R-30 insulation runs about 8.25 to 10 inches thick for standard fiberglass batts, and anywhere from roughly 4.3 inches (closed-cell spray foam) to 13.5 inches (loose blown fiberglass) depending on the material. The thickness comes straight from R-value per inch: divide 30 by the material’s R-per-inch and you get the depth. R-30 is the value most codes assign to insulated 2×10 cathedral rafters and to many floors over unconditioned space, which is why the batt version is sized to fit a nominal 2×10 cavity.
R-30 insulation thickness by material (reference table)
R-30 thickness ranges from about 4.3 inches for closed-cell spray foam to 13.5 inches for loose blown fiberglass. Each material hits R-30 at a different depth because each has a different R-value per inch. The table below gives the R-per-inch and the resulting thickness for the common insulation types, so you can match a product to the cavity or attic depth you have.
| Material | R-value per inch | Thickness for R-30 |
|---|---|---|
| Closed-cell spray foam | R-6.0 to 7.0 | 4.3 to 5 in |
| Fiberglass batt (high-density) | R-3.7 | 8.25 in |
| Open-cell spray foam | R-3.5 to 3.7 | 8 to 8.5 in |
| Mineral wool batt | R-3.3 to 3.4 | 8.75 to 9 in |
| Blown cellulose | R-3.2 to 3.8 | 8 to 9.5 in |
| Fiberglass batt (standard) | R-3.1 to 3.2 | 9.5 to 10 in |
| Blown fiberglass | R-2.2 to 2.7 | 11 to 13.5 in |
Manufacturers round to standard product sizes, so a “R-30” fiberglass batt is commonly labeled 9.5 inches (high-density) or sold as a 10-inch loft. The math and the label rarely match to the tenth of an inch, and that gap is normal.
How many inches is R-30 insulation for batts?
R-30 fiberglass batts are about 8.25 inches thick in high-density form and 9.5 to 10 inches in standard density. High-density batts pack more glass fiber into less depth, so they reach R-30 at 8.25 inches, sized to sit inside a nominal 2×10 rafter or joist (9.25 inches of real depth) without compressing.
Standard-density R-30 batts need 9.5 to 10 inches because their R-value per inch is lower (about R-3.1 to 3.2). Mineral wool batts rated R-30 land near 8.75 to 9 inches. Compressing any batt to fit a shallower cavity lowers its R-value, so a 10-inch batt jammed into a 9.25-inch 2×10 no longer performs at R-30.
Will R-30 batts fit standard rafters?
R-30 high-density batts (8.25 inches) fit a nominal 2×10 rafter or joist, which has a true depth of 9.25 inches, leaving room for a ventilation gap. This pairing is why R-30 is the go-to value for 2×10 cathedral ceilings and sloped rafter assemblies where a deeper cavity is not available.
A standard 9.5 to 10-inch R-30 batt will not fit a 2×10 cavity that also needs a 1-inch vent channel above it. In that case you either use the high-density R-30 batt or install baffles and accept a shallower batt. Framing depth, not the R-30 label, decides which product works, so measure the real cavity before ordering.
R-30 blown insulation depth
Blown R-30 needs roughly 8 to 9.5 inches of settled cellulose or 11 to 13.5 inches of blown fiberglass. Loose fill is measured by settled depth on the attic floor, and installers mark rulers or set depth cards so a blower operator can hit the target coverage across the whole attic.
Blown fiberglass is fluffier and lighter, so it needs the most depth of any R-30 option. Cellulose is denser per inch, so it reaches R-30 in fewer inches but weighs more, which matters on older ceiling framing. Always blow to the initial installed depth the manufacturer specifies, because loose fill settles over the first months and settled depth is what carries the rating.
R-30 fiberglass vs cellulose thickness
At R-30, blown cellulose sits at about 8 to 9.5 inches while blown fiberglass needs 11 to 13.5 inches. Cellulose delivers roughly R-3.2 to 3.8 per inch versus R-2.2 to 2.7 for loose fiberglass, so cellulose reaches the same R-value in noticeably less depth.
The tradeoff is weight and air movement. Cellulose is heavier and resists air infiltration better, which helps in cold, windy attics. Fiberglass is lighter and does not absorb water the way cellulose can, but its lower density lets convective loss creep in at very low temperatures unless it is installed deep. Choose by attic depth, ceiling load, and climate rather than by the R-number alone.
How thick is R-30 spray foam?
R-30 closed-cell spray foam is about 4.3 to 5 inches thick, and open-cell foam is about 8 to 8.5 inches. Closed-cell foam carries the highest R-per-inch of any common insulation (R-6.0 to 7.0), so it hits R-30 in the least depth and doubles as an air and vapor control layer.
Open-cell foam runs about R-3.5 to 3.7 per inch, close to a batt, so it needs roughly twice the closed-cell depth for R-30. In a 2×10 rafter, closed-cell R-30 leaves several inches of open cavity, while open-cell nearly fills it. Spray foam is applied in lifts, so the finished thickness is what the installer sprays to, not a preset panel size.
Do you need R-30, and where?
R-30 is a code-level target for cathedral or sloped rafter ceilings, floors over unconditioned space (crawlspaces, garages, cantilevers), and some knee walls, rather than for flat attic floors, which now typically call for R-49 to R-60. The 2021 IECC sets R-30 as the floor requirement across many climate zones.
Where a full attic floor is being insulated, current code in most of the US asks for far more than R-30. Check the sibling depth targets in our R-38 insulation thickness, R-49 insulation thickness, and R-60 insulation thickness guides. For the per-inch numbers behind every material, see our insulation R-value chart. R-30 remains the right value when cavity depth is capped, as it is in a 2×10 rafter.
Frequently asked questions
How thick is R-30 insulation?
R-30 is about 8.25 to 10 inches thick for fiberglass batts, 8 to 9.5 inches for blown cellulose, 11 to 13.5 inches for blown fiberglass, 4.3 to 5 inches for closed-cell spray foam, and 8 to 8.5 inches for open-cell spray foam. The exact depth depends on the material’s R-value per inch, since thickness equals 30 divided by that figure.
How many inches is R-30 insulation?
The most common R-30 product, a standard fiberglass batt, is 9.5 to 10 inches. A high-density R-30 batt is 8.25 inches so it fits a 2×10 rafter. There is no single inch count for R-30 because each material reaches the value at a different depth, from about 4.3 inches for closed-cell foam to 13.5 inches for loose blown fiberglass.
How thick is an R-30 batt?
An R-30 batt is 8.25 inches in high-density fiberglass, 9.5 to 10 inches in standard fiberglass, and about 8.75 to 9 inches in mineral wool. High-density versions exist specifically so R-30 fits a nominal 2×10 cavity with room for a vent channel. Compressing a batt below its rated thickness cuts the R-value below 30.
How deep does blown insulation need to be for R-30?
Blown insulation needs about 8 to 9.5 inches of settled cellulose or 11 to 13.5 inches of blown fiberglass to reach R-30. Loose fill settles over the first months, so it is installed to a greater initial depth than the settled target. Follow the coverage chart on the bag, which lists installed depth for the R-value.
Is R-30 enough for an attic?
R-30 is enough for cathedral rafters, floors over unconditioned space, and knee walls, but it is below current code for a flat attic floor in most US climate zones, where R-49 to R-60 is typical. R-30 is used where framing depth caps the cavity, such as a 2×10 sloped ceiling that cannot hold a deeper batt.
Why does R-30 thickness vary so much by material?
R-30 thickness varies because each material has a different R-value per inch, from about R-2.2 for loose fiberglass to R-7.0 for closed-cell spray foam. Thickness equals 30 divided by the R-per-inch, so a high-performing material reaches R-30 in fewer inches. Density, air resistance, and how the product is manufactured all drive that per-inch figure.
Reviewed by The Roofing Brief Team. Last reviewed July 2026.