Abstract: An illustrative embodiment of the invention is directed to a thermally insulated exhaust duct structure. In this regard, a typical duct comprises an inner stainless steel conduit and one or more sheets of highly-reflective material that are spirally wrapped around and spaced outwardly from the outer surface of the conduit. The adjacent layers of the sheet material, moreover, are spaced from each other a predetermined distance through corrugated strips that are fastened to one side of the sheet material. Thus, when wound about the conduit in an overlaying relationship, the corrugation peaks bear against the sheet of material that is immediately below and thereby estabish a spacing between adjacent layers that is at least equal to the depth of the corrugations. Other features of the inventon include an outer protective stainless steel casing that fits over the spirally wrapped insulating sheet and a roof penetration for the duct structure.