I have a masonry chimney on an exterior wall that runs straight up without any bends in the flue. I am wondering whether I should install a (1) flex liner wrapped in insulation, (2) install rigid pipe wrapped in insulation, or (3) install double wall pipe with insulation factory installed inside the walls (expensive and probably unnecessary)?? My goal of course is to insulate the chimney as best as possible since it sits on an exterior wall. Any suggestions? Any suggestions on brand?