Fiberglass insulation does not burn as it is composed of inorganic materials, and it is therefore often used as a fire block in wood-frame walls. If the fiberglass insulation is covered in a vapor-retarder facing, though, that part is flammable, and it must not be left exposed.
Unfaced fiberglass insulation is naturally not flammable for the entire life of the product, and no additional chemicals treatments are necessary for fire protection. Faced fiberglass insulation is covered with gypsum board or another approved material to provide fire retardant properties. Overall, fiberglass insulation withstands higher temperatures than cellulose and spray foam insulation.