I have a loft/living space above my garage. Because of that, all of my garage ductwork is metal, no flex or ductboard. The garage heating/cooling system cannot be connected with the living space above or adjacent lving spaces. My ductwork is all concealed in bulkheads in the garage ceiling and will be covered in 5/8 inch drywall as will the rest of the ceiling. All duct joints are taped with UL-181 listed metal foil tape. The ductwork is then wrapped with flame-spread (silver) type insulation, I think R-5.6. The insulation is provided because I also have a/c. In the summer, when the a/c is first turned on, the ductwork will sweat like a sob here in northern va with the high humidity. The duct insulation will reduce the potential for duct sweating and the resulting mold and funk which can occur in the presence of moisture. The garage ceiling is fully insulated with R-30 kraft faced insulation. The "interior ceiling" of the bulkheads is lined with FSK paper since the kraft paper on the R-30 ceiling insulation is flammable and would not be in direct contact with the drywall. All taps and returns are framed tight. As far as I know, the building Code will not let you use plywood framed ductwork for supplies but it can be done for returns. My heating system is in full compliance with local Codes and is being inspected. Hope this helps.
