The subject of this thesis is testing of the air permeability of windows and furthermore, comparing the obtained results with the specified minimum values of air permeability according to Technical regulation on rational use of energy and heat retention in buildings. Tests were carried out on windows in residential building in Osijek. Air permeability of windows depends largely on the quality of windows and installation. Testing is carried out by using BlowerDoor device. In order to properly conduct testing it is necessary to follow the instructions provided by the device manufacturer and the HRN EN 12207. The results are shown for windows air permeability over the overall window area and related to the length of the opening joint. Also, there are diagrams presenting relationship between the pressure difference and air flow. Additionally, airtightness value in the apartment is measured using the decompression and compression method. Using the fan air is blown into the apartment and thus, the results of compression are obtained. When examining decompression, the fan blows air out of the apartement to the outside space. Obtained results are compared with the minimum values given by regulations and classified in the corresponding class.