Advanced heat and flow models (as employed within Building Thermal and Mass Flow Simulation (BSim) and Computational Fluid Dynamics (CFD)) with different degrees of detail were investigated and their modelling deficiencies identified. The CFD technique, which defines the fluid how on a ＇micro＇ scale, was integrated into BSim in which fluid flow is described in a larger scale. The resulting combined approach strengthens the modelling potential of each methodology by overcoming their specific deficiencies. BSim＇s inability to predict air flow property gradients within a single space was surmounted and the difficulty of estimating CFD boundary conditions are now supplied by BSim. The conflated methodology allows the analysis of part of the building via a detailed CFD technique where air property gradients are judged to be crucial while in the rest of the building, where the air can be considered mixed, less resolved BSim approach is employed. The BSim environment, ESP-r, was elected to perform the current work. Two air flow situations encountered within buildings are discussed to demonstrate the combined method＇s applicability when compared with the BSim approach. Finally, general conclusions are presented showing that the developed methodology is very promising.