Thermaflex flexible ducting is UL 181 Listed Class 1 air duct. Strong polyester film laminated and encapsulating a heavy steel wire helix forms the inner core. The outer silver film provides a reflective surface on the inner core. An oversized, loose, steel wire helix supports a thick blanket of fiberglass insulation creating an air gap between the reflective inner core and fiberglass insulation. The outer vapor barrier is a jacket of durable black polyethylene. The smooth airtight inner core of polyester is designed for low to medium pressure systems. Can be used in place of galvanized pipe in most applications for heating, cooling, intake and exhaust needs.