Natural ventilation is one of the most fundamental techniques to reduce energy usage inbuildings. However, due to complicated site plans and building layouts, it is difficult todesign optimal layouts for the enhancement of ventilation without knowledge about the flowpatterns. The employment of computational fluid dynamic (CFD) tools in the design processcan give predictive feedback to the designers, allowing them to optimize airflow around thesite to decide on building placement, orientation, and interior space layout. Since thesimultaneous simulation of the building and the site plan would require large computerresources, this paper attempts to reduce the computing burden by decoupling outdoor flowmodeling from an indoor airflow modeling, through an iterative design procedure. Thedecoupling uses loose and compact integration methods. The former method involves anoutdoor simulation, from which indoor boundary conditions are extracted, while the lattersimulates indoor and partial outdoor flow. An apartment complex in Shanghai is used as themodel in this study.