Fibrous glass has been used as thermal and acoustical insulation in forced air heating, ventilating, and air- conditioning systems ever since these systems became widely available following World War II. Today, fibrous glass duct systems, duct liners, duct wraps, and commercial insulation boards contribute to indoor environmental quality (IEQ) and occupant comfort in residential, commercial, industrial, and public buildings of all kinds. These include homes, apartments, schools, hotels, motels, medical facilities, theaters, shopping centers, office and government buildings, factories, and many other types of structures where occupant comfort and indoor environmental quality are important criteria.

Fibrous glass insulation is used in air duct systems for four basic reasons:

• Temperature control - Delivery of heated or cooled air at comfort levels suited to building occupancy requirements.

• Acoustical control - Absorption of noise generated by central air handling equipment and movement of air through the ducts, as well as "cross-talk" carried by ducts from one space to another.

• Condensation control - Prevents condensation in or on ducts when installed R-value recommendations are followed, reducing the likelihood of moisture damage to ceilings and other interior finishes.

• Energy conservation - Reduction of HVAC system operating costs by controlling heat loss or gain through air duct walls, helping systems run more efficiently.

HVAC systems typically operate with internal air temperatures between 40˚F (4˚C) and 250˚F (121˚C). Systems operating outside this range are not considered in this guide, except that fibrous glass insulation boards may be used in systems operating at internal air temperatures between 0˚F (-18˚C) and 450˚F (232˚C).