The airtightness measurements of the system buildings are very important. They allow, together with the infrared tests, to verify all undesirable system joint leaks, which significantly increase the heating energy needs. In the article the results of airtightness measurements of two flats, built in system W-70, were presented. One of the buildings is after thermal modernization. During the leakage tests the system joints were monitored with the infrared camera. Thermograms, presented in the paper, confirmed the assumption about the negative influence of joints on the total building airtightness. Described tests are a pilot studies of the problem and provide the starting point to the further measurements on the statistical level.