Galvanized mild steel is the standard and most common material used in fabricating ductwork. For insulation purposes, metal ducts are typically lined with faced fiber glass blankets (duct liner) or wrapped externally with fiber glass blankets (duct wrap). When called for, a double walled duct is used. This will usually have an inner perforated liner, then a 1–2" layer of fiberglass insulation contained inside an outer solid pipe.
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Rectangular ductwork commonly is fabricated to suit by specialized metal shops. For ease of handling, it most often comes in 4' sections (or joints). Round duct is made using a continuous spiral forming machine which can make round duct in nearly any diameter when using the right forming die and to any length to suite, but the most common stock sizes range evenly from 4" to 24" with 6"-12" being most commonly used. stock pipe is usually sold in 10' joints. There are also 5' joints of non-spiral type pipe available, which is commonly used in residential applications. These are snapped together with a pipe-lock seam and uses  crimped and beaded small end connections.

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