This paper presents a research framework to study the effects of placing hedgy windbreaks ahead of a residential building on the airflow in and around the building. A CFD simulation is carried out to predict the fluid flow characteristics around the porous hedge and the building resulted from different hedge porosities. Results show that the porous hedge could modify the flow structures behind the hedge, and thus alter the cross ventilation in the building. Three typical flow patterns behind the porous hedge are identified: the penetrating flow, the stagnant flow and the recirculating flow. It is also found that there exists a critical hedge porosity below which the cross ventilation in the first floor of the building is inverse. For the second floor, placing windbreaks results in stronger windflow. Furthermore, hedges with porosities within a certain range have the same effects on the airflow.