How do cellulose and fiberglass compare on R-value?
Both materials are rated in R-value per inch, and both fall in a broadly similar range: blown cellulose typically runs around R-3.2 to R-3.8 per inch, while blown or batt fiberglass typically runs around R-2.2 to R-4 per inch depending on density. In practical terms, that means it usually takes a bit less cellulose than fiberglass to hit the same R-value, though the difference is not dramatic once both are installed at a proper depth.
The bigger factor in real-world performance is usually installed depth and coverage, not the small per-inch gap between the two materials. An attic with the model code's recommended R-49 to R-60 for Climate Zone 5 in mind can hit that target with either material — the question is how many inches of each it takes and whether the existing attic has room for it.
What happens to each material over time?
This is where the two materials genuinely diverge. Loose-fill cellulose will settle from 10 to 20 percent over time due to gravity and vibration — foot traffic near the attic hatch, HVAC vibration, and just years of sitting under its own weight all contribute. A competent installer accounts for that settling by blowing in extra depth up front so the settled R-value still lands where it needs to.
Fiberglass does not settle in the same way. Once it is blown or laid to a given depth, it tends to hold that loft much more consistently over the years, with less need to overfill to compensate. That is a genuine advantage for fiberglass in attics where get-it-right-once depth matters, and a genuine tradeoff worth knowing about before choosing cellulose.
How do the two materials handle fire and moisture?
Cellulose is made largely from recycled paper, so it is treated with a fire retardant, usually a borate compound, to meet fire-safety standards. Fiberglass is spun glass and is non-combustible on its own, without needing a chemical treatment to resist ignition. Both are considered safe attic insulation when installed correctly; the difference is in how each material achieves that safety.
Moisture behavior differs too. Cellulose can absorb and hold more moisture than fiberglass before it dries out, which matters most in attics with roof leaks, poor ventilation, or ice-dam history. Fiberglass sheds moisture faster and dries quicker, though neither material is a substitute for fixing the actual moisture source. In either case, insulation should go in after ventilation and leak issues are addressed, not instead of addressing them.
When does each material typically make more sense?
Cellulose is often chosen for attics that need to be topped up densely in a limited number of inches, or where its somewhat better resistance to air movement through the material is a priority. Its recycled content also appeals to some homeowners. Fiberglass is often chosen where consistent long-term loft matters, where a homeowner wants batts for a wall or floor cavity rather than loose-fill, or where cost per R-value is the deciding factor on a given project.
Neither material is categorically better across every attic — the right choice depends on the existing insulation already up there, the depth available, ventilation conditions, and budget. A contractor doing an in-home assessment should be able to explain which material fits the specific attic rather than defaulting to whichever one they install most often.
Sources
What the data says
According to the Building America Solution Center, “loose-fill cellulose will settle from 10 to 20 percent over time due to gravity and vibration.”
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 5.”
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What happens after you request a quote?
An installer should measure existing insulation depth, note attic ventilation and any moisture issues, and walk through why they are recommending cellulose, fiberglass, or a mix for the specific attic before quoting a price.
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We've got your info and are routing it to a local pro. When they call, here's what to look for in the quote you receive: (1) R-value target — the model energy code for Climate Zone 5 calls for R-49 to R-60 attic insulation; ask what your contractor is quoting to. (2) Air sealing scope — insulation alone does nothing if air leaks aren't sealed first, and Xcel requires a blower door test to rebate it. (3) Rebate handling — ask which programs your specific address qualifies for. Xcel and Efficiency Works are separate programs with separate paperwork, and not every address qualifies for both. (The federal IRA Section 25C tax credit ended December 31, 2025 — no federal insulation credit applies to 2026 work. Xcel Energy's insulation and air-sealing rebate remains active for its gas and electric customers. Longmont Power & Communications electric customers may separately qualify for an Efficiency Works rebate — check your bill to see which utility serves your address. Boulder County EnergySmart pays a third, income-qualified-only rebate as of January 1, 2026.) (4) Removal scope — older homes often need old insulation removed before new install. If a quote skips all four, get another quote.
Frequently asked
Common questions about cellulose vs. fiberglass
Can cellulose and fiberglass be mixed in the same attic?
Yes, topping one material with the other is common and not a problem structurally. What matters is getting the combined depth to the target R-value, which a contractor should calculate rather than guess at.
Does settling mean cellulose loses R-value permanently?
Settling reduces loft, and R-value tracks loft, so yes, a cellulose layer that has settled has less R-value than the day it was installed unless it was overfilled to account for that settling up front.
Is fiberglass itchy to work with compared to cellulose?
Fiberglass fibers can irritate skin and lungs during installation, which is one reason this is not a do-it-yourself job. Cellulose has its own dust concerns during installation. Both call for proper protective equipment and ventilation.
Which material is better for a rodent-damaged attic?
Rodent damage usually means the existing insulation needs removal regardless of material, since droppings and nesting material are the real problem. See the rodent-damaged insulation page for how that removal step is typically handled.
Does either material help with soundproofing?
Both add some sound dampening compared to an empty cavity, largely because of density and coverage rather than one material having a special acoustic property. Neither is a substitute for a dedicated sound-control assembly.
Is one material cheaper than the other?
Pricing varies by installer, coverage needed, and market conditions, and specific numbers are not something to generalize here. An in-home quote is the reliable way to compare real costs for a given attic.
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