Sprayed Acoustic loose fill fiberglass Insulation/blown in insulation is formed from molten stone or slag spun into a material with the look and consistency of sheep’s wool. Fiberglass is molten glass spun into fibers and formed into clumps that resemble wool. Cellulose is typically made from recycled paper fiber. While fiberglass and rock wool naturally resist burning, fire-retarding chemicals must be added to cellulose to help prevent the spread of fires.