The prediction of leakage rates for commercial sheet metal duct systems is most difficult. The leakage rates of specific components are largely unknown and only crude models have been advanced to estimate the leakage. ASHRAE Technical Committee 5.2, Duct Design and Construction, recognized the need for reliable methods of leakage prediction for sheet metal ducts and commissioned several studies aimed at resolving this design problem. The work reported herein is a part of TC 5.2's effort.