The case of thermal control by means of air natural convection stimulates the investigation of configurations with the aim at improving the thermal performance. The effect of adding adiabatic extensions downstream of a vertical isoflux symmetrically heated channel has been experimentally analyzed. Optimal configurations have been identified through the measured wall temperature profiles, with reference to the extension and expansion ratios (L/L-h and B/b) of the insulated extensions. Conspicuous maximum wall temperature reductions have been achieved by means of these optimal configurations. In details, percent increases of the heat transfer rate (i.e., average channel Nusselt number) were of order 10-20% depending on the channel aspect ratio, L-h/b, and imposed wall heat flux. In any case, quite large extensions should be added to enhance the heat transfer rate, i.e. about 3.0 times the height of the channel, while the optimal expansion ratio was nearly 2.0 for all the configurations. Composite correlations between the average Nusselt number and the maximum dimensionless wall temperatures and Ra*, the Rayleigh channel number, B/b and L/L-h parameters, in the 1.5 less than or equal to L/L-h less than or equal to 4.0, 1.0 less than or equal to B/b less than or equal to 4.0 and 10(2) less than or equal to Ra*B/b less than or equal to 5.0 x 10(6) ranges, have been evaluated.