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 cost of painting. For insulation purposes, metal ducts are typically lined with faced fibreglass 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″ (2.5-5 cm) 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 o any length to suit, but the most common stock sizes range evenly from 4″ to 24″ (10-60 cm) with 6″-12″ (15-30 cm) being most commonly used. Stock pipe is usually sold in 10′ (300 cm) joints. There are also 5′ (150 cm) joints of the non-spiral type pipe available, which is commonly used in residential applications