By The Roofing Brief Team. Last reviewed: October 2026.
R-30 insulation thickness is 9 to 9.5 inches for a standard fiberglass roll or batt and 8.25 inches for a high-density R-30C batt, and ranges from about 4.25 inches (closed-cell spray foam) to 10.8 inches (blown fiberglass) across materials. Those batt figures come from the Owens Corning PINK Next Gen fact sheet (May 2026); stone wool reaches R-30 at 7.25 inches per the ROCKWOOL Comfortbatt data sheet (July 2026). Rigid foam board reaches R-30 at about 5.2 inches of polyiso or 6 inches of XPS. Divide 30 by the material’s R-value per inch and you get the depth.
R-30 insulation thickness by material (reference table)
R-30 thickness ranges from about 4.25 inches for closed-cell spray foam to 10.8 inches for Johns Manville Climate Pro blown fiberglass. Each row below names the product whose maker chart we checked, so you can match it to the bag or data sheet you are buying.
| Material | Product checked | R-value per inch (derived) | Thickness for R-30 | Source |
|---|---|---|---|---|
| Closed-cell spray foam | Typical 2.0 lb/cu ft foam | R-6.1 aged (R-7.1 on newer HFO sheets) | About 4.9 in (4.25 in at R-7.1) | DOE Building America spray foam guide |
| Polyiso board | Sika Sarnatherm ISO | LTTR R-5.7 at 1.0 in | 5.2 in (two 2.6 in boards) | Sika data sheet, Aug 2024 |
| XPS board | Owens Corning FOAMULAR NGX 250 | R-5.0 | 6 in | Owens Corning data sheet, Oct 2025 |
| Stone wool batt | ROCKWOOL Comfortbatt R30 | About R-4.1 | 7.25 in | ROCKWOOL data sheet, July 2026 |
| Fiberglass batt, high-density | Owens Corning R-30C | About R-3.6 | 8.25 in | Owens Corning fact sheet 10025109-B |
| Open-cell spray foam | Typical 0.5 lb/cu ft foam | About R-3.6 | About 8.3 in | DOE Building America spray foam guide |
| Blown cellulose | Greenfiber SANCTUARY (25 lb bag) | About R-3.6 settled | 8.9 in installed, 8.3 in settled | Greenfiber DM-6.3-38 Rev F, May 2026 |
| Fiberglass roll or batt, standard | Owens Corning R-30 | About R-3.2 to 3.3 | 9 in (roll), 9.5 in (batt) | Owens Corning fact sheet 10025109-B |
| Blown fiberglass | Owens Corning ProCat (33.5 lb bag) | About R-2.9 | 10.5 in | Owens Corning ProCat fact sheet, March 2026 |
| Blown fiberglass | Owens Corning AttiCat (27.5 lb bag) | About R-2.8 | 10.75 in | AttiCat data sheet 10011273-L, May 2026 |
| Blown fiberglass | Johns Manville Climate Pro | About R-2.8 | 10.8 in | JM Climate Pro attic card BIC-260 |
Makers round to standard product sizes, so the R-30 label and the 30-divided-by-R-per-inch math rarely match to the tenth of an inch. Owens Corning sells its standard R-30 as a 9.5-inch batt and a 9-inch roll, and the Insulation Institute’s compressed R-value chart (BI506, Oct 2020) also lists a 10-inch R-30 label thickness from other fiberglass makers.
Which R-30 product fits your space? (decision table)
| If your space is | Use | Thickness for R-30 | Why |
|---|---|---|---|
| 2×10 cathedral rafter (9.25 in deep), vented | Owens Corning R-30C high-density batt | 8.25 in | Leaves the 1 in air gap IRC R806.3 requires above insulation at vents |
| 2×10 floor joist over a garage or crawlspace (no vent gap) | Standard R-30 batt | 9.5 in | Compressed into 9.25 in it still rates R-30 on the Insulation Institute chart |
| 2×8 joist or rafter (7.25 in deep) | ROCKWOOL Comfortbatt R30, or closed-cell foam | 7.25 in stone wool; about 4.9 in closed-cell | A 10 in R-30 fiberglass batt crushed into 7.25 in drops to about R-25 |
| Open attic floor in Climate Zone 1 | Blown cellulose or fiberglass | 8.9 to 10.8 in | R-30 is the ENERGY STAR level for an uninsulated Zone 1 attic |
| Open attic floor in Zones 2 to 8 | Go past R-30 to R-49 or R-60 | See our R-49 and R-60 guides | ENERGY STAR recommends R-49 to R-60 for uninsulated attics there |
| Low-slope roof, insulation above the deck | Polyiso board in two layers | 5.2 in | Sika’s thickest single board (4.5 in) stops at R-26.8 |
How many inches is R-30 insulation for batts?
R-30 fiberglass batts are 8.25 inches thick in high-density R-30C form and 9.5 inches in standard density (9 inches for the R-30 roll), per Owens Corning’s fact sheet (publication 10025109-B, May 2026). The R-30C batt comes in 15.5 and 23.75 inch widths, sized to sit inside a nominal 2×10 rafter or joist (9.25 inches of real depth) without compressing.
Standard R-30 batts need 9.5 inches because their R-value per inch is lower, about R-3.2. Stone wool is denser: ROCKWOOL lists Comfortbatt R30 at 7.25 inches for wood studs, which fits a 2×8 bay. Compressing a fiberglass batt lowers its R-value: the Insulation Institute chart rates a 10-inch R-30 batt at R-29 in a 9.25-inch 2×10 cavity and about R-25 in a 7.25-inch 2×8.
Will R-30 batts fit standard rafters?
R-30C high-density batts (8.25 inches) fit a nominal 2×10 rafter or joist, which has a true depth of 9.25 inches, and leave exactly the 1-inch ventilation gap that IRC R806.3 requires between insulation and roof sheathing at the vent. That pairing is why R-30C exists for 2×10 cathedral ceilings and sloped rafter assemblies where a deeper cavity is not available.
A standard 9.5-inch R-30 batt will not fit a 2×10 cavity that also needs a 1-inch vent channel above it, because 9.5 plus 1 is 10.5 inches against 9.25 inches of depth. In that case use the R-30C batt, or install baffles and closed-cell foam, or fur the rafters down by 1.25 inches. Measure the real cavity before ordering: a 2×10 is 9.25 inches and a 2×12 is 11.25 inches.
R-30 blown insulation depth
Blown R-30 needs 8.9 inches installed and 8.3 inches settled of Greenfiber SANCTUARY cellulose at 38.6 bags per 1,000 sq ft, or 10.5 to 10.8 inches of blown fiberglass: Owens Corning ProCat 10.5 inches (12.4 bags), Owens Corning AttiCat 10.75 inches (14.6 bags) and Johns Manville Climate Pro 10.8 inches (13.7 bags). Coverage charts rate loose fill at its settled depth, but the depth markers required by the 2021 IRC (Section N1101.10.1.1) show the minimum initial installed thickness, so the blower operator fills to that higher line across the whole attic. The settled thickness goes on the installer’s certificate (Section N1101.10.1).
| Product (bag weight) | Installed depth for R-30 | Settled depth | Bags per 1,000 sq ft | Min. weight per sq ft | Source |
|---|---|---|---|---|---|
| Greenfiber SANCTUARY cellulose (25 lb) | 8.9 in | 8.3 in | 38.6 | 0.97 lb | Greenfiber DM-6.3-38 Rev F |
| Owens Corning ProCat fiberglass (33.5 lb) | 10.5 in | 10.5 in | 12.4 | 0.416 lb | Owens Corning 10019591-C |
| Owens Corning AttiCat fiberglass (27.5 lb) | 10.75 in | 10.75 in | 14.6 | 0.402 lb | Owens Corning 10011273-L |
| Johns Manville Climate Pro fiberglass | 10.8 in | 10.8 in | 13.7 | 0.41 lb | JM BIC-260 |
Blown fiberglass is lighter, so it needs the most depth of any R-30 option: about 0.40 to 0.42 lb per sq ft on the three fiberglass charts versus 0.97 lb for Greenfiber cellulose, roughly 2.4 times the ceiling load. Blow cellulose to the 8.9-inch installed depth, not the 8.3-inch settled figure. In Minnesota, the AttiCat sheet adds 1.0 inch and 1.6 bags per 1,000 sq ft at R-30 to meet the state’s winter design temperature rule.
R-30 fiberglass vs cellulose thickness
At R-30, Greenfiber SANCTUARY cellulose settles at 8.3 inches while Owens Corning AttiCat blown fiberglass needs 10.75 inches, per their May 2026 charts. Cellulose delivers about R-3.6 per inch settled versus about R-2.8 for AttiCat (30 / 10.75), so cellulose reaches the same R-value in about 2.5 fewer inches.
The other tradeoffs are weight and moisture. Cellulose loads the ceiling with about 0.97 lb per sq ft at R-30 against about 0.40 lb for AttiCat. Fiberglass does not absorb water the way cellulose can, while the AttiCat Minnesota column shows loose fiberglass needs 1.0 extra inch at R-30 to hold its rating at winter design temperatures. Choose by eave clearance, 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 4.9 inches thick at the R-6.1 per inch the DOE Building America spray foam guide lists for typical 2 lb foam, or about 4.25 inches for an HFO foam rated R-7.1 per inch. Open-cell foam at about R-3.6 per inch needs about 8.3 inches. Closed-cell doubles as an air and vapor control layer at typical thickness.
The DOE guide notes closed-cell is usually sprayed in 2 to 3 inch passes, so R-30 takes 2 passes, while open-cell can go on up to 10 inches in a single pass. In a 2×10 rafter, about 4.9 inches of closed-cell R-30 leaves about 4.35 inches of open cavity, while 8.3 inches of open-cell nearly fills it.
How thick is R-30 rigid insulation?
R-30 rigid insulation is about 5.2 inches of polyiso board or 6 inches of XPS board. Polyiso reaches R-30.0 with two 2.6-inch boards on Sika’s published chart, because its thickest single board (4.5 inches) stops at R-26.8. XPS rated at R-5.0 per inch needs 6 inches. Either way, R-30 in rigid board is a two-layer job.
The Sika Sarnatherm ISO polyiso data sheet (August 2024) rates each board by long-term thermal resistance (LTTR) under ASTM C1289: R-5.7 at 1.0 inch, R-15.0 at 2.6 inches, R-17.4 at 3.0 inches and R-26.8 at 4.5 inches. Two 2.6-inch boards give R-30.0, and so do a 3.0-inch and a 2.2-inch board (R-17.4 plus R-12.6). Owens Corning rates FOAMULAR NGX 250 XPS (October 2025 data sheet) at R-5.0 per inch at a 75°F mean temperature, so R-30 takes 6 inches of board.
| Rigid board | Rated R-value | Thickness for R-30 | Source |
|---|---|---|---|
| Polyiso (Sika Sarnatherm ISO) | LTTR R-5.7 at 1.0 in to R-26.8 at 4.5 in | 5.2 in (two 2.6-in boards, R-30.0) | Sika data sheet, Aug 2024 |
| XPS (Owens Corning FOAMULAR NGX 250) | R-5.0 per inch at 75°F | 6 in | Owens Corning data sheet, Oct 2025 |
Rigid board R-values are product specific, so check the data sheet for the board you buy: Sika’s 2.6-inch polyiso is R-15.0, about R-5.8 per inch. Polyiso is commonly installed above the deck under single-ply membranes such as TPO; our TPO roof installation cost guide works through the insulation math per square foot.
Do you need R-30, and where?
R-30 is the attic target only in Climate Zone 1. The ENERGY STAR retrofit table, based on 2021 IECC Table R402.1.3, recommends R-30 for an uninsulated Zone 1 attic and R-49 to R-60 in Zones 2 through 8. It lists R-30 as the floor level over unheated space in Zones 4C, 5 and 6 (R-13 to R-19 in warmer zones, R-38 in Zones 7 and 8). Our guide to garage attic insulation covers when a garage ceiling needs it.
R-30 is also the code fallback for cathedral ceilings. Under 2018 IRC N1102.2.2 (IECC R402.2.2), where the roof/ceiling assembly has no room for the required R-value, R-30 is allowed if it extends uncompressed over the wall top plate, limited to 500 sq ft or 20 percent of the insulated ceiling area, whichever is less, per the ICC’s Significant Changes to the 2018 IRC. For full attic floors, check the sibling depth targets in our R-38 insulation thickness, R-49 insulation thickness, and R-60 insulation thickness guides, and our insulation R-value chart.
Frequently asked questions
How thick is R-30 insulation?
R-30 insulation is 9 to 9.5 inches thick for a standard Owens Corning fiberglass roll or batt, 8.25 inches for the high-density R-30C batt, 7.25 inches for ROCKWOOL Comfortbatt stone wool, 8.9 inches installed (8.3 settled) for Greenfiber SANCTUARY cellulose, 10.5 to 10.8 inches for blown fiberglass, and about 4.25 to 5 inches for closed-cell spray foam.
How many inches is R-30 insulation?
The most common R-30 product, a standard fiberglass batt, is 9.5 inches thick on Owens Corning’s May 2026 fact sheet, and the R-30 roll is 9 inches. The high-density R-30C batt is 8.25 inches so it fits a 2×10 rafter with a 1-inch vent gap. Across materials, R-30 runs from about 4.25 inches of closed-cell foam to 10.8 inches of blown fiberglass.
How thick is an R-30 batt?
An R-30 batt is 8.25 inches in high-density fiberglass (R-30C), 9.5 inches in standard fiberglass (9 inches for the roll), and 7.25 inches in ROCKWOOL Comfortbatt stone wool. Compressing a 10-inch R-30 batt into a 7.25-inch 2×8 cavity cuts it to about R-25, per the Insulation Institute’s compressed R-value chart.
How deep does blown insulation need to be for R-30?
Blown R-30 needs 8.9 inches installed and 8.3 inches settled of Greenfiber SANCTUARY cellulose (38.6 bags per 1,000 sq ft), 10.5 inches of Owens Corning ProCat, 10.75 inches of Owens Corning AttiCat (14.6 bags) or 10.8 inches of Johns Manville Climate Pro. Install both the minimum depth and the minimum bag count on the bag chart.
Is R-30 enough for an attic?
Only in Climate Zone 1. ENERGY STAR’s retrofit table, based on the 2021 IECC, recommends R-30 for an uninsulated attic in Zone 1 and R-49 to R-60 in Zones 2 through 8. R-30 is ENERGY STAR’s floor level in Zones 4C, 5 and 6, and the 2018 IRC allowed R-30 in a cathedral ceiling without room for more, limited to 500 sq ft or 20 percent of the ceiling area.
Why does R-30 thickness vary so much by material?
Thickness equals 30 divided by the material’s R-value per inch. Blown fiberglass at about R-2.8 per inch needs 10.75 inches (Owens Corning AttiCat), cellulose at about R-3.6 needs 8.3 inches settled (Greenfiber SANCTUARY), stone wool at about R-4.1 needs 7.25 inches (ROCKWOOL Comfortbatt) and closed-cell foam at R-6.1 to 7.1 needs about 4.25 to 4.9 inches.