Natural convection in differentially heated rectangular cavities has been investigated. One of the vertical walls of the cavity is cooled, and the opposing vertical wall is heated. The cold and hot vertical walls have constant and time periodic temperatures, respectively. The other four walls are adiabatic. Heat transfer is by convection only because surface emissivities of the walls have been reduced. Experiments are performed on rectangular cavities with aspect ratios of 1, 2.09, 3, 4, 5 and 6. All six cavities have a height of 340 mm and a depth of 210 mm. The cavity length is changed to obtain different rectangular cavities. The cavity is closed and filled with air. Thermocouples are used to measure the temperature. For each cavity, the temperature distribution between the cold and the hot vertical walls is obtained at 35 positions along the length, three positions along the height and one position along the depth directions. The contour maps of the dimensionless temperature are presented at the mid-depth of the cavity. The local temperature difference is highest, and the local Nusselt number is lowest at the mid-height. Heat transfer correlations are presented for the Rayleigh number range of 4.51 x 10(5)-1.13 x 10(8), and for the aspect ratio range of 1-6. The Nusselt number largely decreases when the temperature on the heated wall is changed from constant to sinusoidally varying. (C) 2018 Elsevier Ltd. All rights reserved.