This study deals with the ventilation performance of two storey dwellings employing stack effect ventilation to satisfy the overall ventilation requirement. The SRF (Supply Rate Fulfillment) index was used as a ventilation performance index. The experiments were implemented to measure effective fresh air rate by using tracer gas in a test house. Following are the results of the experiments and the theoretical calculations. The measured air change rate and the SRF value were fairly close to the theoretical calculation. The ventilation factors such as the ventilation rate, the effective fresh air supply rate and the SRF value were measured by using two tracer gases in transient conditions. The relationships among the equivalent leakage area of dwellings, the total ventilation rate and the overall SRF value are quantified on the basis of the measurements and theoretical calculations.