Poor building airtightness results in excessive air infiltration and resultant uncontrolled energy loss. The energy use increase is dependent on the infiltration flow rate and the amount of conditioning of the air that is necessary to achieve thermal comfort. The infiltration flow rate depends on the building airtightness and natural driving forces by wind and stack effect. For the same building airtightness, the energy impact is larger when the building is more exposed to high wind speeds and large temperature differences, and when the building is higher.

As an approximation, an increase of the air permeability of the building envelope with 1 m³/(h.m²)@50Pa has a similar energy impact as an increase of the average building envelope thermal transmittance (U-value) with 0.02 W/(m²K).