Rigid duct is almost always preferable to flex duct, but if flex duct must be used insulated flex duct is preferable to uninsulated duct. When installing flex duct take care to avoid dips, sags, and bends as much as possible, since poorly installed ductwork will reduce performance of the heating, cooling, or ventilation system. This is particularly important with longer duct runs, as flex duct has twice the resistance to air flow compared to rigid duct. In other words, the airflow resistance created by 1 foot of flex duct is equivalent to the air flow resistance created by 2 feet of rigid duct.
This insulated duct consists of a strong polyester film laminated and encapsulating a heavy, corrosion resistant steel wire helix, that forms the inner core and supports a 1.5" thick blanket of fiberglass insulation (R-4.2) over a fiberglass scrim. The outer vapor barrier is a jacket of durable black polyethylene. It is designed for low to medium pressure systems, with airflow velocity up to 5,000 feet per minute.