Air infiltration adds to the quantity of air entering the building but may also distort the intended ventilation air flow pattern to the detriment of overall indoor air quality and comfort. The consequences are inferior performance, excessive energy consumption, and inability to provide adequate heating (or cooling) .
Because air infiltration is uncontrolled, poor envelope airtightness may affect:
- Indoor air quality: Some rooms may be largely under-ventilated while other are over-ventilated as a consequence of distorted air flow patterns. Also the air circulating in the wall may bring inside pollutants from outside and from building product emissions.
- Energy use: Air leakage may inadequately increase the total ventilation airflow rate; or it may decrease the relative impact of heat recovery (in case of systems with heat recovery devices, the unit will only recover heat on the airflow passing through it).
- Moisture performance: air leaking through the envelope from a warm, humid environment to a cold environment, may cause condensation damage as it flows along materials with a temperature below its dew point. In cold climates condensation damage may occur in materials at the outside of the thermal insulation when air exfiltrates, while in warm climates damage may occur at the inside of the insulation when air infiltrates.
- Acoustic performance: airborne sound may propagate through leakages. It is found that sound transmission loss can be degraded by up to 15 dB under field conditions, mainly because of sound leaks.
 Liddament M.W., 1996. A Guide to Energy Efficient Ventilation. AIVC, 1996.