Tough aluminized Mylar outer wrap|Spun-bonded nylon and helical wire core|This consists of ten foot long return air duct made out a sound attenuating material that is anywhere from 12 to 20 in ID|1 in fiberglass insulation
The Unico System Return Air Duct, Insulated, 12 in Inside Dia, 10 ft Length, R4.2 Insulation Value, For Use With: M1218 Return Air Cooling Module, Aluminum