Describes experimental studies of the natural ventilation of four similar houses with different ventilating systems. Describes houses and gives experimental procedure and results of measurements of air-change-rates using hydrogen as a tracer gas.Shows variation in air-change-rates are due mainly to changes in wind speed and that wind direction and temperature difference are secondary factors. Estimates rate of heat loss as a functionof wind speed. Discusses relationship between measured pressure differences and wind speed and direction. Estimates rate of airflow through windows for a known pressure.