By The Roofing Brief Team. Last reviewed: October 2026.
Rockwool attic insulation is mineral wool (stone wool, also called rock wool) made from spun basalt rock and recycled slag, delivering about R-4.0 to R-4.3 per inch, a non-combustible rating under ASTM E136 with a flame spread index of 0 under ASTM E84, and water vapor sorption of just 0.05 percent by volume, per the ROCKWOOL Comfortbatt technical data sheet (July 2026). In an attic it usually shows up as ROCKWOOL Comfortbatt friction-fit between the ceiling joists, in R-15, R-23, R-30, and R-38 thicknesses. It costs more than fiberglass or blown-in cellulose, and for a large open attic floor blown cellulose (about R-3.7 per inch) or fiberglass is often the cheaper way to hit code. Where rockwool earns its premium is fire performance, sound control, and staying dry.
What is rockwool attic insulation?
Rockwool attic insulation is a batt made of stone wool, spun from molten basalt and steel-industry slag into dense fibers. The residential product most people mean is ROCKWOOL Comfortbatt, a semi-rigid, unfaced batt with a density above 1.8 lb per cubic foot, sized 15.25 in wide for 16 in framing and 23 in wide for 24 in framing, 47 in long. “Rockwool” is a brand name that has become the common word for mineral wool the way “Kleenex” stands in for tissue. In North America the company sold the same stone wool batts as Roxul until January 1, 2018, when it adopted its parent company’s ROCKWOOL name, so older labels and spec sheets that say Roxul Comfortbatt describe the same product line. The generic terms for the material are mineral wool, stone wool and rock wool.
In an attic, rockwool goes down as batts laid or wedged between the ceiling joists, or in a cathedral or conditioned-attic assembly, between the rafters. ROCKWOOL’s data sheet describes a flexible edge that compresses as the batt goes in and springs back against the framing, so it holds its place in a 16 in or 24 in bay without stapling. Because it is unfaced, it carries no vapor barrier of its own: the data sheet lists 42 perm vapor transmission at 3.5 in, which matters for how you detail the assembly.
For the full material-by-material picture, see our attic insulation types and R-value guide, which puts mineral wool next to fiberglass, cellulose, and spray foam at R-2.8 to R-6 per inch.
What is the R-value of rockwool insulation?
Rockwool insulation has an R-value of about R-4.0 to R-4.3 per inch, per ROCKWOOL’s Comfortbatt data sheet: R-15 at 3.5 in (R-4.29 per inch), R-23 at 5.5 in (R-4.18), R-30 at 7.25 in (R-4.14) and R-38 at 9.5 in (R-4.0), all tested to ASTM C518. That sits above fiberglass attic batts (about R-3.2 to R-3.5 per inch on Owens Corning’s PINK Next Gen chart) and above loose-fill cellulose (about R-3.7 per inch settled on Greenfiber’s SANCTUARY chart).
Comfortbatt comes in fixed R-value thicknesses. Picking the right one, then getting depth right on the attic floor, is what actually determines performance.
| Comfortbatt product | Nominal thickness | R per inch | Typical use | Source |
|---|---|---|---|---|
| R-15 | 3.5 in | 4.29 | 2×4 walls, added attic layer | ROCKWOOL data sheet, July 2026 |
| R-23 | 5.5 in | 4.18 | 2×6 walls, rafters | ROCKWOOL data sheet, July 2026 |
| R-30 | 7.25 in | 4.14 | Attic floor, cathedral ceiling | ROCKWOOL data sheet, July 2026 |
| R-38 | 9.5 in | 4.00 | Attic floor | ROCKWOOL data sheet, July 2026 |
Most U.S. attics need R-49 to R-60: ENERGY STAR’s retrofit table, based on the 2021 IECC, recommends R-49 for an uninsulated attic in zones 2 and 3 and R-60 in zones 4 through 8. That is more than a single Comfortbatt gives you, so a rockwool attic floor is usually two layers: an R-30 or R-38 between the joists, then a second layer run perpendicular across the top to cover the joists and cut thermal bridging. For the full depth-by-zone breakdown, use our insulation R-value chart.
Rockwool attic insulation for R-49 and R-60: layer combinations
| Target | Layer 1 (between joists) | Layer 2 (perpendicular) | Total R | Total depth | Weight, at least |
|---|---|---|---|---|---|
| R-49 (zones 2 to 3) | R-30, 7.25 in | R-23, 5.5 in | R-53 | 12.75 in | 1.9 lb per sq ft |
| R-49 (zones 2 to 3) | R-38, 9.5 in | R-15, 3.5 in | R-53 | 13.0 in | 2.0 lb per sq ft |
| R-60 (zones 4 to 8) | R-30, 7.25 in | R-30, 7.25 in | R-60 | 14.5 in | 2.2 lb per sq ft |
| R-60 (zones 4 to 8) | R-38, 9.5 in | R-23, 5.5 in | R-61 | 15.0 in | 2.25 lb per sq ft |
Roofing Brief arithmetic from the ROCKWOOL Comfortbatt data sheet (R-values and thicknesses; density above 1.8 lb per cubic foot). For comparison, Owens Corning’s AttiCat data sheet lists blown fiberglass at 0.866 lb per sq ft at R-60 (20.5 in), and Greenfiber’s SANCTUARY chart lists cellulose at 2.16 lb per sq ft at R-60 (17.3 in installed).
How does rockwool compare to fiberglass and cellulose for an attic?
Rockwool beats both fiberglass and cellulose on fire behavior and R-value per inch (R-4.0 to R-4.3 versus about R-2.9 for blown fiberglass and R-3.7 for cellulose), while fiberglass and cellulose win on price and installed speed across a big open floor. For a standard vented attic floor where fire and sound are not the priority, blown fiberglass or cellulose usually gives the same code R-value for less money: HomeGuide prices installed blown-in insulation at $1.10 to $1.90 per sq ft at R-49. Rockwool makes the most sense where those extra properties earn their keep.
| Property | Rockwool (stone wool) | Fiberglass | Cellulose (blown) |
|---|---|---|---|
| R-value per inch | About 4.0 to 4.3 (Comfortbatt) | About 2.8 to 2.9 blown (AttiCat); 3.2 to 3.5 attic batts | About 3.7 settled (Greenfiber SANCTUARY) |
| Fire behavior | Non-combustible, ASTM E136; FSI 0, SDI 0 (ASTM E84); ROCKWOOL rates it to 2,150°F | Non-combustible glass, ASTM E136 (AttiCat); kraft facing on faced batts is combustible | Borate fire-retardant treated; organic fiber that can char and smolder |
| Water | Water vapor sorption 0.05 vol% (ASTM C1104); passes fungi resistance (ASTM C1338) | Does not absorb moisture (ASTM C1104, AttiCat) | Absorbs moisture, can mat and lose loft if wetted |
| Density | Above 1.8 lb per cubic foot | About 0.5 lb per cubic foot blown (AttiCat R-60: 0.866 lb per sq ft at 20.5 in) | About 1.5 to 1.6 lb per cubic foot (Greenfiber R-60: 2.16 lb per sq ft) |
| Settling over time | Batt; no settling allowance needed | AttiCat chart lists settled depth equal to installed | Greenfiber chart: 14.3 in installed settles to 13.3 in at R-49 |
| Material cost, attic | About $2.93 per sq ft for R-30 Comfortbatt | About $2.27 per sq ft for R-49 batts; $1.01 for R-49 blown (AttiCat) | See HomeGuide installed range |
| Recycled content | Basalt plus steel-slag | Some recycled glass | Recycled paper fiber |
| Form in an attic | Semi-rigid batt, friction-fit | Batt or blown loose-fill | Loose-fill blown across the floor |
Sources: ROCKWOOL Comfortbatt data sheet (July 2026); ROCKWOOL product selection guide (2,150°F); Owens Corning AttiCat data sheet (May 2026); Greenfiber SANCTUARY chart (Rev F 05-26); R-30 Comfortbatt at $87.39 per 29.87 sq ft bag at Superarbor, October 2026; fiberglass prices from Home Depot and HEP Sales listings, October 2026. Density figures are Roofing Brief arithmetic from the charts.
The settling point matters in an attic. Greenfiber’s chart has you blow cellulose to 14.3 inches so it settles to 13.3 inches at R-49, and the FTC R-value rule (16 CFR Part 460) is why loose-fill bags print a minimum settled thickness. A rockwool batt is sold at its rated thickness, and its stiffness (above 1.8 lb per cubic foot) keeps it from slumping in a rafter bay or knee wall. If your attic doubles as storage, keep boxes off any insulation; a crushed batt of any material loses depth. For the fiberglass side in detail, see our fiberglass attic insulation guide.
Cellulose is often the value pick for a wide open attic floor or for topping up existing insulation, because it flows around obstructions and reaches R-49 at 13.3 inches settled. Its weak spot is moisture (it mats and loses loft if a leak wets it), which rockwool handles better with 0.05 percent water vapor sorption. Fiberglass sits between the two on price and performance. For R-value, settling and cost detail on the cellulose side, see our cellulose attic insulation guide.
Rockwool vs blown insulation for an attic
Blown rockwool exists but is less common: HomeGuide prices blown-in rockwool at $1.90 to $4.20 per sq ft installed, against $1.10 to $1.90 per sq ft for blown-in insulation at R-49. In batt form, R-30 Comfortbatt costs about $2.93 per sq ft in material alone, so a two-layer R-60 attic floor runs about $5.86 per sq ft before labor. That gap is why blown fiberglass or cellulose wins most open, vented attic floors, and why rockwool batts win where a stiff batt has to stay in a vertical or sloped bay.
Rockwool attic insulation pros and cons
Rockwool’s advantages in an attic are fire, moisture, density, and dimensional stability. Its drawbacks are cost (about $2.93 per sq ft for R-30 batts), weight (at least 2.2 lb per sq ft at R-60), and slower coverage across a large floor. The table lays out the trade-off against fiberglass and cellulose.
| Pros | Cons |
|---|---|
| Non-combustible (ASTM E136), rated to 2,150°F by ROCKWOOL | R-30 batts cost about $2.93 per sq ft, versus $1.01 for R-49 blown fiberglass material |
| Semi-rigid batt does not sag in rafter bays or knee walls | At least 2.2 lb per sq ft at R-60, about 2.5 times blown fiberglass at R-60 |
| Water vapor sorption 0.05 vol%; passes ASTM C1338 fungi resistance | Slower and pricier to cover a large open floor than blown-in |
| Dense fiber (above 1.8 lb per cubic foot) absorbs airborne sound | Still needs clearance from non-IC lights and flue pipes |
| GREENGUARD Gold certified, per ROCKWOOL’s data sheet | Cutting stone wool releases dust, so an N95 is needed |
| Flame spread 0 and smoke developed 0 (ASTM E84) | No built-in vapor barrier (42 perm at 3.5 in), so the assembly must be detailed |
What is the fire rating of rockwool insulation?
Rockwool is non-combustible, and ROCKWOOL’s product selection guide says its stone wool can withstand temperatures up to 2,150°F (1,177°C). The US Comfortbatt data sheet lists a flame spread index of 0 and a smoke developed index of 0 under ASTM E84 (UL 723), which meets Class A under the IBC, and non-combustibility at 750°C under ASTM E136. In a fire it acts as a barrier, buying time rather than feeding the flames.
This is the clearest reason to choose rockwool over fiberglass or cellulose in an attic. Attics carry knob-and-tube legacy wiring, recessed can lights, chimney chases, and flue pipes. A non-combustible batt around those heat sources behaves differently than a fiberglass batt with a kraft-paper facing, which is combustible: Owens Corning’s PINK Next Gen fact sheet says standard kraft facing must be in contact with a thermal barrier. Cellulose is borate-treated but is still an organic material that can char and smolder, unlike stone wool. Any insulation still needs clearance from non-IC-rated recessed lights (NEC 410.116 calls for 3 in) and flues, but the base material not being fuel is a real safety margin.
Is rockwool good for moisture and sound in an attic?
Yes. ROCKWOOL lists Comfortbatt’s water vapor sorption at 0.05 percent by volume (ASTM C1104) and a pass on the ASTM C1338 fungi resistance test, because the mineral fibers give mold nothing organic to eat. If a roof leak wets a rockwool batt, the material can dry in place, and at 42 perm (3.5 in, ASTM E96) it is vapor-open, so moisture moves through rather than getting trapped. This is a clear edge over cellulose, which is organic and mats down when wet; blown fiberglass is also rated as non-absorbent under ASTM C1104 by Owens Corning.
On sound, density above 1.8 lb per cubic foot absorbs airborne noise better than low-density fill, which is why ROCKWOOL markets a separate Safe’n’Sound batt for interior walls and floors and why rockwool is a common pick under a bonus room or home theater. The trade-off is weight: a two-layer R-60 rockwool floor is at least 2.2 lb per sq ft, versus 0.866 lb for AttiCat blown fiberglass at R-60. On older or undersized ceiling framing, that weight is worth a second look before you pile it deep.
How much does rockwool attic insulation cost?
ROCKWOOL R-30 Comfortbatt costs about $2.93 per square foot in material ($87.39 per 29.87 sq ft bag at Superarbor, October 2026), so two layers to R-60 run about $5.86 per square foot before labor. HomeGuide prices blown-in rockwool at $1.90 to $4.20 per square foot installed, and installed blown-in fiberglass or cellulose at $1.10 to $1.90 per square foot at R-49. Because most attics need two batt layers to reach R-49 or R-60, the material premium compounds across the job.
| Option, 1,000 sq ft attic floor | Per sq ft | Job total | Source |
|---|---|---|---|
| Rockwool batts, R-60 (2 x R-30), material only | About $5.86 | About $5,860 | Superarbor listing, Oct 2026 |
| Blown-in rockwool, installed | $1.90 to $4.20 | $1,900 to $4,200 | HomeGuide blown-in insulation cost guide |
| Blown-in insulation at R-49, installed | $1.10 to $1.90 | $1,100 to $1,900 | HomeGuide blown-in insulation cost guide |
| Blown fiberglass at R-49, DIY material | About $1.01 | About $1,009 | AttiCat chart; Home Depot listing, Oct 2026 |
The cost gap is why rockwool is rarely the default for a big, open, easy-access attic floor. For that job, blown cellulose or blown fiberglass covers the same square footage to the same code R-value for less, at $1.10 to $1.90 per sq ft installed. Rockwool earns its price in specific spots: attics with fire concerns, sound-sensitive rooms below, sections prone to leaks or condensation, and knee walls or rafter bays where a stiff batt stays put better than loose fill. To pick the right material for your situation and budget, use our best attic insulation guide.
How do you install rockwool batts in an attic?
Rockwool attic batts install by friction-fit between the joists or rafters, cut to length, with a second layer run perpendicular to cover the framing. The semi-rigid batt, 15.25 in or 23 in wide and 47 in long, holds itself in place without staples. The core steps below apply whether you are insulating a vented attic floor or a rafter assembly.
- Air seal first. Seal top plates, penetrations, the attic hatch, and any bypasses before insulating; ENERGY STAR’s 15 percent heating and cooling savings estimate assumes sealing and insulating together.
- Keep clearance. Keep insulation at least 3 in from non-IC recessed lights (NEC 410.116), away from chimney and flue clearances, and leave at least 1 in of air space at soffit vents (IRC R806.3).
- Measure and cut. Cut batts with a serrated blade or bread knife against a straightedge, sizing them slightly long for a snug friction-fit against the 47 in batt length.
- Lay the first layer. Set 15.25 in batts in 16 in bays or 23 in batts in 24 in bays, tight to each other with no gaps at the ends, pressed to full contact with the drywall below.
- Cross the second layer. Run R-15 (3.5 in), R-23 (5.5 in) or R-30 (7.25 in) batts perpendicular over the joists to cover the 1.5 in framing tops and cut thermal bridging.
- Wear PPE. Long sleeves, gloves, eye protection, and an N95 respirator, the same as with fiberglass, since cutting stone wool releases dust; IARC Monograph 81 classes rock wool in Group 3, not classifiable as to carcinogenicity.
Rockwool is DIY-friendly for accessible attics because the semi-rigid batts do not sag or slump in a bay, and a 29.87 sq ft bag of R-30 covers about 3 percent of a 1,000 sq ft floor per layer, so plan on about 34 bags per layer. For deep multi-layer jobs, low-clearance attics, or when you would rather not crawl a hot attic, a pro crew is faster. Our attic insulation installation guide walks through DIY steps, tooling, and when hiring out pays off. If you are comparing it with fiberglass, our guide to attic insulation rolls lists the common sizes and brands.
Which attic insulation should you pick?
Match the material to the attic, not the other way around. Rockwool is the sharper choice wherever fire, sound, moisture, or a stiff batt is the point, and blown-in at $1.10 to $1.90 per sq ft installed is usually cheaper for a plain open floor. The table maps common attic situations to the material that tends to win, with the number that decides it.
| If your attic… | Best pick | Why, with the deciding number |
|---|---|---|
| Is a large open floor, easy access, budget-driven | Blown cellulose or fiberglass | R-49 installed at $1.10 to $1.90 per sq ft (HomeGuide) versus about $5.86 per sq ft material for R-60 rockwool batts |
| Has fire concerns near flues, a chimney or knob-and-tube | Rockwool | Non-combustible under ASTM E136, FSI 0, rated to 2,150°F |
| Sits over a bedroom or theater | Rockwool | Density above 1.8 lb per cubic foot damps airborne noise |
| Has leak-prone or condensation-prone bays | Rockwool | 0.05 vol% water vapor sorption; passes ASTM C1338 fungi test |
| Has knee walls, rafter bays or a cathedral ceiling | Rockwool batts | R-23 fills a 2×6 rafter at 5.5 in; R-30 fits a 2×8 at 7.25 in |
| Already has 3 to 4 in and needs a cheap top-up | Blown cellulose | ENERGY STAR says add R-38 to R-49; cellulose reaches R-38 at 10.4 in settled (Greenfiber) |
| Has old ceiling framing or sagging drywall | Blown fiberglass | 0.866 lb per sq ft at R-60 (AttiCat) versus at least 2.2 lb for R-60 rockwool |
Rockwool attic insulation: is it worth it?
Rockwool is worth the premium when fire resistance, sound control, or moisture tolerance matter more than the lowest price. For a plain vented attic floor where the only goal is hitting R-49 or R-60 cheaply, blown-in fiberglass or cellulose usually wins on cost ($1.10 to $1.90 per sq ft installed at R-49) and speed. Rockwool is the sharper choice for conditioned attics, rooms below a noisy attic, leak-prone bays, and any assembly where a non-combustible, dimensionally stable batt is the point.
Price out blown-in first if you are covering a wide open floor. If you are insulating knee walls, rafters, or a spot near heat sources, rockwool’s stiffness and fire rating often justify a material premium of about $4.60 per sq ft at R-60 over AttiCat blown fiberglass (31.5 bags per 1,000 sq ft at $40.34, about $1.27 per sq ft). The quiet advantage is that a batt sold at 7.25 in for R-30 stays at 7.25 in, with no settled-thickness allowance to blow over.
Frequently asked questions
What is the R-value of rockwool per inch?
Rockwool Comfortbatt runs about R-4.0 to R-4.3 per inch: R-15 at 3.5 inches, R-23 at 5.5 inches, R-30 at 7.25 inches and R-38 at 9.5 inches on ROCKWOOL’s July 2026 data sheet. That beats fiberglass attic batts at about R-3.2 to R-3.5 per inch and loose-fill cellulose at about R-3.7 per inch settled.
Is rockwool better than fiberglass for an attic?
Rockwool is better for fire resistance, density and R-value per inch, while fiberglass is cheaper and lighter. For a plain vented attic floor, blown fiberglass reaches R-49 for about $1.01 per square foot in material versus about $2.93 per square foot for a single R-30 rockwool batt. Rockwool is the stronger pick where fire safety near flues or wiring, noise control under a room, or leak tolerance justify the cost.
Is rockwool better than cellulose for an attic?
Rockwool wins on fire rating, moisture, and stability, while cellulose is cheaper to blow across a large open floor and gives about R-3.7 per inch settled. Greenfiber’s chart has cellulose blown to 14.3 inches to settle at 13.3 inches for R-49, and cellulose mats if it gets wet, whereas a rockwool batt holds its 7.25 inch R-30 thickness and has 0.05 percent water vapor sorption. For a big attic floor on a budget, cellulose usually wins on cost.
Is rockwool fireproof?
Rockwool is non-combustible, not technically fireproof. ROCKWOOL says its stone wool withstands temperatures up to 2,150 degrees F, and the Comfortbatt data sheet lists a flame spread index of 0 and smoke developed index of 0 under ASTM E84 and non-combustibility under ASTM E136. It still needs code-required clearance from non-IC recessed lights and flue pipes.
Does rockwool absorb water in an attic?
Very little. ROCKWOOL lists Comfortbatt’s water vapor sorption at 0.05 percent by volume under ASTM C1104, and it passes the ASTM C1338 fungi resistance test because the mineral fibers are not food for mold. A wetted batt can dry in place, which makes it a good choice for leak-prone or condensation-prone attic bays where wet cellulose would mat down.
How much does rockwool attic insulation cost?
ROCKWOOL R-30 Comfortbatt costs about $2.93 per square foot in material, based on an October 2026 listing at $87.39 per 29.87 square foot bag, so a two-layer R-60 floor runs about $5.86 per square foot before labor. HomeGuide prices blown-in rockwool at $1.90 to $4.20 per square foot installed and blown-in insulation at R-49 at $1.10 to $1.90.
How many layers of rockwool do I need in an attic?
Most attics need two layers to reach the R-49 to R-60 that ENERGY STAR recommends by climate zone. R-30 plus R-23 gives R-53 at 12.75 inches for zones 2 and 3, and two R-30 layers give R-60 at 14.5 inches for zones 4 through 8. Run the second layer perpendicular across the joists to cover the framing and cut thermal bridging.
How deep is rockwool attic insulation at R-60?
Two layers of R-30 Comfortbatt reach R-60 at 14.5 inches, and R-38 plus R-23 reaches R-61 at 15 inches, per ROCKWOOL’s data sheet thicknesses. At a density above 1.8 pounds per cubic foot, that is at least 2.2 pounds per square foot on the ceiling, versus 0.866 pounds for Owens Corning AttiCat blown fiberglass at R-60.