Insulated flexible ducts are supported at intervals as recommended by manufacturers, but at a distance not greater than 5 feet (1.5 m), with maximum permissible sag of 1/2 in. per foot (15mm per 300 mm) of spacing between support points.

Details of flexible duct connection and support are available in Air Diffusion Council (ADC) Flexible Duct Performance & Installation Standards, 4rd edition, 2003.