R-38 insulation thickness runs from about 5.5 inches to 16 inches depending on the material. A standard fiberglass batt hits R-38 at roughly 12 inches, high-density fiberglass batt at 10.25 inches, blown cellulose at 10.5 to 12 inches, blown fiberglass at 13.5 to 16 inches, and closed-cell spray foam at just 5.5 to 7 inches. The number that matters is R-38 (the thermal resistance), not the depth. Any material reaches it at the depth its R-value per inch requires.
R-38 is the U.S. Department of Energy’s recommended attic level for most cold and mixed climates (roughly Climate Zones 5 through 8 under the 2021 IRC). Because material density varies, “how thick is R-38” has no single answer. Use the reference table below to match the depth to the product you are actually installing.
R-38 insulation thickness by material (reference table)
R-38 thickness depends entirely on the insulation’s R-value per inch. Denser, higher-R materials reach R-38 in less depth. Below is the depth each common attic material needs to hit R-38, with the R-value per inch each is rated at. Closed-cell spray foam is the thinnest at about 5.5 to 7 inches; loose-fill fiberglass is the thickest at up to 16 inches.
| Material | R-value per inch | Thickness for R-38 |
|---|---|---|
| Closed-cell spray foam | R-6.0 to 7.0 | 5.5 to 7 in |
| Fiberglass batt (high-density, R-38C) | R-3.7 | 10.25 in |
| Blown cellulose | R-3.2 to 3.8 | 10.5 to 12 in |
| Mineral wool (rock wool) batt | R-3.3 to 3.4 | 11.25 to 11.5 in |
| Open-cell spray foam | R-3.5 to 3.7 | 10.5 to 11.5 in |
| Fiberglass batt (standard density) | R-3.1 to 3.2 | 12 in |
| Blown (loose-fill) fiberglass | R-2.2 to 2.7 | 13.5 to 16 in |
These figures assume the insulation is installed at its rated density and not compressed. Compression lowers R-value, so a 12-inch batt crushed into a 10-inch cavity no longer delivers R-38. For a full breakdown of every material’s rating, see our insulation R-value chart by material, zone, and thickness.
R-38 insulation: how many inches for batts?
R-38 fiberglass batts are 12 inches thick in standard density and 10.25 inches in high-density (labeled R-38C, designed for 2×12 cathedral and vaulted cavities). Both are sold pre-cut to R-38, so you install one layer and you are done. The standard-density batt is the common attic product; the high-density R-38C exists specifically to fit R-38 into a nominal 12-inch framing bay.
Mineral wool batts reach R-38 at about 11.25 to 11.5 inches because rock wool runs a slightly higher R-value per inch than standard fiberglass. When you buy a batt labeled “R-38,” the manufacturer has already sized the thickness to that rating, so you match the batt to your joist depth rather than measuring loose depth. More on batt selection is in our guide to fiberglass attic insulation: batts vs blown, R-value, and cost.
Will R-38 batts fit standard attic joists?
Standard 2×10 attic joists are 9.25 inches deep, so a 12-inch R-38 batt sits about 2.75 inches proud of the framing. That is normal and correct: the excess loft above the joists is where the R-value lives. Do not compress the batt down to joist height, because compressing a 12-inch batt to 9.25 inches drops it from R-38 to roughly R-30. If you need R-38 flush inside a 2×10 rafter cavity, use high-density R-38C batt or closed-cell spray foam instead.
R-38 blown insulation depth
Blown cellulose reaches R-38 at 10.5 to 12 inches of settled depth, while blown (loose-fill) fiberglass needs 13.5 to 16 inches. The gap comes from density: cellulose is rated R-3.2 to 3.8 per inch, loose-fill fiberglass only R-2.2 to 2.7 per inch. Blown insulation is measured by installed depth and by coverage (bags per square foot), both printed on the bag’s coverage chart.
Every bag of blown insulation carries a manufacturer coverage table listing the settled depth, minimum bags per 1,000 square feet, and installed weight required to hit each R-value. For R-38, follow that chart rather than eyeballing it, then set depth rulers (attic cards) around the attic so you can verify the finished depth. Settling matters most with cellulose: blow to the initial depth the bag specifies for R-38 so the settled depth still meets it. See our full guide to blown-in insulation: cost, R-value, and DIY vs pro.
R-38 fiberglass vs cellulose thickness
Cellulose reaches R-38 in less depth than blown fiberglass: about 10.5 to 12 inches for cellulose versus 13.5 to 16 inches for loose-fill fiberglass. Cellulose is denser and rated higher per inch (R-3.2 to 3.8 versus R-2.2 to 2.7), so it hits the same R-38 in roughly 3 to 5 fewer inches. In a low-clearance attic where depth is limited, that difference decides which material fits.
| Blown material | R-value per inch | Depth for R-38 | Best when |
|---|---|---|---|
| Cellulose | R-3.2 to 3.8 | 10.5 to 12 in | Low headroom, air-sealing priority |
| Loose-fill fiberglass | R-2.2 to 2.7 | 13.5 to 16 in | Open attics, moisture concerns |
Depth is not the only tradeoff. Cellulose settles more over time and is heavier per square foot, which matters on older ceiling framing. Loose-fill fiberglass resists moisture better and does not settle as much, but it loses more R-value in extreme cold. Match the material to the attic, not just to the depth chart.
How thick is R-38 spray foam?
Closed-cell spray foam reaches R-38 at 5.5 to 7 inches, the thinnest of any attic material, because it is rated R-6 to R-7 per inch. Open-cell spray foam needs 10.5 to 11.5 inches at R-3.5 to 3.7 per inch. Closed-cell also adds a vapor barrier and structural rigidity, which is why it is used where depth is tight, such as under a roof deck in an unvented (conditioned) attic assembly.
Spray foam is installed by depth passes, so the applicator builds up lifts until the rated R-38 thickness is reached. Because closed-cell foam is expensive per board foot, many jobs use it as a thinner base layer for air-sealing and vapor control, then top it with cheaper blown insulation to reach the full R-38. That hybrid approach is common in retrofit attics.
Do you need R-38, and where?
R-38 is the Department of Energy’s recommended attic insulation level for most of the northern and central United States (roughly IRC Climate Zones 5 through 8). Warmer southern zones often call for R-30, while the coldest zones may require R-49 or R-60. Attic ceilings carry the highest recommended R-values in the house because heat rises and escapes through the roof plane first.
If your attic already has some insulation, you add depth to reach R-38 rather than starting over: measure existing depth, identify the material, and top up with the same or a compatible product. To size the exact amount and cost for your attic, use our guide to how much it costs to insulate an attic and the broader attic insulation types, R-value by zone, and cost overview.
Frequently asked questions
How many inches is R-38 insulation?
R-38 insulation is 5.5 to 16 inches thick depending on material. Standard fiberglass batt is about 12 inches, high-density fiberglass batt 10.25 inches, blown cellulose 10.5 to 12 inches, blown fiberglass 13.5 to 16 inches, and closed-cell spray foam only 5.5 to 7 inches. The R-38 rating stays constant; the depth changes with the material’s R-value per inch.
How thick is an R-38 fiberglass batt?
A standard-density R-38 fiberglass batt is 12 inches thick, and a high-density R-38C batt is 10.25 inches thick. The high-density version is made to fit R-38 into a nominal 12-inch framing cavity, such as a 2×12 cathedral ceiling. Both are pre-cut to R-38, so a single layer delivers the full rating when installed without compression.
How deep does blown insulation need to be for R-38?
Blown cellulose needs 10.5 to 12 inches of settled depth for R-38, and blown fiberglass needs 13.5 to 16 inches. Always follow the coverage chart printed on the insulation bag, which lists the exact settled depth and bags per square foot for R-38. Set depth rulers across the attic so you can verify the finished depth after blowing.
Is R-38 cellulose or fiberglass thinner?
Cellulose is thinner for R-38: about 10.5 to 12 inches versus 13.5 to 16 inches for loose-fill fiberglass. Cellulose is denser and rated R-3.2 to 3.8 per inch, while loose-fill fiberglass is only R-2.2 to 2.7 per inch, so cellulose reaches the same R-38 in roughly 3 to 5 fewer inches. That can matter in a low-clearance attic.
Will R-38 batts fit in 2×10 joists?
A 12-inch R-38 batt sits about 2.75 inches above 2×10 joists (9.25 inches deep), which is normal for an attic floor. Do not compress the batt to joist height, because crushing a 12-inch R-38 batt to 9.25 inches cuts it to roughly R-30. If R-38 must fit flush inside a 2×10 cavity, use high-density R-38C batt or closed-cell spray foam.
Why does R-38 thickness vary so much by material?
R-38 thickness varies because each material has a different R-value per inch. R-38 is a fixed thermal resistance target, so depth equals 38 divided by the material’s R-per-inch. Closed-cell spray foam at R-6 to R-7 per inch reaches it in 5.5 to 7 inches, while loose-fill fiberglass at R-2.2 to 2.7 per inch needs up to 16 inches for the identical R-38.
Reviewed by The Roofing Brief Team. Last reviewed July 2026.