Six test buildings were extensively instrumented for measuring heating loads and indoor comfort and were exposed to a winter heating season and an intermediate heating season.

The test buildings were 20 ' 20 ft (6.1 ' 6.1 m) one-room buildings constructed at Gaithersburg, MD. The buildings had the same floor plan and orientation, and were identical, except for the wall construction, which was as follows: insulated lightweight wood frame; uninsulated lightweight wood frame; insulated masonry with outside mass; uninsulated masonry; log; and insulated masonry with inside mass. The construction of the buildings generally was selected to be representative of current practice.

During the winter season, when some space heating was supplied each hour of the test, measured heating loads were predicted with a steady-state heat-transfer model that did not include the effect of thermal mass, thereby indicating no thermal mass effect. The indoor comfort was not affected by wall mass.

During the intermediate heating season, when solar window and occupancy heat gains caused the indoor temperatures to float above the thermostat set temperatures during warm day periods, a significant thermal mass effect was observed. Heavyweight buildings were observed to consume less heating energy than comparable lightweight buildings having equivalent wall thermal resistance. The effect was greater when wall mass was positioned, inside as opposed to outside, the wall insulation. Wall mass was observed to reduce overheating considerably during warm day periods, thereby producing less variation in indoor comfort.