A prefabricated outdoor thermal duct system is provided having a metal formed ductwork air passage, a hard board insulation layer glued to the metal duct leaving custom cut space for duct connections, a custom prefabricated cladding formed and adhered to the hard board insulation, a custom fabricated insert matching the insulation material, installed over the duct connection in the field to seal the joint, and a final fabricated cap adhered to the cladding.