Air permeability pressure testing and passivhaus n50
To comply with UK non-domestic Building Regulations (Part L2) it is necessary to carry out an air permeability pressure testing on all buildings with a usable floor area > 500 m2. Whilst for the domestic regulations (Part L1) a representative sample of dwellings on each new build site must be tested. The result of this test is expressed as the AP50 value. This is the measure of a volume of air (m3) that flows through the building fabric (m2) every hour at a reference pressure of 50 Pascals (Pa) – hence the AP 50 units are m3/(h.m2).
In order to calculate the air permeability the ATTMA (Air Tightness Testing and Measurement Association) TS1 standard defines the envelope air barrier line as being ‘along the line of the component to be relied upon for air sealing’. In practice this could be anywhere within the building envelope but typically measurements are taken up to the internal wall surface. The ATTMA AP50 test result is commonly referred to as the ‘air permeability’ however technically it is defined as the rate of air flow (m3/h) at a pressure difference of 50Pa (which is analogous to the
v50 referred to in Passivhaus terminology).
The 2010 Building Regulations state that the airline and the thermal line of a building should be contiguous. It is worth noting that a building regulations test can be either a pressurisation or depressurisation test, but for Passivhaus both tests with the final result being the average of the two. The actual testing procedure is largely identical however and both the Building Regulations and Passivhaus compliance tests may be carried out at the same time.
Passivhaus n50 in m3/m3.h or h-1 or ach/h
For Passivhaus certification the airtightness target is expressed differently as n50, which is defined as the number of air changes per hour in the building at a reference pressure differential of 50 Pascals. Since the result is calculated using the building’s internal air volume (m3), rather than using its envelope area (m2) the n50 units are expressed as m3/m3.h which may be simplified to h-1 or ach/h.
Note: a standardised conversion between Air Permeability (AP50) and Air Change Rate (n50) values is not possible as they do not have a direct relationship with one another. Furthermore each test uses differentmeasurement and testing protocols.
Excerpt from “Passivhaus primer: Airtightness Guide” by BRE