How do cellulose and fiberglass actually differ?
Cellulose is recycled paper treated with boric acid for fire and pest resistance, blown into attics and walls through a hose. It adds roughly 3.5 R per inch and installs at 1.5-1.7 pounds per cubic foot on an attic floor. Fiberglass is spun glass fiber — lightweight at 0.5-0.8 pounds per cubic foot blown, roughly 2.5 R per inch, and indifferent to moisture in a way paper-based material can never be.
Both run through the same blower machine, both fill the gaps and irregular framing that batts can't, and both are covered in depth on the blown-in insulation hub. The practical differences live in four places: wind resistance, moisture behavior, fire and pest treatment, and settling. Each one is below.
Which handles Denver wind-washing better?
Cellulose, and it isn't close. Wind-washing — air movement through the surface of fibrous insulation that strips effective R-value — hits hardest at the attic perimeter where soffit airflow enters, and it can strip 20-40% of effective R-value in the affected zones. Low-density loose-fill fiberglass is the most vulnerable material; cellulose's denser fiber pack and inter-fiber friction resist air movement through the blanket, and dense-pack cellulose at roughly 3.5 pounds per cubic foot is highly wind-resistant.
This matters more here than in most metros. Homes west of I-25 and anywhere foothills-adjacent — Golden, west Lakewood, west Arvada — take chinook wind events that find every soffit gap. The full mechanics, including what baffles do and how to spot wind-washed insulation, are in the Front Range wind guide. The short version: if your perimeter rooms run cold and your home catches wind, material density at the attic edges is not a tiebreaker — it's the point.
Which handles moisture and freeze-thaw better?
Fiberglass. Glass fiber doesn't absorb water; cellulose does, and once wet it compresses, loses R-value, and dries slowly. That's why blown-in cellulose installs must happen in dry conditions and why pre-existing roof or plumbing leaks get remediated before any cellulose goes in — not after.
Two Denver-specific qualifiers keep this honest. First, Denver's climate is dry: the chronic-humidity conditions that make moisture indifference decisive in, say, the Gulf states are rare here. Second, the moisture event that actually damages Denver attics is the freeze-thaw cycle working on roof edges and the condensation that air leakage carries into cold attics — problems that air sealing solves and material choice doesn't. The freeze-thaw guide covers that mechanism. If your attic has a known moisture history, fiberglass buys forgiveness; if the moisture source is unfixed, neither material is a fix.
What about fire, pests, and settling?
Fire: cellulose is treated with boric acid and meets current fire-code requirements; fiberglass is inherently non-combustible glass. Both pass. The boric-acid treatment earns extra weight in foothills-adjacent areas where wildland-urban-interface fire risk runs higher.
Pests: the same boric-acid treatment that handles fire deters insects and rodents. Fiberglass is fire-resistant but carries no pest deterrence — a real difference in older homes with a rodent history.
Settling: cellulose settles 15-20%, which sounds alarming and mostly isn't. Reputable installers blow extra material above the target so the settled depth still hits the rated R-value — the spec to ask for is "settled R-49" or similar. Blown fiberglass holds its loft and doesn't meaningfully settle, which removes one spec conversation. An installer who can't explain settled-depth compensation is telling you something more important than any material comparison can.
Which costs less installed in Denver?
Fiberglass, usually — by a margin that rarely decides anything. Denver attic installs typically price cellulose at $1.50-$3.00 per square foot and fiberglass at $1.25-$2.75, with whole-attic jobs for most 1,500-2,500 sq ft homes landing between $1,500 and $4,500 before rebates either way. The overlap in those ranges is the honest story: access, existing conditions, and air-sealing scope move the price more than the material does.
Fiberglass's lower R per inch also claws back some of its price edge — hitting the same R-49 takes more depth, more bags, more blowing time. Run your own square footage through the cost calculator for the project-level math, and see the blown-in cost guide for what moves a blown-in quote up or down. The R-value targets those numbers chase — and why Denver's altitude makes them non-negotiable — live in the R-value at altitude guide.
When is each the wrong choice?
Cellulose is wrong when the trusses can't carry its density, when the attic has a chronic unresolved moisture problem, or when the homeowner has a documented cellulose allergy. It's also the wrong tool below grade and against masonry — that's foam territory.
Fiberglass is wrong at low density in a wind-exposed perimeter without proper baffles, in homes where sound dampening between floors matters, and where pest deterrence carries weight. It's a fine field material that underperforms exactly where Denver attics get attacked.
Worth saying plainly: blown-in retrofit installs across Denver are predominantly cellulose, which is why this site has a blown-in hub anchored on cellulose and no dedicated fiberglass page. That's a reflection of what local crews actually install, not a verdict against fiberglass — when the cases above apply, fiberglass is the right call and a good contractor will say so. And when the job is a rim joist, a crawl space wall, or an unvented roof assembly, neither fibrous material is the answer — that comparison is the open-cell vs closed-cell guide. For the broader question of whether your attic needs anything at all, start there.
Sources
What the data says
According to the Department of Energy, “loose-fill cellulose insulation typically settles 15-20% over its lifetime, reducing effective R-value at the same nominal depth.”
According to the International Energy Conservation Code, “the 2021 IECC (R402.1.2) sets attic insulation minimums at R-49 to R-60 for Climate Zone 5B, which covers the Denver metro area.”
According to the Department of Energy, “adequate insulation and air sealing can reduce heating and cooling costs by 10% to 20% in typical homes.”
Take the next step
Want the right material for your actual attic?
The cellulose-or-fiberglass call depends on what is already up there — depth, density, moisture history, and how much wind your perimeter sees. The free in-home estimate settles it with measurements: current R-value, the right material for each zone, and the rebate-adjusted cost to fix it.
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We've got your info. A local pro is reviewing it now. Expect a call within a few hours, or by tomorrow at the latest. While you wait, here's what to look for in the quote you receive: (1) R-value target — current Colorado code is R-49 to R-60 for attics, anything less is under-spec. (2) Air sealing scope — insulation alone does nothing if air leaks aren't sealed first. (3) Rebate handling — Xcel rebate paperwork should be handled for you, not by you. (The federal IRA Section 25C credit expired in 2025 and Colorado HEAR closed for the Front Range — Xcel programs are now the active rebate stack.) (4) Removal scope — pre-1990 homes often need old insulation removed before new install. If a quote skips all four, get another quote.
Frequently asked
What do Denver homeowners ask about cellulose and fiberglass?
Is cellulose a fire risk?
No. Cellulose is treated with boric acid and meets all current fire-code requirements — the recycled-paper origin makes people nervous, but the treated product chars rather than flames. In foothills-adjacent Denver areas with elevated wildfire exposure, the boric-acid treatment is actually a point in cellulose's favor. The fire question worth asking isn't about the material; it's whether recessed lights and flues get proper clearances during install.
Does fiberglass really itch forever?
It itches whoever handles it, every time — install crews wear protection for a reason. Once it's blown into the attic and left undisturbed, you have no exposure; insulation on the attic floor isn't in your living space air. The itch matters if you use the attic for storage and dig through it regularly, in which case ask about decking over the storage zone rather than switching materials over it.
Can cellulose and fiberglass be layered?
Yes, and it's one of the most common retrofit patterns in Denver: existing fiberglass batts from the 1960s-80s, topped with blown-in cellulose to reach R-49. Dry, intact existing material can typically stay in place and be topped — the new layer fills gaps the old batts left. What can't be papered over is a dirty ceiling plane: if air sealing is needed, it has to happen before any new material buries the leak paths.
Which material do Denver pros actually install?
Predominantly cellulose for attic retrofits — it's the most common attic-retrofit material in North America, and most 2026 Xcel Energy Insulation and Air Sealing Rebate applications use blown-in cellulose or fiberglass as the primary material. Fiberglass shows up where its strengths apply: weight-limited trusses, moisture-prone spots, and homeowner preference. If a contractor pushes one material for every house, that's an inventory decision, not an engineering one.
What's the R-value per inch for each?
Cellulose roughly 3.5 R per inch; blown fiberglass roughly 2.5. At the Climate Zone 5B attic target, that's about 14 inches of cellulose for R-49 — fiberglass needs proportionally more depth for the same number, which matters in attics with low clearance at the eaves. Depth is the spec to verify at the final walkthrough either way: R-value lives in installed inches, not in the product brochure.
My home was built after 2010 and feels fine — do I need either?
Probably neither. Post-2010 homes built to recent code typically carry attic insulation at or near current targets, and topping an already-adequate attic buys almost nothing. If a newer home has cold rooms or climbing bills, the likely culprits are air sealing gaps, duct issues, or a construction defect — which is the builder's warranty problem, not a material choice. Diagnose before buying anything.
Does cellulose's recycled content actually matter?
For thermal performance, no — R-value is R-value. It matters if embodied footprint factors into your decision: cellulose is recycled paper with low-energy processing, while fiberglass is melted glass. It's a legitimate tiebreaker when the application doesn't already decide the question. Just don't pay a premium for the label: the green choice that actually moves your energy use is whichever material gets installed correctly over a sealed ceiling plane.
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