Moderate Rayleigh number thermal convection for horizontal rectangular enclosure is experimentally studied for 1.85 x 10(6) <= Ra <= 1.04 x 10(11). The best fit for the investigated range of Rayleigh numbers is found to be Nu = 0.067Ra(0.330) when a sidewall material of very low thermal conductivity is employed. The effect of sidewall conductance and aspect ratio on the Nu - Ra relationship is also scrutinized. (C) 2019 Elsevier Ltd. All rights reserved.