Very low air leakage rates are required by the Passivhaus standard and must be demonstrated for each certified building by means of a “blower door” air tightness test. The air change rate must be less than or equal to 0.6 air changes per hour, under test conditions. Passivhaus airtightness (n50)≤0.6 h-1@ 50 Pa. Note: The limiting value of 0.6 h-1@ 50 Pa should be used as the default design airtightness value in all preliminary Passive House Planning Package (PHPP) calculations until a verified pressure test reading is available.
Heating demand is important
It is important to note that achieving a heating demand of 15kWh/m2 yr or a peak load of ≤ 10 W/m2 often requires the elemental specifications to be far better than the recommended Passivhaus limiting values. This is particularly the case for buildings that do not have an optimum form. Therefore the limiting value of n50 being 0.6 h-1@50 Pa should be seen as the maximum air leakage permissible. Attaining the Passivhaus standard of airtightness is challenging, but achievable when a clear and well thought out strategy is arrived at during the design stage.
This strategy must be maintained and fully implemented during the construction phases. It is therefore imperative that the contractors clearly understand the airtightness strategy and are able to easily implement the sequence of work on site. It is worth noting that in practice an airtightness level of n 50 = 0.6 h-1 @ 50 Pa is roughly equivalent to having a hole in the envelope area of the building less than the size of a 5 pence piece for every 5 m2 of building envelope. In comparison a building that achieves the limiting figure for airtightness to comply with the Building Regulations Part L (2013) (Section 6 of the Building Standards, Scotland and Building Regulations Part F, Northern Ireland) will have an equivalent hole the size of a 20 pence piece for every 1 m2 of envelope.
Excerpt from “Passivhaus primer: Airtightness Guide” by BRE