Flexible duct, known as flex duct, is very convenient ducting
for attaching supply air outlets to rigid trunk ductwork, and,
in many cases, flex duct comprises the entire duct system. Flex
duct made for HVAC applications is typically constructed of a
plastic inner liner attached to a metal wire helix (or coil) to
make round, flex duct. The duct comes with a layer of fiberglass
blanket insulation already attached around the duct. The
insulation is covered and protected by a polyethylene or foil
vapor barrier. Flex duct is typically available in insulation
values of R-4, R-6, and R-8. For residential HVAC systems,
insulated flex duct typically comes in diameters of 4 inches
through 10 inches; above 10 inches, it comes in even sizes of
12, 14, 16, etc., up to 22 inches measured at the radius of the
metal helix. For proper performance, flex ducts in unconditioned
space should not be installed in cavities smaller than the outer
diameter of the duct, including the insulation. Compression of
the insulation around th