Serving Nationwide, all 50 statesAttic Insulation Help connects you with licensed local companies. We are not a contractor and do not perform the work ourselves.
info@atticinsulationhelp.com
Attic Insulation Help
Email Us
United States

How deep does blown-in insulation need to be for its rated R-value?

Blown-in fiberglass gives R-2.2-R-2.7 per inch; cellulose gives R-3.2-R-3.8. See how depth, settling, and R-value work together.

Request a Quote
How deep does blown-in insulation need to be for its rated R-value?

What R-value does blown-in insulation give per inch?

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 per inch. Divide your target R-value (R-30 to R-60 depending on climate zone) by the material's per-inch figure to estimate the depth it needs - cellulose reaches a given target in noticeably less depth than fiberglass.

What R-value does blown-in insulation give per inch?

Why doesn't R-value scale evenly with thickness?

Because settled density rises as depth increases, the R-value of loose-fill does not change proportionately with thickness (U.S. DOE, Energy Saver). That is also why installers blow cellulose in thicker than the final target: it settles about 13% after installation, so 17 inches of settled thickness typically means installing roughly 20 inches (GreenBuildingAdvisor). Fiberglass settles far less, so its installed depth stays closer to its rated depth. Measure the depth you actually have, not the depth that was installed years ago.

Sources

3 cited
  1. Loose-fill fiberglass and cellulose R-value per inch: U.S. Department of Energy, Building America Solution Center, Loose-Fill Insulation (checked 2026-08-19).
  2. R-value-vs-thickness behavior: U.S. Department of Energy, Energy Saver, Types of Insulation (checked 2026-08-19).
  3. Cellulose settling (~13%) and installed-vs-settled depth: GreenBuildingAdvisor, Blown-in cellulose (settling) (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.

Request a Quote
  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.

Request a Quote
How many inches of blown-in insulation equal R-38?

It depends on the material - cellulose (R-3.2-R-3.8 per inch) reaches R-38 in roughly 10-12 inches, while fiberglass (R-2.2-R-2.7 per inch) needs roughly 14-17 inches. Check the specific product's coverage chart, since brands vary.

Does cellulose lose R-value as it settles?

The rated R-value already accounts for settled density, so a properly installed job (blown in about 13% deeper than the target) should still hit its rated value once settled - but measuring the actual current depth is the only way to confirm it.

Get matched with a licensed local insulation company

Request a QuoteEmail Us
Request a QuoteEmail Us