Ventilation in a building enables to renew the air it contains by means of a natural exchange of air (depending on weather conditions and climate) or a forced exchange using mechanical appliances. This exchange of air must range between minimum air purity and maximum economical limit of dispersion (ventilating means cooling) without causing currents of air, unbearable for the people in the room, which would worsen thermal comfort. It is difficult to envisage the exchange because it depends on numerous variables: - airtightness of the building; - difference between internal and external temperature; - speed and direction of the wind which influences the difference between internal and external pressure. The studies for the Energy Project at the ICITE the Central Institute for Building Industrialization and Technology of the National Research Council have enabled the development of a method to measure air exchange in homes with the aid of a gas electromechanical analyzer which examines the decreasing curve, in relation to time, of the gas concen~ration with which the home was saturated. In this document the authors describe the method used and the results obtained by using two different types of gas: water vapour and oxygen. The use of oxygen has given the most interesting results which have enabled significant statistical analysis repeatable with any type of climate. The method used resulted effective although at present there is insufficient data to define the accuracy (estimated around 10%). Experiments were carried out on a modular prefabricated housing unit built with highly insulating material complete with forced ventilation.