This research aims to quantify occupants’ window behaviour impact to the energy performance gap. Occupants’ window behaviour poses a real challenge to energy demand control in mixed-mode buildings. A window being left open, may compromise the efficiency of the ventilation system. Applying a mixed-method approach, this study was carried out over the summer of 2017, in a mixed mode office building at the University of Southampton. Dry bulb temperature, radiant temperature, relative humidity, CO2 and window movement were recorded. Concurrently a weekly questionnaire gathered environmental perception from 35 participants. Using TRNSYS, the results of the monitoring were compared to standard assumptions. Results indicate that windows activity plays a significant part in bridging the performance gap between design and actual energy consumption. Furthermore, the results of the questionnaires revealed participants’ rationales for window opening and closing behaviours. Although this study comprises of a small sample in temperate climate, implications of this research addresses key issues for researchers investigating behaviour modelling and practitioners initiating building design.