Results of a CFD simulation of the wind-assisted stack ventilation of a single-storey enclosure with high and low-level ventilation openings are presented and compared with both the laboratory measurements and the analytical model of the flow and thermal stratification developed by Hunt and Linden (2001). Comparisons show that close quantitative agreement is obtained between the thermal stratification predicted by the CFD and the analytical model and experimental measurements. A key consideration in the CFD modelling work is how to specify appropriate boundary conditions at the inlet and outlet locations of the enclosure. This paper investigates the use of constant pressure boundary conditions imposed over an opening whose physical area has been reduced to account for the effects of discharge and expansion. The close agreement with the analytical predictions and experimental results gives confidence in this approach and offers guidance on how to model wind-assisted stack ventilation flows using CFD.