Normalized air leakage rate q50

What is the q50 value and what does it measure?

The q50 value is the air leakage through a building envelope at a 50 pascal pressure difference, divided by the area of the envelope instead of the interior volume. It is written in m3/(h·m2), which reads as cubic metres of air per hour for each square metre of envelope. The Passive House criteria describe it as airtightness based on the envelope area. It comes from the same pressure test used for n50, the test vzduchotesnosti known in English as the blower door test.

The divisor is the building's outer skin, not the air it holds. q50 is one way to express airtightness, next to n50.

The name needs a note. The standard ISO 9972:2015 calls the area-based value air permeability at 50 Pa and writes it qE50, and q50 is its former name from EN 13829. The Passive House criteria still write q50. The Air Infiltration and Ventilation Centre (AIVC) lists the same units, m3/(h·m2) for qE50 and 1/h for n50, and gives no formula that links the two. This page therefore gives no conversion between them, because the conversion depends on the volume and the envelope area of each building.

How does q50 differ from n50?

Both values start from the same measured airflow at 50 Pa. The difference is the divisor. n50 divides by the air volume of the heated and cooled building, so its unit is 1/h. q50 divides by the envelope area, so its unit is m3/(h·m2). The Passive House criteria take the n50 air volume, called Vn50, room by room over the entire heated or cooled volume, multiplying floor area by the average clear height. A reduction factor applied to gross volume is not allowed, and the base area is not the Treated Floor Area used for energy calculations.

Metric	Divided by	Unit	Role in the Passive House criteria
n50	Air volume of the heated and cooled building (Vn50)	1/h	Binding limit: 0.6 1/h or lower
q50	Area of the building envelope	m3/(h·m2)	Recommended target of 0.6 m3/(h·m2) or lower for buildings of 4000 m3 or more

The practical consequence is scale. A larger building has more air volume for the same envelope area, so the same leakage through the envelope gives a lower n50. The joints may be no tighter, yet the number improves. The Passive House Institute recommends a q50 target for buildings of 4000 m3 or more.

Why do Passive House criteria use n50 but also mention q50?

The binding Passive House limit is set on n50. Table 1 of the criteria sets a pressurisation test result of n50 of 0.6 1/h or lower, and section 3.2.10 keeps that limit for large buildings as well. q50 appears only in the extended version of the criteria, as a recommendation. For buildings of 4000 m3 or more, the Passive House Institute recommends aiming for a maximum q50 of 0.6 m3/(h·m2).

The two values therefore do different jobs. n50 is the number that decides certification. q50 is a recommended target for large buildings. A designer who wants to compare buildings of different size can ask for both numbers in the test report.

What are the Passive House airtightness requirements?

The criteria set the limit, the test method and the exceptions in one document. The values below come from version 10c of the Passive House, EnerPHit and PHI Low Energy Building criteria, valid with PHPP version 10 and revised in January 2023.

Item	Value in the criteria	Section
Passive House limit	n50 of 0.6 1/h or lower	Table 1, section 2.1
Test method	ISO 9972 method 1, with positive and negative pressure measured and averaged	Section 3.2.10
Extensive leak detection	Required for EnerPHit and PHI Low Energy Building when n50 is between 0.6 and 1.0 1/h, and for pre-certification	Section 3.2.10
Cooling calculation assumption	n50 of 1.0 1/h instead of 0.6 1/h, for EnerPHit and PHI Low Energy Building	Section 1.3.2 (changes from version 10b)
Large-building q50 target	0.6 m3/(h·m2) recommended for buildings of 4000 m3 or more	Section 3.2.10, extended version

The pressure test must cover the total heated or cooled volume of the enclosing envelope. Basements, porches and conservatories that are not integrated into the thermal envelope are left out. The test report must document how the net air volume was calculated. The criteria also recommend testing while the airtight layer is still accessible, so that any leaks found can be repaired before finishes cover them.

Who is allowed to carry out the airtightness test?

The criteria bar two groups from testing their own work. The first is planners and tradespeople who were partly or fully responsible for building the airtight envelope. The second is the building owner, including close relatives and the employees of a company that the owner works for or majority-owns. Energy and PHPP consultants and the certifier may carry out the test, provided they belong to neither group.

Is the Passive House standard a legal requirement in Slovakia?

No. The Passive House criteria come from the Passive House Institute and are not a Slovak act. Slovak energy performance rules sit in zákon č. 555/2005 Z. z. and its implementing decree 364/2012 Z. z. The consolidated text of the decree in force from 10 March 2020 contains no n50 or q50 value. This page does not cover the Slovak standard STN 73 0540-2 and states no Slovak airtightness target. For a Slovak design brief, a practical sequence is to agree the n50 test first, then ask for q50 on large buildings, and to check that the report documents the net air volume.