Rock wool or mineral wool is a building insulation made of melted rock fibers. It has existed in various manufactured forms since the 1800s; today it's often made of basalt rock, iron slag and limestone melted together in a furnace heated to nearly 3,000 degrees Fahrenheit and blown through a spinning screen that turns it into strings of fine fiber. The fiber strings are mixed with binder material, compressed into soft mats and heat-cured to bind the fibers together. After curing, the mats are cut into batts and slabs or are shredded into loose fill.
Rock wool, like other forms of fiber insulation, greatly slows heat loss by trapping air in tiny pockets separated from one another by the rock wool fibers. Heat can't travel easily from one pocket to the next. Rock wool's resistance to heat transfer gives it an insulation value of R-3.1 to R-4. Rock wool is noncombustible and can act as a fire barrier to slow the spread of flames. It muffles sounds, repels water and won't support growth of bacteria, mildew or mold.
Rock wool fibers are irritating to the skin and eyes. Installers should wear long pants, long sleeves, goggles, gloves and a breathing mask before handling rock wool.