What is duct that you put on insulation?

You insulate any ductwork where you want to control the temperature diffences between the inside and outside of the duct. One example would be the combustion air duct, usually a round sheet metal line that comes from the outside and ends near the furnace and/or water heater. Typically this would be insulated with fiberglass padding held in place with a plastic coating. Larger, rectangular ductwork can be insulated with internal or external insulation, not always fiberglass, of verying thicknesses.