Ventilation rates in two test home were measured using helium as a tracer gas. Pressure differences across the exterior walls of the house were recorded using pressure taps. Gives results for air infiltration tests and the calculated air infiltration. Finds that during summer air infiltration rates varied approximately linearly with wind velocity. During the winter, the pattern and extent of air infiltration were influenced by both house stack action and furnace operation. Stack effect alone was determined for one house and air change rate found to vary linearly with the square root of the inside to outside temperature difference. Also finds that wind and temperature effects are not independent.