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Cooling energy use of residential buildings in Hong Kong has been increasing in the past two decades. Natural ventilation is an effective means to reduce cooling energy use. To encourage wider and more efficient use of natural ventilation, previous works have been focused on window designs and the influence of window opening on the discharge coefficient. There is virtually no works on identifying the window opening degree for efficient natural ventilation. In this study, walk-through surveys, site measurements, CFD simulations and statistical analyses were adopted to ascertain Hong Kong households' window opening habits and to identify the optimum window opening degree. Three window types, operating under nine sets of representative wind conditions in Hong Kong, four possible window orientations and two ventilation modes (single-sided and cross ventilation), to become 1944 cases, were studied. Air change per hour (ACH) was used to quantity the indoor natural ventilation performance. Regression analysis was employed to quantify the influence of different operating characteristics on ACH. The results concluded that Hong Kong households had limited and inefficient use of natural ventilation, while the optimum window opening degree, taking into account seasonal wind conditions and all design options, should be in the range of 0.6 to 0.9. [ABSTRACT FROM AUTHOR]