Duct insulation and sealing, especially insulated supply ducts delivering conditioned air within a building, save energy. The intent of energy efficiency codes, as related to duct insulation and sealing, is to keep mechanically warmed or cooled air as close to a constant, desired temperature as possible and prevent the conditioned air from escaping the duct system while it is being moved to spaces where it is needed. If reduced heat transfer through insulated ducts is accounted for in the heating, ventilating, and air conditioning (HVAC) load calculations, it may even be possible to reduce the size of HVAC equipment.