Galvanized mild steel is the standard and most common material used in fabricating ductwork because the zinc coating of this metal prevents rusting and avoids the cost of painting. For insulation purposes, metal ducts are typically lined with faced fiberglass blankets (duct liner) or wrapped externally with fiberglass blankets (duct wrap). When necessary, 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. 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 produce round duct in nearly any diameter when using the right forming die, and to any length to suit. However, the most common stock sizes range evenly from 4" to 24", with 6"-12" being the most commonly used. Stock pipe is usually sold in 10' joints. There are also 5' joints of the non-spiral type of pipe available, which is commonly used in residential applications.
