Hart & Cooley does not recommend screws be used to fasten the polyester core of the air ducts because they weaken the polyester. Polyester is a very strong material as long as there are no holes or tears in it. As soon as a tear or hole is introduced, it's strength drops. To maintain our UL approval status, our air ducts must pass a tension test (25 pounds hanging from one end of the duct), a torsion test (one end rotated 180° or to 25 foot-pounds whichever comes first) and then a leakage test. In all of these tests, both ends must be connected to collars per our installation instructions. Flexible duct connected with screws would not pass these tests.