How is the building airtightness quantified?

The airtightness of a building is quantified by means of the measured air leakage rate through the building’s envelope in m³/h at a given reference pressure difference, often 50 Pa. The building airtightness may be expressed by other quantities, derived from the air leakage rate at a reference pressure difference, and normalised using, for example, one of the following measures for the size of the building:

• Internal building volume V. At 50 Pa, the associated indicator is called air change rate at 50 Pa and noted n₅₀ [unit h^-1].
• Envelope area A_E. At 50 Pa, the associated indicator is called air permeability at 50 Pa and noted q_E50 (former: q₅₀ in EN 13829) [unit m³/(h.m²⁾[.
• Net floor area A_F. At 50 Pa, the associated indicator is called specific leakage rate and noted q_F50, (former: w₅₀ in EN 13829) [unit m³/(h.m²)].

Other indicators of airtightness exist in national regulations.