Recently a number of new design tools have been developed for research and industry to predict natural ventilation effects in buildings. This paper shows the use of CFD to predict the cross ventilation rate of a single-zone building with two dominant openings. To validate the CFD results, full-scale experiments had been undertaken under various weather conditions. RANS model predictions proved reliable when wind directions were near normal to the vent openings. However, when the fluctuating ventilation rate exceeded the mean flow, RANS models were incapable of predicting the total ventilation rate. Improved results are expected by applying more sophisticated turbulence models, such as LES or weighted quasi-steady approximations.