**UK Methodology**

The UK standard measures Air Permeability, in m3/hr/m2@50Pa (the q_{50 }measurement), or in other words the air leakage per square metre of building envelope. The ATTMA (Air Tightness Testing and Measurement Association) TS1 standard defines the building envelope as everything within the air barrier line *‘along the line of the component to be relied upon for air sealing’*. This could be anywhere within the building envelope (even the external render). This is a measure of building envelope airtightness.

**Passivhaus Methodology**

The Passivhaus standard measures the Air Change Rate (ACH) @50Pa (the n_{50 }measurement), or in other words the number of times the volume of air within the building is changed in an hour. So, it is a purely volumetric measure. The Passivhaus methodology considers the volume of air which needs to be heated. Therefore internal walls and floors are excluded. This is a measure of air infiltration, and hence the heating energy cost of the building.

**Calculating the Passivhaus Air Test Volume**

A straightforward way to calculate the volume required for the Pressure Test is as follows:

- Start with the Treated Floor Area (TFA)
- Add the space occupied by any stairs (imagine that they do not exist and that a standard floor construction occupies the void)
- Any areas treated at 60% for TFA purposes should be treated at 100% for this volume calculation
- The total TFA for each floor can be multiplied by each floor to ceiling height (averaged if necessary for sloping ceilings) to give a volume for each storey (not including internal floors)
- The total for all the storeys equals the Pressure Test volume
- Divide the measured air flowrate, in m3/hr, by the Pressure Test Volume (m3) to get the air change rate, n50

**Note**

The two measures, Air Permeability and Air Change Rate, do not have a direct relationship with each other (so you can’t apply a conversion factor to one to get the other). Therefore the building needs to be measured to the right protocol, and the results calculated in the right way (n_{50 }not q_{50}), to get a result which can be used for certification. It is essential to use the correct volume for testing, even before the interior spaces are fully finished. Voids within wall and floor constructions **cannot** be counted.