Mixed-mode ventilation that combines natural ventilation and mechanical ventilation has great potential to save cooling energy when compared to mechanical systems and is more reliable than natural ventilation systems. This paper presents a semi-empirical model for studying the impact of window opening area, insulation, and thermal mass on the cooling energy saving of mixed-mode ventilation for three office buildings in different types of US climates using EnergyPlus simulations. The results show that electricity use can be reduced by 6-91% depending on the climate. In addition to climate, thermal mass has a large impact on the performance of mixed-mode ventilation. This investigation developed a semi-empirical model to predict the impact of thermal mass on energy use, and optimized the thermal mass for maximum monetary return based on the model. (C) 2013 Elsevier B.V. All rights reserved.