Speedi-Products FD-25R4 18 18-Inch Diameter by 25-Feet Length R4.2 Insulated Flexible Duct with Metalized Jacket R4 fiberglass insulation for improved energy efficiency & noise absorption Polyethylene sleeve separates airflow from insulation Cut to length for each specific application Can be used in Supply, Return and standard exhaust applications Ideal for duct systems within the conditioned space