Displacement ventilation and mixing ventilation are methods of air distribution that create different airflow patterns in rooms. Moreover, these two methods of air distribution involve diverse processes, such as turbulent flow, heat transfer and mass transfer. It is challenging to describe and compare the impact of these processes on the natural environment in a simple manner. Entropy generation, which represents energy degradation, is a direct and effective parameter for assessing irreversibility in multi-process systems. In this study, heat transfer irreversibility is found to be the primary source of entropy generation in displacement ventilation and mixing ventilation systems. In the tested scenarios, the entropy generation in a displacement ventilation system is approximately 30 % less than that in a mixing ventilation system. During space cooling, this difference in entropy generation is evidenced by the tendency of air to flow towards a uniform mixing state in mixing ventilation. When indoor air can be assumed to be completely mixed, a simplified model can effectively predict entropy generation. Considering entropy and exergy balance, the exergy efficiency for a displacement ventilation system is 25.2 %, whereas that for a mixing ventilation system is only 9.2 %.