The most common materials used to fabricate ducts are metal, fiberglass ductboard, and flexible wire-and-plastic “flex-duct.” All three materials can be used with good results, but size requirements vary from material to material because of differences in surface characteristics that affect resistance to airflow. Air-sealing and connection methods also vary between materials. Flex-duct and fiberglass ductboard may be self-insulating, whereas metal duct typically should have added insulation (see Insulating Ducts in Insulating and Air-Sealing).

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