Laminar free convection is studied experimentally in a box with differentially heated endwalls that is divided by a barrier into two connected chambers. We investigate how the steady-state circulation, the temperature field and the transport of heat and mass depend on the height and position of the barrier. We find that the temperature fields in the two chambers are very different and that a complex flow structure develops, featuring a fast overflow of the barrier, a number of shear layers above the barrier, partial boundary layer detrainment on the cold endwall, and a slow upwelling in the cooler of the two chambers. The heat transport is found to have little dependence on the barrier height or barrier position, and begins to decrease rapidly only when the gap above the barrier becomes small. (c) 2006 Elsevier Ltd. All rights reserved.