SMACNA shares some important information on the proper installation of a duct system.
(The following is taken from HVAC Duct Construction Standards, Metal and Flexible, published by the Sheet Metal and Air Conditioning Contractors' National Association.)
A duct system is an assembly whose primary function is to convey air between specified points. ASHRAE (the American Society of Heating, Refrigerating and Air-Conditioning Engineers) categorizes duct systems as either single path or dual path. Systems should be designed using accepted engineering practice and data such as that in the four ASHRAE handbooks and the SMACNA HVAC Duct Systems Design manual.