The bachelor thesis is aimed at acquiring and summarizing knowledge about the airtightness of the building envelope. The theoretical part describes ventilation and methods of ventilation, infiltration and its influence on the indoor environment of buildings, lifetime of construction, energy demand of the building during operation. The principle of continuos airtight layer, airtight construction and materials commonly used materials are presented. Description and principle of measurement of air tightness, the most common errors and leakages as they arise and the reasons for their occurrence are discussed. The practical part deals with the calculation of heating demand for a standard family house, and the effect of the air-tightness of an external envelope on the total energy demand for heating is then analyzed.