What is building airtightness?
Building airtightness (also called envelope airtightness) is a general descriptive term expressing the resistance to inward or outward air leakage through unintentional leakage points or areas in the building envelope. This air leakage is driven by differential pressures across the building envelope due to the combined effects of stack, external wind and mechanical ventilation systems . The smaller the air leakage rate at a given pressure difference across a building envelope, the greater the airtightness .
In standards  as well as publications for non-experts, building airtightness is also called air permeability of buildings.The terms however are not synonymous: the more airtight the envelope, the less permeable it is.
 G. Guyot, F. R. Carrié and P. Schild, “Project ASIEPI – Stimulation of good building and ductwork airtightness through EPBD,” 2010.
 ISO 9972:2015 Thermal performance of buildings - Determination of air permeability of buildings - Fan pressurization method