Batt insulation carries an R-value of roughly R-3.1 to R-3.4 per inch for standard fiberglass and R-3.7 to R-4.3 per inch for high-density fiberglass and mineral wool. In practice you buy batts by their labeled R-value, not by the inch: a 3.5-inch batt sized for a 2×4 wall reads R-13 or R-15, a 6.25-inch batt for a 2×6 wall reads R-19, and a 12-inch attic batt reads R-38. The number on the bag is what the batt delivers only when it is installed at full thickness in a cavity that matches its depth.
Batt insulation R-value per inch by material
Per inch, batt R-value depends on the fiber and its density. Standard fiberglass batts run about R-3.1 to R-3.4 per inch. High-density fiberglass batts reach R-3.7 to R-4.3 per inch by packing more glass fiber into the same space. Mineral wool (rock or stone wool) batts sit at R-3.7 to R-4.3 per inch, close to high-density fiberglass but with better fire and sound performance.
| Batt material | R-value per inch | Typical use |
|---|---|---|
| Standard-density fiberglass | R-3.1 to R-3.4 | Walls, floors, attic |
| High-density fiberglass | R-3.7 to R-4.3 | 2×6 walls, cathedral ceilings |
| Mineral wool (rock/stone wool) | R-3.7 to R-4.3 | Walls, sound and fire assemblies |
Fiberglass batt R-values are tested to ASTM C665. Because per-inch numbers vary by brand and lot, the labeled R-value on the batt, not a per-inch estimate, is the figure to size your job around.
Batt insulation R-value and thickness chart
Every standard batt R-value maps to a set thickness. Standard fiberglass batts give the familiar R-11, R-13, R-19, R-30, and R-38 grades. High-density fiberglass and mineral wool hit higher R-values (R-15, R-21, R-23) at the same or thinner profiles, which is how a 5.5-inch batt can read R-21 instead of R-19.
| Nominal R-value | Actual thickness | Material type | Designed cavity |
|---|---|---|---|
| R-11 | 3.5 in | Low-density fiberglass | 2×4 wall |
| R-13 | 3.5 in | Standard fiberglass | 2×4 wall |
| R-15 | 3.5 in | High-density fiberglass / mineral wool | 2×4 wall |
| R-19 | 6.25 in | Standard fiberglass | 2×6 wall (see compression note) |
| R-21 | 5.5 in | High-density fiberglass | 2×6 wall |
| R-23 | 5.5 in | Mineral wool | 2×6 wall |
| R-25 | 8.25 in | Standard fiberglass | 2×8 floor/ceiling |
| R-30 | 8.25 in (high-density) or 10 in (standard) | Fiberglass | 2×8 to 2×10, attic floor |
| R-38 | 10.25 in (high-density) or 12 in (standard) | Fiberglass | 2×10 to 2×12, attic floor |
Batts also come in two widths: 15 inches for framing 16 inches on center, and 23 inches for framing 24 inches on center. Width does not change R-value; it only affects fit and friction hold between studs or joists. For a broader cross-material view, see the full insulation R-value chart by material and climate zone.
What R-value batt fits each framing cavity
Match the batt to the cavity depth, not the other way around. A 2×4 wall has a 3.5-inch cavity, a 2×6 wall has 5.5 inches, and deeper floor or ceiling joists take thicker batts. Overstuffing a cavity with a higher-R batt does not add R-value; it compresses the fiber and loses performance.
| Framing | Cavity depth | Best-fit standard batt | Best-fit high-density batt |
|---|---|---|---|
| 2×4 wall | 3.5 in | R-13 | R-15 |
| 2×6 wall | 5.5 in | R-19 (compresses to ~R-18) | R-21 or R-23 |
| 2×8 joist | 7.25 in | R-25 | R-30 (high-density) |
| 2×10 joist | 9.25 in | R-30 | R-38 (high-density) |
| 2×12 joist | 11.25 in | R-38 | R-38 |
The 2×6 wall is where batt choice matters most. An R-19 fiberglass batt is 6.25 inches thick, so it gets squeezed into a 5.5-inch cavity and settles at about R-18. An R-21 high-density batt is built at 5.5 inches and holds its full rating in the same wall, which is why many builders spec R-21 over R-19 in 2×6 framing.
Fiberglass vs mineral wool batt R-value
Mineral wool batts deliver a higher R-value per inch than standard fiberglass, roughly R-4.0 versus R-3.2, so they reach R-15 in a 2×4 wall and R-23 in a 2×6 wall where standard fiberglass tops out at R-13 and R-19. Mineral wool is also denser, does not sag, resists fire to about 1,800 F, and blocks more sound. It costs more per square foot than fiberglass.
Fiberglass batts are cheaper, lighter, and easier to cut, which keeps them the default for most walls and attics. If you need maximum R-value in a fixed cavity, a fire-rated assembly, or quieter interior walls, mineral wool earns the premium. Compare the two directly in fiberglass batts versus blown-in fiberglass.
Standard vs high-density batts
High-density batts pack more fiber into the same thickness, so they hit a higher R-value without a thicker profile. A standard 3.5-inch fiberglass batt reads R-11 or R-13; a high-density 3.5-inch batt reads R-15. In a 2×6 cavity, high-density fiberglass reaches R-21 at 5.5 inches while standard fiberglass needs 6.25 inches to reach R-19.
- Use standard batts when the cavity depth already matches the R-value you need and cost is the priority.
- Use high-density batts when you want the highest R-value a fixed cavity can hold, such as a 2×4 exterior wall or a 2×6 wall where R-19 would compress.
- Use mineral wool when fire rating, sound control, or the highest per-inch R-value drives the choice.
The compression penalty: why the label is not always the R-value
Compressing a batt lowers its R-value, and the loss is larger than most homeowners expect. Squeezing a 6.25-inch R-19 batt into a 3.5-inch 2×4 cavity does not give R-19; it delivers about R-13. The trapped air that does the insulating gets pushed out, so more material in a tight space performs worse, not better.
| Batt (rated) | Full thickness | Compressed into | Effective R-value |
|---|---|---|---|
| R-19 fiberglass | 6.25 in | 5.5 in (2×6) | ~R-18 |
| R-19 fiberglass | 6.25 in | 3.5 in (2×4) | ~R-13 |
| R-30 fiberglass | 10 in | 8.25 in | ~R-27 |
| R-38 fiberglass | 12 in | 10 in | ~R-33 |
Two rules follow from this. First, never cram a thicker batt into a shallow cavity to gain R-value; buy the batt built for that depth. Second, a partially compressed batt still beats an uncompressed batt of the same original thickness at a lower rating, per the Insulation Institute compressed R-value data, so an R-30 pressed to 8.25 inches (about R-27) still outperforms a true R-25 batt in the same space.
Does faced or unfaced batt change the R-value?
Facing does not change a batt’s R-value. A kraft-paper or foil facing is a vapor retarder and a stapling flange, not insulation, so an R-13 faced batt and an R-13 unfaced batt insulate the same. Choose faced batts where code requires a vapor retarder on the warm side, and unfaced batts for a second attic layer or where a separate vapor barrier is used.
Attic floors that already have one insulated layer should get unfaced batts on top, laid perpendicular to the joists, so no second vapor retarder is trapped in the assembly.
What R-value batt do you actually need?
The R-value you need depends on the assembly and your climate zone, not on the batt alone. The U.S. Department of Energy recommends R-13 to R-21 for walls, R-25 to R-30 for floors, and R-30 to R-60 for attics, with the higher end applying to colder zones. Batts can reach the wall and floor targets in a single layer; attic targets above R-38 usually need a second layer or blown-in insulation on top.
| Assembly | DOE recommended R-value | Typical batt solution |
|---|---|---|
| 2×4 exterior wall | R-13 to R-15 | R-13 standard or R-15 high-density |
| 2×6 exterior wall | R-19 to R-21 | R-21 high-density |
| Floors over unconditioned space | R-25 to R-30 | R-30 batt in 2×10 joists |
| Attic (warm zones) | R-30 to R-38 | R-38 batt or R-30 plus blown-in |
| Attic (cold zones) | R-49 to R-60 | R-38 batt plus blown-in top-up |
For attic floors, compare batt grades directly in R-30 vs R-38 insulation, and size the full assembly by climate zone with the attic insulation guide by zone and cost. If you are matching an exact thickness, see how thick R-19 insulation is by material.
Frequently asked questions
What is the R-value of batt insulation per inch?
Standard fiberglass batt insulation delivers about R-3.1 to R-3.4 per inch. High-density fiberglass and mineral wool batts reach R-3.7 to R-4.3 per inch. Because per-inch figures vary by brand and density, size a job by the labeled R-value on the batt rather than multiplying a per-inch estimate by thickness.
How thick is R-13, R-19, and R-30 batt insulation?
An R-13 fiberglass batt is 3.5 inches thick and fits a 2×4 wall. An R-19 batt is 6.25 inches thick and is designed for a 2×6 wall. An R-30 batt is about 8.25 inches in high-density form or 10 inches in standard fiberglass, used in 2×8 to 2×10 joists and attic floors.
Is R-13 or R-19 better for a 2×4 wall?
R-13 is the correct batt for a 2×4 wall. The cavity is only 3.5 inches deep, so a 6.25-inch R-19 batt would be compressed to roughly R-13 anyway, wasting the material and money. To gain R-value in a 2×4 wall without changing framing, use a high-density R-15 batt built at 3.5 inches.
Can you compress batt insulation to fit a cavity?
Compressing a batt lowers its R-value, so it should be avoided for gaining performance. An R-19 batt forced into a 3.5-inch cavity drops to about R-13. Light compression to fit around wiring is acceptable, but you should never stuff a thicker batt into a shallow space expecting its full rating. Buy the batt sized for the cavity depth.
What R-value batt fits a 2×6 wall?
A 2×6 wall has a 5.5-inch cavity. An R-19 fiberglass batt is 6.25 inches and compresses to about R-18, so many builders instead use a high-density R-21 batt built at 5.5 inches, which holds its full rating. Mineral wool R-23 batts also fit a 2×6 wall at 5.5 inches for the highest single-layer value.
Does a faced batt have a higher R-value than unfaced?
No. Facing is a vapor retarder and stapling flange, not insulation, so a faced and unfaced batt of the same rating insulate identically. Use faced batts where code requires a warm-side vapor retarder, and unfaced batts for a second attic layer or where a separate vapor barrier already exists.
Reviewed by The Roofing Brief Team. Last reviewed July 2026.