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Blown-in cellulose vs fiberglass: which one insulates better?

Cellulose gives R-3.2-R-3.8 per inch vs fiberglass's R-2.2-R-2.7, but settles ~13%. Compare R-value, settling, moisture and cost.

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Blown-in cellulose vs fiberglass: which one insulates better?

Blown-in cellulose vs fiberglass: which insulates better?

Per inch, cellulose wins on paper. The U.S. Department of Energy lists loose-fill cellulose at about R-3.2 to R-3.8 per inch and loose-fill fiberglass at R-2.2 to R-2.7 - so cellulose reaches a target R-value in less depth. Fiberglass is lighter, resists moisture better, and settles less, which is why "better" depends on your priorities rather than the spec sheet alone.

Blown-in cellulose vs fiberglass: which insulates better?

How do settling, moisture and cost compare?

Cellulose settles about 13% after installation, so installers blow it in deeper than the target depth (GreenBuildingAdvisor). Fiberglass settles very little, so its installed depth stays close to rated. On price the two are close: fiberglass runs $0.50 to $1.10 per square foot and cellulose $0.60 to $1.80, up to $2.30 for dense-pack (HomeAdvisor, 2025). Cellulose is made mostly from recycled paper treated for fire resistance; fiberglass is spun glass and handles moisture better since it will not absorb and hold water the way paper-based cellulose can.

Sources

3 cited
  1. R-value per inch for both materials: U.S. Department of Energy, Building America Solution Center, Loose-Fill Insulation (checked 2026-08-19).
  2. Cellulose settling behavior: GreenBuildingAdvisor, Blown-in cellulose (settling) (checked 2026-08-19).
  3. Cost per square foot for both materials: HomeAdvisor, Cost to Install Blown-In Insulation, 2025 (checked 2026-08-19).
How it works

How blown-in insulation is actually installed

Four steps, done in order. The Department of Energy is explicit that skipping the air-sealing step is the most common way a new insulation job still leaves an attic underperforming.

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  1. 1

    Air seal first

    The EPA recommends sealing attic-floor penetrations before insulating, because loose-fill does not stop air moving through gaps around wiring, pipes and fixtures.

  2. 2

    Add baffles and depth markers

    Rafter baffles keep blown material off the eaves where it could get wet, and depth rulers mark the target R-value before the blower starts.

  3. 3

    Blow to depth

    A pneumatic machine feeds loose fiberglass or cellulose over the attic floor, covering the tops of the joists so heat cannot bypass the insulation.

  4. 4

    Verify and clean up

    The settled depth gets checked against the target R-value for your climate zone, then tools and debris are cleared.

How we work

What this site will and will not do

We explain the R-value target and the real cost drivers first, and we do not sell you the installation - which is why we can say plainly when topping up beats a full replacement.

01

R-value before a quote

ENERGY STAR recommends R-30 to R-60 depending on your climate zone. Knowing your target before you call means you can hold every bid to the same number.

02

Air sealing first, always

The Department of Energy is explicit that insulation slows heat flow but does not stop air leaks - sealing gaps first is what makes the insulation you pay for actually perform.

03

We are not the installer

We do not install anything ourselves, so we have nothing to gain from steering you toward the bigger job. Licensed local companies do the work and quote it themselves.

04

Nothing we cannot source

Every figure on this site traces to a named, dated source - HomeAdvisor, the DOE, ENERGY STAR, the EPA or the FTC. Where we have not sourced a number, we say so instead of guessing.

Questions

Frequently asked

The questions people actually type before calling a company.

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Is blown-in cellulose better than blown-in fiberglass?

Cellulose has a higher R-value per inch, so it reaches a target R-value in less depth. Fiberglass is lighter, settles less, and handles moisture better. Neither is universally better - it depends on your attic and priorities.

Does cellulose or fiberglass cost more?

They're close. Fiberglass runs $0.50-$1.10 per square foot and cellulose $0.60-$1.80 (up to $2.30 for dense-pack), per HomeAdvisor's 2025 data.

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