Muffler-Duct? is a fully flexible, thermally insulated ducting that is specially designed for the HVAC market. Muffler-Duct? is composed of three layers that work together to stand up to the most demanding applications. The inside is made of aluminum foil duct, in the middle there is a flame-retardant pearl wool, then we use a white reinforced PVC sleeve with an encapsulated high tensile steel wire helix. Engineered ducting that reduces noise and condensation. Oversized to fit easily onto connections. Each Muffler-Duct? comes in 15 ft lengths with 2 duct clamps included.