Rockwool attic insulation is mineral wool (stone wool) made from spun basalt rock and recycled slag, delivering about R-4.2 per inch, a non-combustible fire rating tested to CAN/ULC S102 with a flame spread of 0, and water-repellent performance that fiberglass cannot match. 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, and for a large open attic floor blown cellulose 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 sized to friction-fit between framing. “Rockwool” is a brand name that has become the common word for mineral wool the way “Kleenex” stands in for tissue.
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. It is denser and stiffer than a fiberglass batt, so it holds its shape and stays put without stapling. Because it is unfaced, it carries no vapor barrier of its own, 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.
What is the R-value of rockwool insulation?
Rockwool insulation has an R-value of about 4.2 per inch at 75 degrees F, per ROCKWOOL’s published data for Comfortbatt. That sits above most fiberglass batts (roughly 2.9 to 3.8 per inch) and near dense-packed cellulose. Unlike fiberglass, rockwool holds that R-value when compressed, because its rated thickness already accounts for its density rather than relying on maximum loft.
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 | Typical use |
|---|---|---|
| R-15 | 3.5 in | 2×4 walls, added attic layer |
| R-23 | 5.5 in | 2×6 walls, rafters |
| R-30 | 8.0 in | Attic floor, cathedral ceiling |
| R-38 | 9.5 in | Attic floor to code minimum |
Most U.S. attics need R-49 to R-60 under the 2021 IECC, depending on climate zone. 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 of R-15 or R-23 run perpendicular across the top to cover the joists and cut thermal bridging. Two R-30 layers land near R-60. For the full depth-by-zone breakdown, use our insulation R-value chart.
How does rockwool compare to fiberglass for an attic?
Rockwool beats fiberglass on fire resistance, moisture, sound, and R-value per inch, while fiberglass wins clearly on price and weight. For a standard vented attic floor where fire and sound are not the priority, fiberglass or blown-in usually gives the same code R-value for less money. Rockwool makes the most sense where those extra properties earn their keep.
| Property | Rockwool (stone wool) | Fiberglass |
|---|---|---|
| R-value per inch | About 4.2 | About 2.9 to 3.8 |
| Melting point | Around 2,150 F, non-combustible | Softens near 1,100 F |
| Fire test | CAN/ULC S102: flame spread 0, smoke 0 | Combustible facing on faced batts |
| Water | Hydrophobic, sheds water, dries out | Absorbs and mats down |
| Sound (STC benefit) | Higher, denser fiber | Lower |
| Material cost | About $1.20 to $2.00 per sq ft | About $0.60 to $1.20 per sq ft |
| Weight on ceiling | Heavier per batt | Lighter |
The compression point matters in an attic. Fiberglass loses R-value when it is squashed by stored boxes, wiring, or a second batt pressing down. Rockwool holds its rating because it is already dense. If your attic doubles as storage, that stability can be worth the upcharge. For the fiberglass side in detail, see our fiberglass attic insulation guide.
What is the fire rating of rockwool insulation?
Rockwool is non-combustible and rated to withstand temperatures around 2,150 degrees F without melting, smoking, or igniting. ROCKWOOL Comfortbatt tests to CAN/ULC S102 with a flame spread index of 0 and a smoke developed index of 0, and stone wool broadly meets non-combustibility criteria like 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 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 or foil facing, which is combustible and must be kept away from heat-producing fixtures. Any insulation still needs clearance from non-IC-rated recessed lights and flues per manufacturer and code, but the base material not being fuel is a real safety margin.
Is rockwool good for moisture and sound in an attic?
Yes. Rockwool is hydrophobic, so it sheds liquid water instead of soaking it up, and it resists rot, mold, and mildew because the fibers give mold nothing organic to eat. If a roof leak wets a rockwool batt, the material dries and recovers its R-value rather than staying saturated and matted like wet fiberglass. It is also vapor-open, so moisture moves through rather than getting trapped.
On sound, the higher density damps airborne noise better than standard fiberglass, which is why rockwool is a common pick under a bonus room, home theater, or any attic-adjacent living space where rain drumming or HVAC noise is a nuisance. The trade-off is weight: a rockwool batt is heavier, so a deep two-layer attic floor puts more load on the ceiling drywall and joists than the equivalent fiberglass. 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 material runs about $1.20 to $2.00 per square foot, roughly double fiberglass at $0.60 to $1.20 per square foot. Installed, batt insulation commonly lands between $2.00 and $6.00 per square foot depending on thickness, layers, and regional labor. Because most attics need two layers to reach R-49 or R-60, the material premium compounds across the job.
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, and installs faster. 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 stiff, semi-rigid batt 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, because insulation slows heat but does not stop air leaks.
- Keep clearance. Maintain the required gap around non-IC recessed lights, chimneys, and flue pipes, and keep insulation off soffit vents so intake airflow stays open.
- Measure and cut. Cut batts with a serrated blade or bread knife against a straightedge, sizing them a touch long for a snug friction-fit.
- Lay the first layer. Set batts between the joists, tight to each other with no gaps at the ends, pressed to full contact with the drywall below.
- Cross the second layer. Run the second layer of batts perpendicular over the top of the joists to cover the framing 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.
Rockwool is DIY-friendly for accessible attics because the batts do not sag or slump. 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.
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 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.
Match the material to the job. If you are covering a wide open floor, price out blown-in first. If you are insulating knee walls, rafters, or a spot near heat sources, rockwool’s stiffness and fire rating often justify the extra dollar. The stable R-value under compression is the quiet advantage most comparisons skip.
Frequently asked questions
What is the R-value of rockwool per inch?
Rockwool insulation has an R-value of about 4.2 per inch at 75 degrees F, based on ROCKWOOL Comfortbatt data. That beats most fiberglass batts, which run about 2.9 to 3.8 per inch. Rockwool also holds its R-value when compressed, because its rating is based on density rather than maximum loft, so squashing it under stored items costs less performance than it would with fiberglass.
Is rockwool better than fiberglass for an attic?
Rockwool is better for fire resistance, sound, moisture, and R-value per inch, while fiberglass is cheaper and lighter. For a plain vented attic floor, blown fiberglass or cellulose usually hits code R-value for less money. Rockwool is the stronger pick where fire safety near flues or wiring, noise control under a room, or leak tolerance justify paying roughly double the material cost.
Is rockwool fireproof?
Rockwool is non-combustible, not technically fireproof, and resists temperatures around 2,150 degrees F without melting or igniting. Comfortbatt tests to CAN/ULC S102 with a flame spread and smoke developed index of 0, and stone wool meets non-combustibility standards such as ASTM E136. It slows fire spread and buys evacuation time, but still needs code-required clearance from non-IC recessed lights and flue pipes.
Does rockwool absorb water in an attic?
No. Rockwool is hydrophobic, so it sheds liquid water rather than soaking it up, and it resists rot, mold, and mildew because the mineral fibers are not food for mold. If a roof leak wets a rockwool batt, it dries and recovers its R-value instead of staying saturated. That makes it a good choice for leak-prone or condensation-prone attic bays where wet fiberglass would mat down and fail.
How much does rockwool attic insulation cost?
Rockwool material costs about $1.20 to $2.00 per square foot, roughly double fiberglass. Installed batt insulation commonly runs $2.00 to $6.00 per square foot depending on thickness, layers, and labor. Because most attics need two layers to reach R-49 or R-60, the premium adds up, which is why blown-in is often cheaper for a large, open, easy-access attic floor.
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 the 2021 IECC calls for by climate zone. A common build is an R-30 or R-38 Comfortbatt between the joists, then a second R-15 or R-23 layer run perpendicular across the top to cover the framing and cut thermal bridging. Two R-30 layers land near R-60. Check your zone against an R-value chart before buying.
Reviewed by The Roofing Brief Team. Last reviewed July 2026.