The duct leakage in forced-air distribution systems has been recognized for years as a major source of energy losses in residential buildings. Recently, a measurement technique for estimating the total air leakage from residential duct systems, called Delta-Q, has been proposed. It focuses on measuring the total supply and return air leakage flows outside at operating conditions that are required for energy loss calculations. This paper discusses the Delta-Q measurement technique and proposes improvements on its model to increase the accuracy of the total leakage estimations and to provide appropriate estimation of local leakages. Accurate estimations of the total and local leakages can help to focus the choice for both the right house and location in the duct system for performing the potential repair job. Proper information as to where the leakages are located inside the duct system can reduce the time required for the duct sealing task. This study uses detailed laboratory measurements to validate the original and improved Delta-Q technique procedure and calculations. The air duct leakage laboratory (ADLL) has two different air duct configurations and a wide range of leakage levels controlled by holes created at several locations in the ductwork. This work also includes a set of simulation results to provide an insight into the performance of Delta-Q under different conditions. The simulation and experimental studies showed that the proposed Delta-Q models can improve the accuracy of total leakage estimation and provide useful information when these leakages are located inside the duct. DOI: 10.1061/(ASCE)EY.1943-7897.0000038. (C) 2011 American Society of Civil Engineers.