Air tightness testing to Passivhaus standards differs from a standard Building Regulations test in both rigour and frequency – a Passivhaus build is typically tested at three separate stages, against a threshold roughly ten times tighter than the Part L backstop.
In this blog you’ll learn:
- What Passivhaus is, and how the Passive House Institute’s certification tiers differ
- The airtightness standard your build needs to hit for certification
- Why Passivhaus air testing differs from a standard Part L air test
- The methods and stages used to test a Passivhaus during construction
What is Passivhaus?
Passivhaus is a building standard for energy efficiency, developed in Germany from 1988 by physicist Wolfgang Feist working with Swedish professor Bo Adamson, who formulated the standard’s foundational concepts through collaborative research. Construction on the world’s first Passivhaus – a four-unit housing project in Darmstadt-Kranichstein, Germany – began in 1990 and completed in 1991. By pairing continuous insulation and extreme airtightness with mechanical ventilation heat recovery (MVHR), Passivhaus buildings drastically reduce heating and cooling energy needs while maintaining indoor air quality and consistent thermal comfort.
Passivhaus buildings are certified through the Passive House Institute (PHI)‘s quality-assured process. In the UK, the Passivhaus Trust (PHT) is PHI’s official affiliate body, and our testing aligns with the standards PHT promotes nationally.
PHI defines Passivhaus 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)
PHI’s certification tiers are:
- Passivhaus Classic: the foundational level, emphasising airtight construction and insulation, and minimal reliance on heating/cooling systems.
- Passivhaus Plus: goes beyond Classic by adding renewable energy generation, allowing the home to produce as much energy as it consumes.
- Passivhaus Premium: the most demanding level, requiring the home to generate significantly more energy than it uses, with the potential to feed excess energy back into the grid.
Certification is also possible for exceptionally low-energy retrofit projects, via EnerPHit – a slightly relaxed standard used where existing architecture or conservation constraints rule out full Passivhaus compliance.
Key features of a Passivhaus include:
- Super insulation
- Stringent levels of airtightness
- Minimal thermal bridging
- Optimisation of passive solar gain
- Mechanical ventilation with heat recovery
- A simple, compact shape
Passivhaus standards and requirements:
- Annual heating and cooling demand of no more than 15 kWh/m²/a
- Or a peak heat load of 10 W/m² as calculated in the Passivhaus Planning Package (PHPP)
- Total primary energy consumption of no more than 60 kWh/m²/a as calculated in PHPP
- Air changes per hour no greater than 0.6 m³/h·m³ @50Pa
- Summer overheating of less than 10% of hours
What is an Air Test?
An air test measures the total conditioned air lost through leaks in a building’s envelope. Part L of the Building Regulations has required an air test on new developments since 2006, carried out towards the end of construction and before occupation.
How to Air Test a Passivhaus
Because the standard is so much tighter, Passivhaus buildings are commonly tested at three stages during construction, rather than once at completion:
- Stage 1: once the initial air barrier sealing works are complete
- Stage 2: once secondary fix works are complete
- Stage 3: at completion
This diagnostic-then-final approach is what we used on a bespoke steel-frame Passivhaus Premium build in the North Cotswolds, featured on Channel 4’s Grand Designs – testing ahead of completion to catch any remaining leakage paths, then returning for the final ATTMA-certified test.
Passivhaus Air Testing Methods
Passivhaus projects are tested using Method A, rather than the Method B used for standard UK Building Regulations testing:
- Method A (standard European practice, used for Passivhaus) measures the same airflow but divides it by the building’s volume rather than its envelope area – expressed as air changes per hour, m³/h·m³ @50Pa.
- Method B (standard UK practice) measures the volume of air passing through the fan per hour to create a 50 Pascal pressure differential, divided by the building’s envelope area – expressed as air permeability in m³/h·m² @50Pa.
Valid Passivhaus Air Testing Steps
A valid test follows this sequence:
- Pre-test environmental readings – barometric pressure, internal and external temperature (the difference between which should be no greater than 10°C), with wind speed under 12 mph.
- Zero-flow baseline readings – 10 readings at 3-second intervals with the fan fully covered.
- Building and flow pressure readings – 10 readings at a maximum interval of 10 Pa, waiting 20 seconds after each fan speed change for pressure to stabilise.
- Post-test zero-flow readings.
- Post-test environmental readings.
Both a pressurisation and a depressurisation test are required. The building is tested in one direction, the fan is then reversed and repositioned, and the test is repeated. The final air changes per hour figure is the mean of the two results – for example, a depressurisation result of 0.5 m³/h·m³ @50Pa and a pressurisation result of 0.4 m³/h·m³ @50Pa would give a final figure of 0.45 m³/h·m³ @50Pa.
What’s Next?
Passivhaus certification demands a level of precision a standard Building Regulations test doesn’t require – testing at multiple stages, to a tighter threshold, using a different calculation method entirely. Getting this right from the first stage test avoids costly surprises at final certification.
If you’re planning a Passivhaus or EnerPHit project, we’d be glad to talk through your testing programme.
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