Rectangular ducts are easily shipped when broken down or nested. They provide flat surfaces for tap-ins and hangers and they are conveniently fabricated. Disadvantages to using rectangular ducts (in contrast to other shapes) are: they create higher pressure drop; they use more pounds of metal for the same air-flow rate as round duct; joint length is limited to the sheet widths stocked by the contractor; and joints are more difficult to seal. Also, rectangular transverse joints are more costly to install than round ones.
