In many cases natural ventilation may be sufficient to ensure acceptable comfort levels in occupied buildings. In these cases, installation of energy-intensive active environmental control systems will not be necessary. This will result in considerable energy and cost savings and also indirectly in a reduced burden on the environment, since the use of energy is always associated with the production of waste materials. This paper describes the development of a new model to predict natural ventilation flow rates in buildings. The model is based on the concept of a flow network where openings are represented by non-linear flow resistances. It takes into account the effect of both wind-induced pressures and pressures due to thermal forces. The model draws on a healthy balance between purely theoretical equations and empirical data. The new flow model was linked to an existing thermal model to produce a new integrated design tool for naturally ventilated buildings. It takes into account the important interaction between the flow model and the thermal model and can therefore be used to predict the natural ventilation Bow rates as well as the resulting indoor air temperatures. The applicability of the new tool is illustrated through a case study. From this it is clear that the tool can be used successfully to optimize passive building design. The optimized design illustrated here not only resulted in minimum initial cost, but also in improved passive thermal comfort in summertime and a reduction in winter heating energy consumption.