The paper presents a numerical study of three-dimensional buoyancy-driven flow in a half-scale model of a two-floor building model. The model consists of an upper compartment and a lower compartment with a stairway connecting the two floors. The model forms a closed system, with no inlet or outlet. The flow is driven by a single heat source placed in the lower compartment. The study is linked closely to a previously published experimental study by the present author, which provided the details of the geometry and the boundary conditions as well as data for comparison with the present numerical results. The numerical method is large eddy simulation with the dynamic kinetic energy transport subgrid model. Radiation exchange is modelled using the discrete ordinates (DO) radiation model. The thermal boundary conditions on the model walls are set as heat flux. It is shown that the air temperature level is sensitive to the initial conditions for temperature, but air velocity is unaffected. In order to study this effect further, with the aid of the k-epsilon model, the measured wall temperatures are set as boundary conditions, which removes the dependency on initial temperature. For the cases studied, comparisons are made between the measured and computed wall temperatures, wall heat fluxes, air temperature and air velocity. There is a general agreement between the two results. (C) 2010 Elsevier Ltd. All rights reserved.