How deep is R-49 insulation?

It depends on the material: about 14 inches of blown cellulose (3.5 R/in), about 19.6 inches of blown fiberglass (2.5 R/in), or about 7.5 inches of closed-cell foam (6.5 R/in). Depth = target R ÷ R-per-inch.

What R-value should attic insulation be?

The DOE recommends roughly R-38 to R-60 in attics by climate zone — colder zones toward R-60, milder toward R-38 to R-49. R-49 is a common target for existing homes.

How many bags of blown insulation do I need?

Bags = attic area ÷ the coverage-per-bag on your product label at the target R, rounded up. For 1,000 sq ft at 42.7 sq ft/bag, that is 24 bags. Coverage per bag falls as the target R rises.

Does R-per-inch change by material?

Yes. Roughly: fiberglass batt 3.2, blown fiberglass 2.5, cellulose 3.5, open-cell foam 3.7, closed-cell foam 6.5. The same R target needs different depths for each.

Attic insulation: how deep for R-38, R-49 and R-60

Insulation is rated in R-value, but you install it by depth. The bridge is R-per-inch, which differs by material — so the same R target is a different number of inches for cellulose than for fiberglass.

By Francesco Zinghinì

R-value, depth and R-per-inch

R-value measures resistance to heat flow — higher is better. But you buy and install insulation by depth (inches), and each material delivers a fixed R per inch. So depth for a target is simply:

inches = target_R ÷ R_per_inch

Because R-per-inch differs by material, hitting R-49 takes very different depths depending on what you use. That is why "how deep should my insulation be?" has no single answer without naming the material — the same R number can be 14 inches or 20 inches of actual product.

R-per-inch by material

Typical values (stable reference points): fiberglass batt about 3.2, loose-fill blown fiberglass about 2.5, blown cellulose about 3.5, open-cell spray foam about 3.7, and closed-cell spray foam about 6.5. The R-value chart lists these with the depth each needs for common targets, and the insulation depth calculator computes any target and material. Note that these are nominal values; a product's label rating is what governs.

Depths for R-38, R-49 and R-60

Working the formula for cellulose (3.5 R/in): R-38 needs 38 ÷ 3.5 = 10.9 in, R-49 needs 49 ÷ 3.5 = 14.0 in, and R-60 needs 60 ÷ 3.5 = 17.1 in. For blown fiberglass (2.5 R/in) the same targets need more depth: R-49 is 49 ÷ 2.5 = 19.6 in — over five inches deeper than cellulose for the same R. Closed-cell foam (6.5 R/in) reaches R-49 in just 7.5 inches, but at a very different cost and with professional application. Depth, not R, is what you actually verify with a ruler stuck into the attic insulation.

Which R target should you aim for?

The US Department of Energy recommends roughly R-38 to R-60 in attics, depending on climate zone — colder zones toward R-60, milder ones toward R-38 to R-49. R-49 is a common whole-country target for existing homes and a sensible default if you're unsure. Aim for the value your climate zone calls for; adding depth past that gives diminishing returns, because savings scale with the difference in 1/R, not R itself — the jump from R-13 to R-38 saves far more than the jump from R-38 to R-60 (see insulation savings).

Batts vs blown, and settling

Two common ways to add attic insulation are batts (pre-cut rolls or sheets laid between joists) and blown loose-fill (cellulose or fiberglass blown in with a machine). Blown fill is usually faster over an irregular attic and gets into corners batts miss, while batts are easy to add in a clean, open attic. One caution with loose-fill: some products settle over time, so install to the labeled installed thickness for the target R, not just to a fresh-blown height that will later compress. The bag's coverage chart accounts for this — follow it.

Blown-in: from depth to bags

For loose-fill insulation you order by the bag, and the coverage per bag depends on the target R — it is printed on the bag as a coverage chart. Bags follow from area and that coverage:

bags = ceil( attic_area ÷ coverage_per_bag )

Worked example: 1,000 sq ft of attic, with a label coverage of 42.7 sq ft per bag at the target R, is 1,000 ÷ 42.7 = 23.4, rounding up to 24 bags. Always read the label for your target R, because coverage per bag drops as the target rises — the deeper you go, the fewer square feet each bag covers. The blown-in bags calculator uses your label figure.

Cost and performance by material

Depth isn't the only difference between materials — cost, installation and behavior vary too, and the cheapest R-per-inch isn't always the best buy. Blown cellulose is inexpensive, has a high R-per-inch (3.5), and packs well around obstructions, though some settling means installing to the labeled thickness. Blown fiberglass is light and won't settle much, but its lower 2.5 R-per-inch means noticeably more depth for the same R. Fiberglass batts are cheap and DIY-friendly in a clean, open attic, but they leave gaps if the attic is irregular. Spray foams deliver the most R per inch (open-cell 3.7, closed-cell 6.5) and also air-seal as they go, but they're the priciest and are professionally installed. For most attic top-ups, blown cellulose or fiberglass gives the best cost-to-R; foam earns its premium where space is tight or air-sealing and insulating in one step is worth it. Match the material to your attic and budget, then set the depth from its R-per-inch.

Air-seal first, and other cautions

Insulation slows heat conduction, but air leaks bypass it entirely, so air-sealing the attic floor (around penetrations, top plates, and the attic hatch) often matters as much as the last few inches of R — do it before you add depth. Don't compress insulation to fit, because compression cuts the R-value. Keep it clear of recessed-light housings unless they're IC-rated, and maintain soffit-to-ridge airflow with baffles so you don't block attic ventilation. These are planning estimates from typical R-per-inch values; confirm your product's rating on its label.