A building air tightness test ("Blower Door test") is an essential method for assessing construction quality and energy efficiency. It measures uncontrolled air leakage through the building envelope. The test is especially important before purchasing an existing property to identify potential heat losses and hidden defects. It is also performed after commissioning to verify compliance with design requirements. Air tightness testing is highly recommended during construction works, when deficiencies can still be corrected efficiently. Common issues are often found around window and external door sealing areas. Early detection helps improve construction quality and prevent future problems. The test provides valuable data for reducing energy consumption and improving indoor comfort. This assessment supports informed decisions about long-term building performance and maintenance costs.