3. Airtightness of buildings

The airtightness of a building corresponds to the capacity of the building envelope to let air through by parasitic infiltration when there is a pressure difference between the inside and outside of the building. This pressure difference may be caused by wind, by a temperature difference between the inside and outside of the building, or by a mechanical ventilation system.

Good airtightness is essential to guarantee the effective energy performance of buildings. The existence of holes in the envelope generates uncontrolled through-air flows, in addition to voluntary air renewal through ventilation.

For single-family homes, semi-detached houses and apartment buildings, the air permeability of the envelope is characterized by a coefficient called "Q4Pa-surf". This represents the leakage rate per m ² ...