An experimental program was conducted to measure thepressure loss attributed to internal reinforcem ents installed ina 22 × 22 in. (559 × 559 mm) phenolic duct system connectedwith four-bolt flanges and cleats, and having 10 ft (3.0 m)section lengths. It was determined that the zero-length losscoefficient for an individual internal cross reinforcement wasC= 0.080 for the specific geometry of the reinforcement andduct cross section considered, assuming that each reinforcementassembly contributed equally to the overall pressure lossin the test section. The reinforcement loss coefficient determinedin this study was comparable to a value predicted usingthe ASHRAE Duct Fitting Database. When evaluated at thesame volume flow rate as unreinforced ducts, the presence ofinternal cross reinforcements more than doubled the measuredpressure loss per unit length.