Measurements of the airtightness of blocks of flats in Lithuania were carried out between 1995 and 1997 to assess the effectiveness of energy saving measures. At that time there was no real data on the airtightness of such dwellings available. The aim of the measurements was to evaluate how much heat could be saved by diminishing the air change rate. Since there are no thermostats installed in the dwellings, 'tightening' becomes the only measure available to increase indoor temperature.