Mineral wool insulation in an attic

Rock is melted at high heat, then spun into fiber the way candy floss is spun from sugar. A binder holds the fiber in a mat. That mat is cut into stiff batts, or shredded into loose fill. The rock is usually basalt or a similar volcanic stone. Slag wool uses the slag left over from making steel instead, which is why the material is often described as recycled content.

Four names, one family. Mineral wool is the umbrella term. Stone wool and rock wool mean the version spun from rock. Slag wool means the version spun from steel slag. In a home center you will mostly meet two brands: ROCKWOOL, whose attic and wall batt is Comfortbatt, and Owens Corning, whose mineral wool line is Thermafiber. Both are sold as stiff rectangles in a plastic-wrapped bundle, not as a floppy roll.

Pick it up once and you will not mistake it again. A mineral wool batt stands on its edge without help. A glass fiber batt flops over. The color runs gray-green to brown rather than pink or yellow. The fibers feel coarse and short. If you are trying to work out what is already in your attic, the identification guide runs through every material with the same kind of cue.

One warning before you go up there. Loose gray-brown pebbles that look like shiny gravel are not mineral wool. That is vermiculite, and it can hold asbestos. Stop and read the vermiculite page before you disturb anything.

Mineral wool batts hold more R-value per inch than glass fiber batts, by a small margin. Loose-fill mineral wool sits between blown fiberglass and blown cellulose. Neither margin is large enough to be the reason you buy it.

The price column is material only, at retail shelf price, for one square foot of ceiling at R-30. No labor, no delivery, no air sealing. Every price was read off a live product page at The Home Depot on 2026-08-11. A contractor buys below shelf price, and this site holds no measured trade discount, so nothing here is marked down.

Read the price column across and the whole argument of this page falls out of it. At R-30, mineral wool batts cost about 1.9 times glass fiber batts, about 3.3 times blown fiberglass, and more than 5 times blown cellulose. For the same R-value, on the same ceiling. That is a jump between price tiers, not a small premium.

Two limits on that table. First, one product is one point, not a market: A second stone wool product at the same R-value works out about a third cheaper a square foot, so the premium over glass fiber varies by brand as well as by material. Second, this site holds no read price at all for loose-fill mineral wool. It is not stocked the way blown fiberglass and cellulose are, and no retail listing was available to read on 2026-08-11. Rather than publish a guess, the row says so. If a contractor quotes you blown mineral wool, ask what the material costs per square foot at your target R-value and compare it with the rows above.

Depth follows from R per inch. Reaching R-49 takes about 11.7 to 14.8 inches of mineral wool batt, or about 13.2 to 17.5 inches of the loose fill. An R-30 batt lands at 7.1 to 9.1 inches, which is why the product this page prices is 7.25 inches thick. R-49 and R-60 are the ENERGY STAR levels for a bare attic in most of the country, and ENERGY STAR bases that guidance on the model 2021 IECC table. Energy codes cover new building and work done under a permit. They do not usually cover adding insulation to an attic you already have. Which edition applies where you live depends on your state, and often on your city. Your local building department is the only authority on it.

This is the property people buy mineral wool for, so it is worth stating exactly, in the words the maker publishes rather than in a slogan.

• ASTM E136 is the test for whether a material burns at 750 °C. ROCKWOOL publishes the result for its stone wool as "Combustibility of Materials at 750 °C: Non-combustible".
• ASTM E84 measures how fast flame spreads across a surface and how much smoke comes off it. ROCKWOOL publishes a flame spread index of 0 and a smoke developed index of 0.
• ASTM C665 is the material specification for mineral fiber blanket insulation, which is what a batt is. ROCKWOOL publishes Comfortbatt as Type 1 compliant to it. The loose-fill version is made to ASTM C764.

Nothing is added to make it flame resistant. The fiber is rock, so the fire performance is the material rather than a treatment that could wash out or wear off. Cellulose reaches its flame resistance a different way, through added chemicals, which is a real difference between the two even though both are sold as safe products.

What that does not buy you. A noncombustible material does not create a clearance, and it does not give you permission to fill one. It also does not make an assembly fire rated. A rated ceiling or wall is a tested build-up of specific layers, and the listing for that assembly is the spec, not the insulation inside it. If somebody is insulating your attic because a fire rating is required, the rating belongs in writing on the quote, with the assembly named.

Three situations, and they are the reason this page exists. In each one, mineral wool does something the cheaper materials cannot.

1. Knee walls and attic walls, where a batt has to stand up

A knee wall is the short vertical wall between an attic and a finished room. Attics also have gable walls, and walls around a stair or a mechanical closet. Loose fill cannot go in any of them, because it needs a floor to lie on. That leaves batts, and a batt in a vertical bay has one job beyond R-value: stay put for thirty years without sagging away from the top of the cavity.

Mineral wool is better at that job than glass fiber, for the plain reason that it is stiffer and denser. It friction-fits crisply and holds its shape. On a knee wall that backs living space, the fire behavior is a second reason, because that wall is often the only separation between an attic and a bedroom. Support the batt on both faces so it cannot bow out of the cavity, and air seal the wall first. The batts page covers the fitting rules that apply to any batt, and gaps and compression cost R-value in mineral wool exactly as they do in glass fiber.

2. Around a masonry chimney, a flue, or a heat producing appliance

This is the one where getting it wrong starts a fire, so read the numbers rather than the summary. The model 2021 International Residential Code, section R1003.18, sets a 2 inch air space between a masonry chimney inside the building, or within an exterior wall, and anything that burns. For a chimney entirely outside the exterior walls, it is 1 inch. And it states that the air space is not to be filled, except to provide fireblocking under section R1003.19.

Read that last clause twice, because it is the part that gets missed. The clearance is an air gap. It is not a slot to pack with a material that does not burn. What R1003.19 does ask for is that the space where a chimney passes through a floor or ceiling is fireblocked with noncombustible material, fastened in place, and either self supporting or laid on metal strips. That is where mineral wool belongs, and it is why an insulation crew carries it even on a job that is otherwise all loose fill.

The same logic covers the rest of the hot things in an attic. A furnace, a water heater, a metal flue or a chimney liner each carry a listing, and the clearance is whatever that listing and the maker's instructions state on the label. The code section above also lists exceptions, including lined chimneys tested to UL 1777. Recessed lights are the same story: the fixture label says whether insulation may touch it. Read the label on the thing itself. Do not take a distance off a website, including this one.

The practical job in an attic is usually a dam. You build a noncombustible collar or box that holds loose fill back from the chimney, the flue or the light, at the distance the code and the label require. Mineral wool, sheet metal or backer board all get used for it. The IRC is a model code, and states and cities adopt and change editions on their own timelines, so the clearance your inspector enforces comes from your local building department. Ask them before you build anything near a chimney.

3. Any attic where fire performance is the reason for the job

Sometimes the reason for the work is not the heating bill. A rated ceiling over an attached garage. A wall between two units. A rebuild in a wildfire-exposed area with rules about exterior assemblies. Insurance or a local order asking for a specific separation. In all of those, the material that does not burn is the point, and the price difference stops mattering.

One caution that saves arguments later. Ask for the rated assembly by name on the quote, and ask who is responsible for it passing inspection. A rating is a tested sandwich of layers, and swapping the insulation inside a listed assembly can invalidate it just as surely as leaving a layer out.

Mineral wool is treated to repel water. Splash it and the water beads and runs off. Soak an assembly and the water drains down through the fiber and out rather than being held in it. The fiber itself is rock, so there is nothing in it for mold to eat. ROCKWOOL states its stone wool "does not rot or promote the growth of mildew, fungi or mold", and publishes a pass under ASTM C1338, the test for fungi resistance.

It is also vapor open. Water vapor moves through it freely, so it does not act as a vapor retarder and does not need to be turned a particular way round. Unfaced batts have no facing to point at the warm side. That removes one of the classic attic mistakes, which is stacking a second faced batt over a first one and trapping water between the two facings.

What this does not do. It does not dry your attic and it does not fix a leak. Water that runs through the insulation still lands on the drywall and the framing below, and wet wood rots whatever is lying on top of it. Dust and dirt collect in any insulation over time, and mold can grow on that even when the fiber cannot feed it. If you already have wet or moldy material up there, start with mold in attic insulation, not with a shopping list.

Mineral wool is the heaviest insulation you are likely to put in an attic, and that is worth a number rather than an adjective. ROCKWOOL publishes a density greater than 1.8 lb per cubic foot (28.8 kg per cubic meter) for Comfortbatt. The R-30 batt is 7.25 inches thick. Multiply those and one square foot of that batt weighs at least about 1.09 lb. Spread over a 1,200 square foot ceiling, that is at least about 1,305 lb.

That is a floor, not a value. The maker publishes the density as "greater than", so the real figure is higher, and the arithmetic above is ours rather than a measurement of a bundle on a scale.

Ceilings have a published limit on this. GA-216, the Gypsum Association specification for applying and finishing gypsum panel products, says that where insulation above a ceiling is carried by the gypsum board itself, its weight must not exceed a maximum set by the board type, its thickness, the number of layers and the framing spacing. It also tells the installer to ask the insulation supplier for the weight. The 2010 edition read for this page states the rule and points at a table for the figures. We could not read those table values cleanly, so no pounds per square foot limit is published here. What you can do with that is ask the question: thin board on widely spaced framing carries less, and a ceiling that already sags carries less again.

Three practical consequences. Batts on a knee wall or in a small area are no problem at all. Spreading the heaviest material across a whole attic floor is the case where the limit is worth checking. And the bundles are hard work to carry up a pull-down ladder, which is a real cost on a big job even though it never appears on a quote.

Dense, open fiber absorbs airborne sound well, and mineral wool is dense open fiber. In a joist bay or a wall cavity it soaks up voices, television and traffic better than a light glass fiber batt of the same thickness. That is a genuine advantage and it is why the material is common in interior walls and floors between apartments.

Two honest limits. Airborne sound is only half the problem. Footsteps, a slammed door and a bath fan travel through the structure itself, and no insulation in a cavity fixes structural sound. And an attic floor is rarely the noise path anyway. If a room below is noisy, the cause is more often an uninsulated party wall, a duct that carries sound between rooms, or the fan itself.

Recommendation: if you are already putting batts in a knee wall or a ceiling bay, mineral wool gives you the sound benefit for free. Do not buy it for a whole attic floor on the strength of the sound claim.

It is sold in bundles of rectangular batts, sized to friction-fit between framing. Check the width against your framing spacing before you buy, and check the depth against the bay you are filling. Cut it with a serrated blade, a bread knife or a purpose-made insulation saw, against a straight edge. It cuts cleanly, which is one of the pleasures of the material, and the offcut holds its shape well enough to be useful.

• Wear the gear. A fitted respirator, sealed eye protection, gloves and long sleeves. The dust is heavier and coarser than glass fiber dust. Professional crews work under OSHA respiratory protection rules for a reason.
• Fit it, do not force it. Compressing a batt into a bay that is too small costs R-value. Cut around wiring and boxes rather than squashing the batt over them.
• Air seal first. Every top plate, wire hole, pipe and light. No insulation of any kind is a substitute. Air sealing is the first job on any attic and the cheapest R-value you will ever buy.
• Do not block the eave vents. Keep the air path from the soffit open, with baffles if needed. See baffles.

On a quote, ask for four lines: the product and its R-value, the area in square feet, where the mineral wool goes and where a cheaper material is being used instead, and what is being done at the chimney, the flues and the recessed lights. A quote that names one material for a whole attic is either overpriced or vague about the details that matter.

Find the row that describes your attic and take the recommendation. If two rows apply, the row describing a fire or safety condition wins over the row describing a preference.

The short version: mineral wool for the walls, the hot spots and the rated assemblies. Blown material for the floor. Most attics need a bag or two of the first and a truckload of the second. A contractor quoting mineral wool across a whole attic floor should be asked why.

Want a contractor to price the walls and the floor separately?

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We do not sell insulation and we do not install it, which is why this section can say what it says. If you came here to top up an open attic floor, mineral wool is the wrong buy and you should spend the money elsewhere.

An attic floor asks four things of a material. Enough depth to hit the target R-value. No gaps. Coverage over the joist tops. And a price low enough that you can afford to do the whole floor. Blown fiberglass and blown cellulose answer all four. Mineral wool answers the first three at roughly 3.3 times the material price of blown fiberglass, and it brings two properties the floor never uses. A floor does not need a batt that stands up on its own. And an open attic floor away from the chimney has nothing there to catch fire. R-49 is R-49, whichever fiber delivers it. See blown-in insulation for the material that belongs there, and cellulose against fiberglass for the choice between the two.

Two more cases where the honest answer is to spend nothing this year. If your attic is already within about R-10 of the level recommended for your climate zone, more depth pays back slowly, and air sealing is the better buy. That R-10 figure is an estimate rather than a measurement, set at about one step of the ENERGY STAR table. Measure before you shop: the depth checker turns inches into an R-value range. And if the material up there is dry, clean and simply thin, the cheapest correct job is more of the same over the top, which insulating over existing insulation walks through.

• Every attic insulation material, compared
• Blown-in insulation: what belongs on an attic floor
• Fiberglass batts, and the fitting rules for any batt
• Cellulose against fiberglass, with a verdict
• What is already in my attic?
• How much R-value your climate zone calls for
• What attic insulation costs, built up from parts