Mineral Wool Insulation:
R-Value, Fire Rating,
Cost & Best Uses

Mineral wool insulation, also sold as rock wool or stone wool, is made by melting basalt-type rock and steel-mill slag and spinning it into fiber. It comes as batts for stud walls, rigid boards for exterior continuous insulation, and loose-fill for attics. Its selling points are real: it does not burn, it shrugs off moisture, it is dense enough to deaden sound, and the batts friction-fit better than fiberglass. It also costs more and, like every fibrous insulation, it does not stop air. As a spray foam manufacturer we see mineral wool on a lot of good jobs, often right next to our foam. This guide covers how it performs, what it costs, how it compares, and two tools for wall assemblies and exterior board.

Mineral wool vs fiberglass, cellulose and foam

Mineral wool sits between fiberglass and foam: more R per inch and far better fire performance than fiberglass, less R per inch than foam, and no air sealing either way. Click a heading to sort.

Mineral wool or fiberglass batts?

In a standard 2x4 wall, mineral wool gets you R-15 where standard fiberglass gets R-13; high-density fiberglass also reaches R-15 for 2x4 and R-21 for 2x6 (DOE). So the R-value gap is small if you buy high-density fiberglass. The real differences are fire resistance, sound absorption, and fit: mineral wool is stiff enough to stand in the cavity and press tight around boxes and pipes, which is where most batt jobs lose performance. If none of those matter for the wall in question, fiberglass is the cheaper product.

Fire, sound and moisture: why people pay more for mineral wool

Fire

This is mineral wool's strongest card. ROCKWOOL's Comfortbatt is classed noncombustible under ASTM E136, scores 0 for both flame spread and smoke developed under ASTM E84, and has a melting point of about 2,150°F (1,177°C). It does not need a thermal barrier the way foam plastics do, and it is a common choice for garage-to-house walls, around fireplaces and chases, and in fire-rated assemblies. Fiberglass itself does not burn either, but its kraft or foil facings can.

Sound

Mineral wool's density makes it a good sound absorber inside wall and floor cavities, which is why it is sold in dedicated acoustic versions for interior partitions. Insulation is only part of a quiet wall, though; mass, resilient mounting and sealed gaps do much of the work. Open-cell spray foam is the other common choice for sound in walls and ceilings; see does spray foam reduce noise.

Moisture

Stone wool fibers do not soak up water: ROCKWOOL lists moisture sorption of 0.03% for Comfortbatt (ASTM C1104) and 0.2% by volume for Comfortboard 80. Water drains out and the batt dries without slumping, so it tolerates the odd leak far better than cellulose. It is also vapor-open, which is the point on exterior walls: the board lets sheathing dry outward, while foil-faced foam blocks that route. What it does not do is stop air. Humid air can still move through it and condense on a cold surface behind it, which is why it is the wrong choice against a cold rim joist or a crawl space wall.

Where mineral wool insulation is the wrong call

A good combined approach in many houses: closed-cell spray foam at the rim joists and any tight roof assemblies, mineral wool in interior walls and floors for sound and fire, and blown insulation on the open attic floor. Our wall insulation guide covers how spray foam handles exterior walls when air sealing is the priority.

Other insulation types, compared honestly