Blower Door Testing - Special features of Large Buildings (q50-value)

As Passive House buildings get larger, there is a requirement not just to test for air changes per hour (n50) but also permeability (q50). The larger the building, the easier it is to reach the n50 of 0.6 ACH. In fact, large buildings with an n50 of 0.6 ACH could still have a lot of large leaks which could cause heat loss, discomfort and structural damage. This article distinguishes between n50 and q50 including upper limits for both.

Blower Door Testing - Special features of Large Buildings (q50-value)

In large buildings, an additional calculation based on the envelope area (q50-value) is required. The n50-value alone is no longer significant because of the more favourable EA/Vol ratio (surface area to volume ratio), therefore the permeability of the building envelope (determined in accordance with EN 13829 [ISO 9972]) is a mandatory parameter for Passive Houses certification.

Buildings with an air volume Vn50 ≥ 4,000 m3 [141,250 ft3] are defined as large buildings.

For such large buildings, both values, i.e. the n50-value and the q50-value, need to be indicated in the measurement report.


≥ 4,000 m3

[≥ 141,250 ft3]

n50 and q50 required


Instructions regarding the calculation procedure for the envelope area in accordance with EN 13829 [ISO 9972]:
The envelope area is the total area of all floors, walls and ceilings enclosing the volume under consideration, including all walls and floors that are below ground level. Calculations are based on interior dimensions from inner edge; the facing areas of integrated interior walls are not deducted. See Figure 1 below.

In deviation from EN 13829 [ISO 9972], the envelope area used in the PHPP can also be used to simplify the procedure. The exterior dimensions used in the PHPP only lead to negligible differences.

Figure 1: Interior dimensions from inner edge for calculation of the envelope area (according to EN 13829 [ISO 9972])

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In terraced houses the partition walls of the building also count towards the envelope area; this also applies to all floors, walls and ceilings adjoining neighbouring apartments in apartment blocks. These areas must only be taken into account if each accommodation unit is measured separately. Terraced houses and apartment blocks

Recommended target value for the q50 requirement

An n50-value less than or equal to 0.6 ACH has been set as a limit value for Passive Houses certification.
The requirements for the envelope area of smaller buildings can be used as a reference for the target value for larger buildings (≥ 4.000 m3 [141,250 ft3]), resulting in a target value of:

q50 ≤ 0.6 m3/(h.m2) [2.132 ft3/(h.ft2) or 0.0355 CFM/ft2]

Numerous measurements in large and very large buildings have shown that this target (or even lower values) can be achieved by applying appropriate airtightness concepts. Higher requirements may apply for special types of buildings (e.g. swimming pool).

Note: In smaller buildings, more meticulous airtightness measures may be necessary in order to meet the n50 < 0.6 ACH requirement. Compliance with the q50 criterion alone is thus not sufficient.


n50 ≤ 0.6 ACH

Target value q50

The q50 alone is insufficient




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