The use of natural ventilation systems continues to be a popular feature in low energy, sustainable building design. One feature of natural ventilation is that, depending upon the prevailing climatic or thermal conditions, the airflow through a ventilator can be bi-directional. Aerodynamically, the ventilator, depending upon its construction, may not perform in the same way for the two different flow directions. The geometry of the internal flow-path, the inclination of louver blades and the location of meshes and acoustic linings are features that stop the ventilator being aerodynamically symmetrical for different flow directions. However, in most cases ventilator airflow performance data relate to tests conducted with the ventilator set with an inlet configuration. This is partly because ventilator manufacturers are concerned about rain being driven through the ventilator, and so test for this worst case scenario. This paper describes a series of experimental measurements on commercial louver ventilators that investigated the influence that flow direction and magnitude of pressure differentials can have on airflow performance. Louver face velocities of 0.05, 0.25 and 0.5 m/s were generated in the study. The commercial ventilators were also modelled as CAD files and then imported in to the CFD software FLOVENT v3.2. The experimental data were compared with CFD predicted values of airflow parameters. This analysis tested the feasibility of using CFD / CAD as a tool for designing and optimising the performance of ventilators.