Mineral fiber insulation is a type of building insulation made from natural rock or industrial slag that’s been melted and spun into fine fibers. It delivers R-values around 3.1 to 3.2 per inch in batt form, resists fire up to at least 1,000°C (1,832°F), and works in everything from residential walls to industrial boilers. You’ll also hear it called mineral wool, rock wool, or slag wool, and it’s one of the most widely used insulation materials in commercial and residential construction.
What It’s Made Of
Mineral fiber insulation starts with one of two raw materials: basaltic rock or slag, which is a byproduct of metal smelting. In the United States, most mineral wool production uses slag. Rock wool varieties combine basalt with limestone, clay, and feldspar. Either way, the raw material is melted at extremely high temperatures and then spun through a wheel centrifuge, producing millions of fine, discontinuous fibers averaging 3.5 to 7 micrometers in diameter. Think of cotton candy, but made from stone.
Those loose fibers are then bound together with a resin to hold their shape as batts, boards, or loose-fill products. Traditionally, manufacturers used formaldehyde-based binders, which have drawn scrutiny because formaldehyde is classified as a carcinogen. The industry has been shifting steadily toward formaldehyde-free alternatives over the past couple of decades. Many products now use polyester, bio-based, or other binder systems that eliminate formaldehyde entirely. If this matters to you, look for products labeled “formaldehyde-free” or “no added formaldehyde.”
Thermal Performance
Mineral fiber insulation provides an R-value of roughly 3.1 per inch for standard batts and blown-in applications. Rock wool batts are slightly higher at about 3.2 per inch, while blown rock wool drops to around 2.8 per inch because the fibers pack less tightly. For context, a standard 2×4 wall cavity filled with mineral wool batts gives you approximately R-13 to R-15, which is comparable to fiberglass batts of the same thickness.
Where mineral fiber really separates itself is in temperature extremes. With a melting point at or above 1,000°C, it can insulate equipment running at continuous service temperatures up to about 650°C (1,200°F). That’s why you’ll find mineral wool boards wrapped around boilers, furnaces, ovens, industrial ductwork, and storage tanks in addition to residential walls and attics.
Fire Resistance
Mineral fiber insulation is classified as non-combustible under European fire testing standards, earning an A2-s1,d0 rating. In practical terms, that means it won’t catch fire, produces virtually no smoke, and doesn’t generate flaming droplets. This is one of its biggest selling points over foam-based insulation products, which can melt or burn. In residential construction, mineral wool batts in wall cavities and floor assemblies add passive fire protection, slowing the spread of flames between rooms. In commercial buildings, mineral fiber boards are commonly used as firebreaks and around structural steel to extend fire resistance ratings.
Sound Insulation
Mineral wool is often promoted for its acoustic properties, and it does absorb sound well within the material itself. However, when installed inside a typical residential wall assembly, the differences between insulation types are surprisingly small. Testing by the Insulation Institute found that mineral wool R-15 batts achieved a Sound Transmission Class (STC) rating of 34, essentially identical to fiberglass R-15 batts (also STC 34) and open-cell spray foam filling the full cavity (STC 33). The variation across all insulation types tested was 3 STC points or less.
The takeaway: in a standard wood-framed wall, the studs and drywall control most of the sound transmission. Swapping fiberglass for mineral wool won’t make a dramatic difference. If you need serious soundproofing, you’ll get more benefit from double layers of drywall, resilient channels, or staggered-stud wall designs than from choosing one cavity insulation over another.
Mineral Wool vs. Fiberglass
These two products compete directly in residential and light commercial projects, and each has clear advantages depending on the application.
- Density and rigidity: Mineral wool is significantly denser than fiberglass, which makes it rigid enough to hold its shape without fasteners in some applications. Fiberglass is lighter and more flexible, making it easier to fit into irregular spaces.
- Weight: That extra density means mineral wool weighs more. In attic applications where insulation sits directly on ceiling drywall, the added weight can be a concern, and fiberglass is often the better choice.
- Fire performance: Mineral wool is non-combustible and withstands far higher temperatures. Fiberglass is also fire-resistant but has a lower melting point.
- R-value: Mineral wool delivers a slightly higher R-value per inch (about 3.1 to 3.2 for batts vs. 2.9 to 3.8 for fiberglass, depending on the product and density). The gap is small enough that it rarely drives the decision on its own.
- Cost: Mineral wool batts typically cost more per square foot than fiberglass of comparable R-value.
For exterior wall cavities, basement walls, and anywhere fire resistance matters, mineral wool is a strong choice. For attics, cathedral ceilings, and projects on a tight budget, fiberglass often makes more sense.
Where It’s Used
In homes, mineral fiber insulation shows up as batts in wall cavities and floors, rigid boards on exterior walls, and loose-fill blown into attics. Its rigidity makes it popular for exterior continuous insulation, where boards are fastened to the outside of wall sheathing to reduce thermal bridging through studs.
In commercial and industrial settings, the applications lean heavily on mineral wool’s heat tolerance. Rigid mineral wool boards insulate boilers, furnaces, precipitators, tanks, and ductwork operating at temperatures that would destroy foam or fiberglass products. It’s also used as a fire barrier in curtain wall assemblies, between floors of multi-story buildings, and around penetrations in fire-rated walls.
Working With Mineral Fiber Insulation
Mineral wool fibers can irritate your skin, eyes, and respiratory tract during cutting and installation. Wear long sleeves (or a protective suit), gloves with rubber or silicone palms, safety glasses, and a respirator or dust mask. If you’re installing overhead, a cap or helmet keeps fibers out of your hair.
Cutting mineral wool is different from cutting fiberglass. Standard utility knives and scissors don’t work well. For thin batts (under about 50mm), a serrated bread knife will do the job. For thicker or denser material, you’ll need a long insulation knife, at least 250mm, with teeth designed for mineral fiber. These specialty blades cut cleanly without tearing the material. Score your cut line, press the knife through with steady pressure, and the rigid material separates with a clean edge. The rigidity of mineral wool actually makes it easier to cut accurately than floppy fiberglass batts, once you have the right tool.
Mineral wool batts friction-fit into wall cavities without staples or adhesive. Cut them about half an inch wider than the cavity and press them in. The stiffness of the material holds it in place, which is one of the practical advantages over fiberglass during installation.