The paper describes the results of an experimental study of natural convection in a closed half-scale model of a building comprising two floors connected by a stairway, and a horizontal opening between the two floors. The driving force for the flow is a heat source in the lower floor. The results indicate a complex three-dimensional flow within the model. The rate of heat losses from each wall, wall temperatures, air speed and temperature in the horizontal opening for a range of heat inputs are presented and discussed. The results are then used to suggest boundary conditions for numerical simulation of the flow. (C) 2007 Elsevier Ltd. All rights reserved.