Occupant's interaction with the external fenestration has a large impact on the indoor environment and energy consumption of buildings. The rapid increase in India’s residential cooling demand requires a better understanding of mixed-mode operations in residential buildings. This study attempts to understand occupant behaviour for window operation. This study has been divided into three stages. Stage I- a survey was conducted with 150 occupants to identify the non-thermal drivers that prevented the opening of windows during a favourable outdoor environment. The first part of stage II consists of a field mapping study in which ten residences were monitored for five days. The parameters monitored were humidity, temperature, CO2, window operation and window status. The second part of stage II was the experimental stage to determine the window status by finding the air change rate through the tracer gas experiment. In this experiment, the CO2 decay curves were analysed with various opening percentages; the curves were cross-referenced with the occupant-generated CO2 data to define the window status. Stage III- is Computer simulation, which assisted in identifying the ideal period for which windows could be operated. The possible energy saving was established by analysing the comfort hours. The data collected on-site was used in modelling and simulation. The window operation schedule was fed into the simulation model and simulated for two cases. The study concluded that in 90% of observed residences the window operations were under the influence of non-thermal drivers. This resulted in a 20% reduction in thermal comfort hours which in turn increases the energy costs by 30%.