fiberglass runs R-2.2 to R-2.7 per inch, so fiberglass needs several more inches of depth to hit the same R-value target.
Does cellulose insulation settle?
Yes. All loose-fill insulation settles as it ages, and cellulose settles more than fiberglass because it is denser. Installers compensate by blowing to a greater initial depth than the settled target, and every bag carries a coverage chart showing initial versus settled thickness. A quote should state the settled R-value, not just the day-one depth.
Is cellulose insulation safe?
Cellulose is ground recycled paper treated with borate compounds to meet fire safety codes; the treatment also deters pests and mold. The practical concern is dust during installation, so the crew should seal off the work area and wear respirators. Once installed it sits inert in the attic. Fiberglass has its own install-day irritation issues, so neither material is a hands-bare product.
Can you blow cellulose over fiberglass?
Yes, topping up over existing insulation is standard practice and the layers simply add R-value. You do not need to match materials. Two caveats: wet, moldy, or pest-contaminated insulation should come out first, and the attic floor should be air sealed before anything new goes on top, because burying leaks locks them in.
Get alerts when insulation rebates change in your state
Free weekly digest. Unsubscribe anytime. Privacy policy.
Either material gets cheaper once your state chips in.
Insulation rebates cover a big share of the job in many states. Two minutes to see yours.
Match my rebates