The suitability of night ventilation for cooling is first discussed by presenting a plot of summer weather conditions on the bioclimatic chart and by reporting on the results of energy and ventilation simulations of a typical UK office module. The development of a simplified design tool suitable for the early stages of design process is then described. For this model, user input is limited to a few key variables and the technique allows the designer to explore rapidly the effects of a range of design variables including variable external temperatures, internal gains and ventilation rates during the day and night.