Fiberglass is a very efficient insulator which makes it an ideal material for air conditioning ducts. The material is pressed into boards that can be cut and joined into a variety of shapes.
Fiberglass is stable, but not as strong as steel, so it is not suited for use in air conditioning ducts that must withstand outdoor damage.
Galvanized Steel and Aluminum
Galvanized steel is the material that was used in the some of the earliest air conditioning systems and is the most commonly used material in air ducts. It is a very strong, conductive material that does not insulate well, but is excellent for cooling air.
Steel is commonly used for exposed, outdoor air ducts and ducts in large buildings. Aluminum is another common material used in air conditioning ducts, usually in the tape and connecting parts of the duct.
Polyurethane is a type of high-strength, insulating plastic that is used in a variety of industrial, commercial and residential products, including air conditioning ducts. It is formed into panels, which are then shaped into ducts.
A few other materials are used commonly in air conducting ducts. Synthetic foams and plastic materials, such as phenolic foam and polyethylene terephthalate, are used to make extremely elastic, bendable ducts that can be placed in more intricate arrangements than steel and fiberglass ducts.