Air tightness testing to Passivhaus standards differs from the normal approach in terms of rigour and frequency of testing required.

What is Passivhaus?

Passivhaus is a building standard for energy efficiency that reduces a building’s ecological footprint. This results in ultra-low-energy buildings that require little energy for space heating or cooling. Their purpose is to provide a high level of thermal comfort with minimal environmental impact and extremely low energy costs.

Developed in Germany in the 1990s by a Swedish professor, Passivhaus design is based on well-researched and proven building physics. It is a quality-assured methodology for designing low-energy buildings. It can help create buildings that use around 90% less energy than standard practice for UK new builds. 

Passivhauses are built with meticulous attention to detail and a rigorous approach to design and construction. Each building is certified through an exacting quality assurance process, according to principles developed by the Passivhaus Institute in Germany.

The definition of Passivhaus is driven by air quality and comfort:

“A Passivhaus is a building in which thermal comfort can be achieved solely by post-heating or post-cooling the fresh air flow required for a good indoor air quality, without the need for additional recirculation of air.”

Passivhaus Institute (PHI)

However, Passivhaus certification is also possible for exceptionally low-energy retrofit projects. EnerPHit, for example, is a slightly relaxed standard for retrofit projects. It is used when the existing architecture and conservation issues mean that meeting the Passivhaus standard is not feasible.

Key features of a Passivhaus include:

Passivhaus Standards & Requirements

What is an Air Test?

An Air Test (otherwise known as Air Tightness Testing; Air Permeability Testing; Air Leakage Testing and Air Pressure testing) is used to determine the total conditioned air lost through leaks in a building’s fabric, known as its envelope. All new developments have been required to have an Air Test as stated in Part L of the building regulations, since 2006. The final Air Test is always completed towards the end of the construction process and before the occupier has moved in to meet Building Regulations.

How to Air Test a Passivhaus

Just as is the case for normal new builds, it is common for low-energy buildings to be tested several times during construction to make sure that they are on track. The stages at which they are commonly tested are: 

Air Testing Methods

Passivhaus projects are tested using Method A rather than B. Method B is the most common practice within the UK. It measures ‘the volume of air passing through the fan per hour to create an imposed pressure differential of 50 Pascals (Pa) which is then divided by the envelope area in square meters’.  This results in an Air Permeability measured in m3/h.m2 @ 50 Pa. Test Method A is the most common practice across Europe. It instead expresses Air Leakage as Air Changes Per Hour. This relates to the volume of the building as opposed to its envelope area. It measures ‘The volume of air passing through the fan per hour in cubic meters to create an imposed pressure differential of 50 Pa divided by the volume of the building’. This is expressed as m3/h.m3 @50 Pa.

Valid Testing Steps

The sequence of steps required to undertake a valid test is as follows:

  1. Pre-test environmental readings
    • This includes Barometric Pressure, Internal Temperature, and External Temperature (the difference between which should be no greater than 10oC). Wind speeds do not need to be recorded but they do need to be less than 12 mph. 
  2. Start zero flow pressure differences
    • 10 base readings need to be taken at 3-second intervals with the fan fully covered.  
  3. Building and flow pressure readings 
    • 10 pressure readings need to be taken with a maximum interval of 10 Pa. After each change of fan speed wait 20 seconds for the pressure to stabilise. It is also best practice to take 5 or 10-second times averages for each reading to maximise accuracy. 
  4. Post-test zero flow pressure differences 
  5. Post-test environmental readings 

When testing a Passivhaus, both a pressurisation test and a depressurisation test are required. So, once the first test has been completed turn the fan around, reposition the tubing as required, and undertake a test in the other direction. The final air changes per hour are the mean average of the depressurisation and pressurisation test of the low-energy building. For example, if the depressurisation test achieved a score of 0.5 m3/h.m3 @50 Pa and the pressurisation a score of 0.4 m3/h.m3 @50 Pa then the final figure would be 0.45 m3/h.m3 @50 Pa.

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