We present a simplified model of an underfloor air distribution (UFAD) system consisting of a single source of heat and a single cooling diffuser in a ventilated space. Laboratory experiments were carried out to simulate the flow and a model for the flow in this space is proposed. The model is based on plume theory for the heat source and a fountain model for the diffuser flow, and predicts a steady-state two-layer stratification in the room. The governing parameters are shown to be the buoyancy flux of the heat source, and the volume and momentum fluxes of the cooling diffuser. The results suggest ways to optimize UFAD design and operation. (C) 2004 Elsevier B.V. All rights reserved.