Mineral wool was at one time the most common type of insulation, but by the 1970s, fiberglass overtook its popularity. In the past few years this product seems to be on a comeback. Perhaps because it is known to be excellent at both sound reduction and fire resistance.

Mineral Wool is spun from mineral mining waste into felts, blankets and batts. Post-industrial recycled content averages 75% in keeping with the Environmental Protection Agency’s (EPA) guidelines. Its rating is commonly R19 to R26 and costs about 50¢ per sq.ft.
Mineral wool actually refers to two different materials: slag wool and rock wool. Slag wool is produced primarily from iron ore blast furnace slag, an industrial waste product, and accounts for 80% of the mineral wool industry. Rock wool is produced from natural rocks, such as basalt and diabase and accounts for the other 20%.
Mineral wool is non-combustible and is particularly useful in high heat areas to prevent the spread of possible fires. Mineral wool is known to not support mold, mildew or bacterial growth.
Most mining waste, which is the basis of slag insulation, has heavy metal traces which can most effect producers and installers. It is possible that batt insulations can be an absorbent for VOCs. However, use of an impermeable membrane, called a vapor barrier, can cover the insulation and assist in resolving this problem. Those who are chemically sensitive may be effected by the mineral oil on the fibers and by the binders. Health concerns have included skin and respiratory complaints from the short fibers.
Once properly installed, carefully separated from a living space, mineral wool should pose no health risks to occupants.
Loose-fill rock wool is similarly made but without the addition of the resin binders. Loose fill can offer supperior insulating abilities. Loose fiber inhalation issues should be a consideration, but pose very little problems as long as kept tightly separated from living spaces. Loose fill insulations have a tendency to shift downward over time, diminishing some of their effectiveness.
Rock wool insulating boards are also available. They contain more resin binder to make them denser and more rigid.