An uninsulated flexible duct includes a core made of a polymer material and including a helical wire as a part thereof, the polymer material core having inner and outer surfaces and including a first adhesive. The duct also includes a fire resistant barrier material surrounding the outer surface of the polymer material core, the fire resistant barrier material including a seam running longitudinally along the length of the uninsulated flexible duct, the longitudinal seam having an overlap zone that contains at least one second adhesive to maintain an integrity of the longitudinal seam. The uninsulated flexible duct is one that is free of any vapor barrier and has a flame spread index and smoke developed index of zero (0) as defined in the UL 181 standard. Component parts of the duct are controlled in terms of composition, amount, and/or configuration to achieve the Class 0 rating.