Highly efficient, thermally insulated flexible duct for use in low and medium-pressure heating and cooling systems. Acoustically rated, self-extinguishing CPE core easily expands over fittings. Core is permanently bonded to a coated spring steel wire helix that supports an ample blanket of fiberglass insulation over a fiberglass scrim and a tough polyethylene vapor barrier. Smooth liner, insulation, and tough vapor barrier provide even airflow and a double air seal.