Numerical methods are used to solve the finite volume formulation of the two-dimensional mass, momentum and energy equations for steady-state natural convection inside a square enclosure. The enclosure consists of adiabatic horizontal walls and differentially heated vertical walls, but it also contains an adiabatic centrally-placed solid block. The aim of the study is to delineate the effect of such a block on the flow and temperature fields. The parametric study covers the range 103 <= Ra <= 106 and is done at three Pr namely, 0.071, 0.71 and 7.1. In addition the effect of increasing the size (characterized by the solidity Phi) of the adiabatic block is ascertained. It is found that the wall heat transfer increases, with increase in the Phi, until it reaches a critical value Phi = Phi(OPT), where the wall heat transfer attains its maximum. Further increases in the block size beyond Phi(OPT), reduces the wall heat transfer, for as the block size becomes larger than the conduction dominant core size it reduces the thermal mass of the convecting fluid. A steady-state heat transfer enhancement of 10% is observed for certain Ra and Pr values. Useful correlations predicting this optimum block size and the corresponding maximum heat transfer as a function of Ra and Pr are proposed; these predict within +/- 3%, the numerical results. (c) 2006 Elsevier Ltd. All rights reserved.