Mineral wool, also known as rock wool or stone wool, is a fibrous insulation material manufactured by melting basalt rock and recycled slag at approximately 1500 degrees Celsius and spinning the molten material into fine fibers. The resulting product is formed into batts, boards, or loose-fill and is valued for its combination of thermal performance, exceptional fire resistance, and acoustic absorption. ROCKWOOL, the Danish company founded in 1937 that pioneered modern stone wool production, remains the global market leader with manufacturing facilities across Europe and North America.
R-Value and Thermal Performance
Stone wool insulation achieves R-values ranging from R-3.3 to R-4.2 per inch, depending on product density and application. ROCKWOOL Comfortbatt residential batts are rated at the higher end of this range. The density of mineral wool products varies significantly by intended use: lighter batts (2 to 4 lb per cubic foot) serve cavity insulation in wood and steel framing, while rigid boards (6 to 8 lb per cubic foot) are specified for flat roofing, exterior continuous insulation, and applications requiring compressive strength.
The thermal performance of mineral wool does not degrade over time because its insulating mechanism relies on trapped air within the fiber matrix rather than on blowing agents that can diffuse out of the material. This gives stone wool a stable long-term R-value that building energy models can rely on throughout the service life of the structure.
Fire Resistance
Mineral wool is the most fire-resistant commercially available insulation material. Because it is produced from basalt rock melted at 1500 degrees Celsius, the fibers themselves withstand continuous temperatures exceeding 1000 degrees Celsius without melting, shrinking, or contributing to flame spread. The small percentage of organic binder used to hold the fiber matrix together burns off at around 200 degrees Celsius, but the stone wool structure remains fully intact.
Mineral wool achieves a Euroclass A1 fire rating (non-combustible) and, under ASTM E84 testing, earns a flame spread index of 0 and smoke developed index of 0. A 2-hour fire-rated steel column can be protected with as little as 45 mm of high-density stone wool. This performance makes mineral wool the standard insulation for commercial high-rise construction, firestopping assemblies, and any application where building codes mandate non-combustible materials.
Acoustic Performance
The dense, fibrous structure of mineral wool makes it an outstanding sound absorber. ROCKWOOL Safe'n'Sound, a product specifically designed for interior partition walls, achieves a Noise Reduction Coefficient (NRC) of up to 1.0, meaning it absorbs essentially all sound energy striking its surface. When installed in a standard 2x4 wood stud partition, mineral wool adds 5 to 8 STC (Sound Transmission Class) points compared to an uninsulated cavity. It is effective at reducing both airborne sound (speech, television) and impact sound (footsteps) in floor-ceiling assemblies.
Hydrophobic Properties
Stone wool is inherently hydrophobic. Water beads on the surface of the fibers rather than being absorbed, and ROCKWOOL products are treated with a mineral oil during manufacturing that further enhances water repellency. Under ASTM C1104 testing, water absorption is typically less than 1 percent by volume. This means mineral wool does not support mold growth and maintains its thermal performance in damp conditions, making it suitable for exterior applications where the insulation may encounter rain during construction or condensation within the wall assembly.
Key Applications
Mineral wool is used in wood and steel stud cavity walls, exterior continuous insulation systems, flat roofing assemblies, rainscreen cladding, HVAC duct insulation, and acoustic interior partitions. Its combination of fire safety, thermal efficiency, and sound control makes it particularly valuable in multi-family residential buildings, schools, hospitals, and commercial structures where code requirements for life safety are stringent. ROCKWOOL Comfortbatt and Safe'n'Sound are the most widely specified stone wool products in North American residential and light commercial construction.