Building on a history of duct construction standards nearly 100 years old, the flagship SMACNA HVAC Duct Construction Standard – Metal & Flexible 4th edition represents the state of the art in the sheet metal duct fabrication and installation requirements. Significant changes for the 4th edition include; flat oval tables for positive/negative pressure applications; spray and aerosolized duct sealants; non-fiberglass duct liner applications; added information on spiral duct fittings; updates to duct hanger requirements; flexible duct hanger spacing and support bracing.