This paper reports the results of measurements of inside surface temperatures on a basic double window arrangement consisting of two sheets of glass surrounded by insulated construction. Principal variables were air space width, height, and overall temperature difference. Carefully controlled natural convection conditions were provided on the warm side, with forced convection on the cold side. Results were also obtained for the average surface to surface thermal conductance of each configuration. Under winter conditions, with natural convection on the warm side, basic double windows exhibit a significant vertical temperature gradient on the inside surface, with lowest temperatures at the bottom. The inside surface heattransfer coefficient for natural convection was found to be dependent uponthe distance from the top edge of the window.