Solid inserts located at the corners of differentially heated two-dimensional square enclosures with laminar natural convection are used to control the heat rate within the enclosures. This numerical study employs air as the working fluid, and the variables of interest are the number, location, size and thermal conductivity of the triangular cross-section inserts. The major finding is that significant changes on the thermal performance of the enclosures can be achieved by using the inserts, which, when appropriately selected, can act either as heat transfer enhancers or as insulators. Analyses of the results, when the overall Nusselt number is the assessment parameter, show that it is viable, based on the variables of interest, to optimise the thermal performance of the enclosure. (C) 2003 Elsevier Science Ltd. All rights reserved.