Fiberglass is one of a group of glassy, noncrystalline materials his­torically referred to as man-made mineral fibers (MMMFs) or man­made vitreous fibers (MMVFs). Glass fibers are made from molten sand, glass, or other inorganic materials under highly controlled conditions. The glass typically is melted in high-temperature gas or electric furnaces. The material is then spun or blown into fibers that are then processed into the final product.

Most major manufacturers use 20 to 30 percent recycled glass content. Rock wool (or slag wool) loose-fill insulation is similar to fiberglass except that it is spun from blast furnace slag (the scum that forms on the surface of molten metal) and other rocklike mate­rials instead of molten glass.

Fiberglass loose-fill insulation is available in two forms: processed either from a by-product of manufacturing batts or rolls or from “prime” fibers produced especially for blowing applications. Both must be applied through pneumatic means using a mechani­cal blowing machine, whether it be “open blow” applications such as attic spaces or closed-cavity applications such as those found inside walls or covered attic floors.