R-49 insulation runs about 13 to 20 inches thick, depending on the material. Fiberglass batts and blown cellulose land near 14 inches, blown fiberglass needs roughly 16 to 20 inches, open-cell spray foam about 13 to 14 inches, and closed-cell spray foam only 7 to 9 inches. R-49 is the IECC attic minimum for climate zones 4 and 5, so this is the target most U.S. homeowners are asked to hit.
The exact depth is simple math: divide 49 by the material’s R-value per inch. Because that R-per-inch figure varies by product and settles over time for loose-fill, the table below gives a working range for each material rather than a single number.
How many inches is R-49 insulation by material?
R-49 requires roughly 7 to 20 inches of material, and the spread comes down to how much R-value each product delivers per inch. Denser, higher-R materials like closed-cell spray foam hit R-49 in under 9 inches. Lower-density loose-fill and batts need 14 inches or more. Use the range that matches your specific product’s rated R-per-inch.
| Material | R-value per inch | Thickness for R-49 |
|---|---|---|
| Fiberglass batt | R-3.1 to R-3.4 | about 14 to 16 in |
| Blown fiberglass (loose-fill) | R-2.5 to R-3.0 | about 16 to 20 in |
| Blown cellulose (settled) | R-3.2 to R-3.8 | about 13 to 15 in |
| Mineral wool batt | R-3.3 to R-3.7 | about 13 to 15 in |
| Blown mineral wool | R-3.0 to R-3.3 | about 15 to 16 in |
| Open-cell spray foam | R-3.5 to R-3.8 | about 13 to 14 in |
| Closed-cell spray foam | R-5.6 to R-7.0 | about 7 to 9 in |
Ranges reflect real product variation. A single manufacturer’s data sheet gives one exact figure, so check the bag or batt rating before you buy. The insulation R-value chart lists per-inch values across every common material if you need to compare options side by side.
How thick is R-49 fiberglass batt insulation?
R-49 fiberglass batts run about 14 to 16 inches thick. No single fiberglass batt is rated R-49 on its own in most product lines, so installers usually stack layers: an R-38 batt (about 12 inches) plus an R-11 or R-13 batt, or two R-25 batts, laid perpendicular to close the seams. High-density R-49 batts do exist but are less common on the shelf.
Because standard attic joists are 2×10 (9.25 inches deep) or 2×12 (11.25 inches deep), a 14- to 16-inch batt stack sits well above the joist tops. That is normal and expected for R-49; the insulation should mound over the framing, not compress down into it. Compressing a batt to fit the cavity lowers its R-value.
How many inches of blown insulation for R-49?
Blown insulation for R-49 runs about 13 to 20 inches of settled depth, and the material matters. Blown cellulose reaches R-49 at roughly 13 to 15 inches. Blown fiberglass needs more, about 16 to 20 inches, because it delivers fewer R-value points per inch. Always measure the settled depth, not the freshly blown height.
Loose-fill fiberglass and cellulose settle after installation. Cellulose can lose 15 to 20 percent of its initial height as it compacts. Reputable installers account for this by blowing to a higher fill line and marking the target depth with attic rulers (cardboard depth markers) stapled to the joists across the attic. The bag’s coverage chart lists both the minimum installed thickness and the settled thickness for R-49.
For a full breakdown of loose-fill options, coverage, and DIY versus pro pricing, see the guide to blown-in insulation.
How thick is R-49 spray foam?
R-49 spray foam is about 7 to 9 inches for closed-cell and 13 to 14 inches for open-cell. Closed-cell foam carries the highest R-per-inch of any common attic material (R-5.6 to R-7.0), so it hits R-49 in the least space. That makes it the usual choice when depth is tight, such as insulating a rafter cavity for a conditioned attic.
Open-cell foam is lighter and cheaper per board foot but delivers only about R-3.5 to R-3.8 per inch, so it needs roughly the same depth as batts to reach R-49. A standard 2×10 rafter (9.25 inches deep) cannot hold R-49 of open-cell foam without furring the rafters out or accepting a lower R-value. See spray foam attic insulation for open versus closed cell cost and R-value details.
Why is R-49 the target for zones 4 and 5?
R-49 is the attic insulation minimum in the 2021 International Energy Conservation Code (IECC) for climate zones 4 and 5, which covers most of the central and mid-latitude United States. Zones 6, 7, and 8 (the colder north) call for R-60, and warmer zones 2 and 3 allow R-38 or R-49 depending on the code year. Your local jurisdiction may adopt an older or amended code, so confirm the requirement with your building department.
The number is a floor, not a ceiling. Many homeowners in zone 4 or 5 add R-49 as a baseline and go higher if attic depth allows, because the added material pays back through lower heating and cooling loads. R-49 is a common upgrade target for older homes that were originally built to R-19 or R-30.
R-49 versus R-38 and R-60 thickness
R-49 sits between R-38 and R-60 on both the code ladder and the depth chart. For the same material, each step up adds a few inches. Fiberglass batt runs about 12 inches at R-38, 14 to 16 inches at R-49, and 18 to 19 inches at R-60. Match your target to your climate zone first, then read the thickness off the material row.
| R-value | Typical IECC zones | Blown cellulose depth | Fiberglass batt depth |
|---|---|---|---|
| R-38 | Zones 2 to 3 | about 10.5 to 12 in | about 12 in |
| R-49 | Zones 4 to 5 | about 13 to 15 in | about 14 to 16 in |
| R-60 | Zones 6 to 8 | about 16 to 18 in | about 18 to 19 in |
If your code calls for a different level, compare the full material tables on the R-38 insulation thickness and R-60 insulation thickness pages.
How to hit R-49 when attic depth is limited
When joist depth cannot hold 14 or more inches of batt or loose-fill, you have three practical routes to R-49: switch to a higher-R material, build up the framing, or combine layers. The right choice depends on whether the insulation sits on the attic floor or against the roof deck.
- Use a higher-R material. Closed-cell spray foam reaches R-49 in 7 to 9 inches, fitting where batts cannot. This is the common fix for shallow rafter cavities in a conditioned attic.
- Add furring or a raised platform. Furring strips on the joists or a raised sub-floor create room for the full batt or blown depth without compressing it.
- Layer batt over blown-in. Blow loose-fill into the joist cavities, then lay unfaced batts across the top, perpendicular to the joists, to reach the combined R-49.
- Keep baffles at the eaves. Whatever the depth, install ventilation baffles so insulation does not block soffit airflow. See attic insulation baffles for placement.
Never compress insulation to make it fit. Compression lowers R-value, so a squeezed R-49 batt may perform at R-38 or less. If space is genuinely tight, a higher-R material is the honest way to keep the rated performance.
Frequently asked questions
How many inches is R-49 insulation?
R-49 insulation is about 13 to 20 inches thick, depending on material. Fiberglass batts and blown cellulose land near 14 inches, blown fiberglass needs 16 to 20 inches, open-cell spray foam about 13 to 14 inches, and closed-cell spray foam only 7 to 9 inches. Divide 49 by the product’s rated R-value per inch to get the exact depth.
How thick is R-49 batt insulation?
R-49 fiberglass batt insulation is roughly 14 to 16 inches thick. Most product lines do not offer a single R-49 batt, so installers stack layers, such as an R-38 batt plus an R-11 or R-13 batt laid perpendicular. Mineral wool batts reach R-49 at a slightly shallower 13 to 15 inches because they carry more R-value per inch.
How many inches of blown insulation is R-49?
R-49 needs about 13 to 15 inches of settled blown cellulose or 16 to 20 inches of blown fiberglass. Measure the settled depth, not the freshly blown height, because loose-fill compacts over time. Cellulose can lose 15 to 20 percent of its height. The insulation bag’s coverage chart lists the exact minimum thickness for R-49.
Is R-49 enough for an attic?
R-49 meets the IECC attic minimum for climate zones 4 and 5, which covers much of the central and mid-latitude United States. Colder zones 6 through 8 call for R-60. R-49 is often the code floor rather than a ceiling, so homeowners with attic depth to spare sometimes add more. Confirm the requirement with your local building department.
Can R-49 fit in a 2×10 or 2×12 joist?
Not without mounding above the framing. A 2×10 joist is 9.25 inches deep and a 2×12 is 11.25 inches, while R-49 batt or blown-in needs 14 to 16 inches. The insulation should sit above the joist tops, which is normal for R-49. In a shallow rafter cavity, closed-cell spray foam is the way to reach R-49 without added depth.
Reviewed by The Roofing Brief Team. Last reviewed July 2026.