The method of simulating non-air-conditioned buildings is somewhat different from that of simulating heating and cooling systems. Ventilation in non-air-conditioned buildings is often achieved using either windows or ventilation fans. Simulation tools can model both cases and can predict the thermal conditions of indoors. Mixed-mode buildings, which use both natural ventilation when ambient conditions are moderate and HVAC system when conditions are harsh, can also be modelled by defining the openings of windows and operation of the HVAC system in the simulation model. In this chapter, the method of modelling non-air-conditioned buildings is explained via six tutorials. Various aspects of modelling of non-air-conditioned buildings, such as how to define window openings, how to define scheduled opening and closing of windows and the impact of window openings on indoor conditions, are explained. Design issues such as the sizes of the openable windows and the temperature-controlled automatic opening of windows are also discussed.