Fibrous glass duct liner insulation is designed for application to the interiors of commercial and residential sheet metal heating, ventilating, and air-conditioning ducts operating at internal air temperatures not exceeding 250˚F (121˚C). Duct liner helps to attenuate noise generated by central air handling equipment, air movement within the ducts, and occupant cross-talk. It also serves to reduce heat loss or gain through sheet metal duct surfaces, and helps to prevent the formation of water vapor condensation both inside the duct and on its exterior surface. Fibrous glass duct liner insulation is fabricated along with the sheet metal ductwork in the shop, providing quality workmanship under controlled conditions.