The aim of this paper is to assess the use of mixed mode ventilation for a typical office building under future weather data and realize a design that minimizes energy use while maintaining thermal comfort and workers productivity. For this reason an operation algorithm for windows and AC is summarized and future weather data of Lebanon is generated using the Hadely Center Coupled Climate Model. Under current construction practices and weather data the simulated building was simulated for the use of mixed mode, 33% energy savings were achieved, and under future weather data 26% energy savings resulted. To make natural ventilation viable for the future and maintain energy consumption thickening of walls and including 5 cm (0.16ft) insulation was studied which decreased the energy consumption further and made the mixed mode process save 39%. Then making the glazing double lowered the energy consumption and the savings reached became 40% against the base case. These two measures proved efficient in the present and the future, no negative effect was witnessed neither on the peak summer energy consumption nor on the natural ventilation use. The best way to enhance the use of mixed mode ventilation in the present and the future is then to use a combination of those two measures.