This is the uncontrolled flow of air into a space through adventitious or unintentional gaps and cracks in the building envelope. The corresponding loss of air from an enclosed space is termed 'exfiltration'. The rate of air infiltration is dependent on the porosity of the building shell and the magnitude of the natural driving forces of wind and temperature. Vents and other openings incorporated into a building as part of ventilation design can also become routes for unintentional air flow when the pressures acting across such openings are dominated by weather conditions rather than intentionally (e.g. mechanically) induced driving forces. Air infiltration not only adds to the quantity of air entering the building but may also distort the intended air flow pattern to the detriment of overall indoor air quality and comfort. Although the magnitude of air infiltration can be considerable, it is frequently ignored by the designer. The consequences are inferior performance, excessive energy consumption, an inability to provide adequate heating (or cooling) and drastically impaired performance from heat recovery devices. Some Countries have introduced air-tightness Standards to limit infiltration losses, these are reviewed in AIVC technical note 55 (A review of international ventilation, airtightness, thermal insulation and indoor air quality criteria).
Other air losses include duct leakage, which is the leakage of air from the seams and joints of ventilation, heating and air conditioning circulation ducts. These losses can be substantial, when, as is common, such ducting passes through unconditioned spaces, significant energy loss may occur. Some estimates have suggested this could be as much as 20% of the heat from typical North American domestic warm air heating systems can be lost through duct leakage. Pollutants may also be drawn into the building through these openings. As a consequence, considerable research and development into the performance of duct sealing measures is being undertaken.