An insulated ductwork system particularly adapted for exterior use joins together pre-assembled air circulation duct sections that each have a ductwork passageway that is formed solely with insulation panels of foam insulation material disposed between foil sheets that are in turn covered by additional insulation material and an outer protective jacket to seal the ductwork from weather effects. The air circulation duct sections are used without traditional metal ductwork forming any part of the air circulation duct. Receiving flanges extend from the ends of each air circulation duct section and the duct sections are joined together in abutting relation with at least one slidably engageable sleeve that engages the flanges. A filler insulation is disposed over the sleeve. A closure band is disposed over the filler insulation and joined to exterior surfaces of the air circulation duct sections.