The objective of this study is to investigate the effects of installing a fluid diode window (FDW) in buildings to promote natural ventilation by controlling ventilation paths. An FDW is a porous plate with an internal structure that provides varying resistance to airflow depending on the flow direction. An FDW was installed in the bathroom of an experimental house to prevent polluted air from flowing into the adjacent multipurpose room during ventilation. Computational fluid dynamics simulations with a steady Reynoldsaveraged turbulence model and tracer gas method were performed to evaluate the performance of the FDW. The results show that an open window or FDW in the bathroom reduced the concentration and increased the ventilation rate. In the same wind direction at which the airflow exits through the window or FDW, it flows out at almost the same volumetric flow rate. In the cases where the airflow enters the bathroom through the window or FDW, the effect of preventing backflow from the bathroom window to the multipurpose room can be observed when the FDW was utilised, resulting in a reduction or even prevention of the gas entering the multipurpose room.