1. Flexible duct shall be supported at manufacturer’s recommended intervals, but at no greater distance than 5 feet (1524 mm). Maximum permissible sag is ½ inch per foot of spacing between supports.
2. Hanger or saddle material in contact with the flexible duct should be of sufficient width (minimum 1 ½" (38 mm)) to prevent any restriction of the internal diameter of the duct when the weight of the supported section rests on the hanger. Individual ducts should be separately supported.
3. Flexible ducts may rest on ceiling joists or truss supports. Maximum spacing between supports should not exceed the maximum spacing per manufacturer’s installation instructions.
4. Support the duct between a metal connection and a bend by allowing theduct to extend straight for a few inches before making the bend.
5. Vertically installed duct should be stabilized by support straps at a maximum of 6 feet (1829 mm) on center.
More information can be found at: http://www.dca.state.ga.us/development/constructioncodes/publications/1ONE.pdf