A large number of modem European buildings are equipped with ducted air distribution systems. To investigate the implications of duct leakage, a field study was performed on 42 duct systems in Belgium and France. The measurement data confirm the findings of the few earlier experimental investigations on these matters in Europe. In our sample, the leakage rate appears to be typically three times greater than the maximum permitted leakage adopted in EUROVENT 2/2 (Class A). The advantages of tight ducts are illustrated with a theoretical case study on a balanced ventilation system with heal recovery. It indicates that the overall effectiveness of the system reduces drastically if the ducts are leaky. The savings potential of an airtight duct policy at the European level is calculated (a) based on estimates of tbe number of buildings equipped with mechanical ventilation systems, and (b) assuming market penetration scenarios of rehabilitation techniques. At the European level, the cumulative savings potential over a period of 10 years appears to be in the region of 10 TW h.