SMACNA has issued a free technical paper addressing zinc coatings used on galvanized steel ducts.

The paper refers to “Hvac Duct Construction Standards – Metal and Flexible,” second edition 1995. The paper reviews zinc coating thickness variations, as commonly used in trade practice. Also included are “the burdensome effects of imposing a universal requirement of G-60 and/or G-90 coating for all components.”

The paper addresses rectangular, round and oval duct, and duct hangers. Under “factors to be considered” it states: “Where ductwork exposures do not involve wet and dry cycles, high humidity and chemically polluted environments, the traditionally used zinc coating amounts reviewed herein have provided satisfactory service life in hvac system service… The service life of both G-60 and G-90 zinc coatings in outdoor temperatures is relatively short; G-90 is a recommended minimum on exposed sheet and reinforcements when supplemental paint coatings are not specified. The cost increase for G-90 versus G-60 coated sheet is small but availability varies nationally.”

The document was prepared by SMACNA’s director of technical services John Stratton. It is available for downloading free of charge at in the “technical services” section.