SMACNA’s long-anticipated HVAC Duct Construction Standards – Metal and Flexible, 4th Edition can now be yours. This ANSI approved standard is the flagship publication put out by SMACNA, and represents the state of the art in the sheet metal duct fabrication and installation requirements. A must-own for contractors, engineers, designers, and fabricators, the Fourth Edition of SMACNA’s HVAC Duct Construction Standards – Metal and Flexible builds on a history of duct construction standards nearly 100 years old.