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.
• Energy use: Air leakage may inadequately increase the total ventilation airflow rate; or it may not allow sufficient heat recovery (in case of a systems with heat recovery devices, the unit will only recover heat on the airflow passing through it).
• Building materials: Air leaking out of the envelope may cause condensation damage as its temperature drops below dew point.