How is the building airtightness quantified?

The airtightness of a building is often expressed in terms of the leakage airflow rate through the building’s envelope at a given reference pressure (usually 50 Pa) divided by the:
– heated building volume V. At 50 Pa, it is called the air change rate at 50 Pa and noted n50.
– envelope area AE. At 50 Pa, it is called the air permeability at 50 Pa and noted q50.
– floor area AF. At 50 Pa, it is called the specific leakage rate and noted w50.

The lower the ‘airtightness’ value is, the more airtight the building’s envelope is.