Airtightness strategy and testing.
The 0.6 ACH at 50 Pa target is a continuity problem, not a product you buy. We draw the airtightness line through every junction, coordinate it across every trade, and prove it on a blower door before the finishes close it in. Senan House, a building we designed, came in at 0.15 ACH.
What airtightness strategy delivers.
What it isThe airtightness line is the single continuous barrier wrapped around the heated volume, the layer that stops warm air leaking out and cold draughts pulling in. Passivhaus airtightness asks it to hold 0.6 ACH at 50 Pa on a blower door test, and Passivhaus Premium asks for tighter still. That result is decided long before test day. It is decided by drawing the line on every plan, every section and every junction, and naming the material that carries it through each element. A gap on paper becomes a gap in the building, so the line is unbroken on the drawings first.
The job has two halves and we own both. First we design the line, specifying where it sits in each assembly and how it transfers between elements, then coordinate it across the trades who build it. Then we prove it: a diagnostic blower door test while the layer is still open, smoke and thermal diagnostics to find every leak, and the pre-completion test that goes into the certification evidence pack. Defects are caught before handover, not chased afterwards. The number on the certificate is the end of a process we control. Senan House, a Passivhaus we designed, tested at 0.15 ACH at 50 Pa, a quarter of the limit.
Passivhaus requires 0.6 ACH at 50 Pa, Premium tighter still. EnerPHit deep retrofit is typically held to 1.0 ACH at 50 Pa, because the line has to be threaded through existing fabric. Every figure is measured on a blower door, never estimated.
Testing is carried out to EN ISO 9972, the international method for measuring air permeability by fan pressurisation. The result is recorded and packaged for PHI certification alongside the rest of the fabric evidence, including the thermal bridge catalogue.
The limit is 0.6. We have hit 0.15.
Passivhaus airtightness draws a hard line at 0.6 air changes per hour at 50 Pa. Most projects aim to clear it. Senan House, a Passive House Mosart designed, tested at 0.15 ACH, a quarter of the limit and far inside it. That gap is the difference between an airtightness line drawn through the design and one improvised on site.
What you get.
DeliverablesFive things land on your desk, from the strategy drawing set through to the signed-off test result. Each one is built to go straight into the PHI evidence pack.
The continuous airtightness line drawn as one unbroken layer through every plan, section and junction, with the carrier material named for each element: plaster, membrane, board or slab.
Annotated details for every transfer where the line moves from one element to the next. Penetrations for services are designed in and sealed by detail, not left to improvise on site.
A staged plan of blower door tests to EN ISO 9972: an early diagnostic test while the layer is open, then the pre-completion test that proves the target for certification.
With the building under pressure, every leak located by smoke, an anemometer at suspect junctions and thermography in cold conditions, then marked up for the trade that owns it.
The final ACH at 50 Pa result, the test conditions, the remediation record and the retest, compiled and ready for the certifier. The number on the certificate, evidenced.
Method: how we work.
MethodThe airtightness line goes onto every plan, section and junction as one unbroken layer, with the carrier named for each element: plaster, membrane, board or slab. At each junction we show exactly where the line transfers from one element to the next, because the transfers are where airtightness is won or lost. Trace it with a pencil and the pen never lifts. If it does, the detail goes back before it is built.
The line is only as good as the interfaces between the people who build it. We coordinate it across the structure, the services routes and every trade, so the electrician, the plumber and the ventilation contractor know where the line runs and how to cross it without breaking it. Every penetration is designed in and sealed by detail. The membrane installer works to the same drawing as the certifier reads.
We run an early blower door test before the finishes close the layer in. That is the one moment a leak is cheap to fix. With the building under pressure we trace each leakage path using smoke, an anemometer at suspect junctions and thermography in cold conditions, then mark it for the trade that owns it. A failed junction caught now saves opening a finished wall later. EnerPHit retrofits start here too, with the same test setting the baseline.
The pre-completion blower door test to EN ISO 9972 confirms the building has hit its target. Where the diagnostic flagged work, the remediation and the retest are documented so the result is traceable. The final ACH at 50 Pa figure, the test conditions and the remediation record go into the evidence pack for PHI certification. Pulse confirms in-use performance once the building is occupied.
Convert your airtightness figures.
n50, q50 and air permeability are three readings of the same blower door test. The converter lines them up in seconds, the way every PHPP file and certifier needs them.
Common questions.
FAQWhat is the airtightness line and why draw it on the drawings?
The airtightness line is the single continuous barrier that wraps the heated volume and stops uncontrolled air leaking through the fabric. Run a pencil around every plan and section and you should be able to trace it without lifting the pen. We draw it as a coloured line on every plan, section and junction detail, and we name the material that carries it through each element: the wet plaster coat on a masonry wall, the membrane in a timber frame, the poured slab below ground. A gap on the drawing becomes a gap in the building. So the line is resolved on paper before anyone builds it.
What counts as a good air tightness test result?
Passivhaus certification requires 0.6 air changes per hour at 50 Pa (0.6 ACH at 50 Pa) on a blower door test. Passivhaus Premium is tighter still. EnerPHit deep retrofit is typically held to 1.0 ACH at 50 Pa, because an existing structure is harder to seal. A new build to baseline regulations often tests near 5 ACH at 50 Pa, so the Passive House figure is close to ten times tighter. Numbers below the limit are normal once the line is designed: Senan House, a building Mosart designed, tested at 0.15 ACH at 50 Pa, a quarter of the certification limit. That comes from one continuous line, protected through construction, not from sealant at the end.
What is the difference between ACH at 50 Pa, q50 and air permeability?
They are three ways of normalising the same blower door measurement. ACH at 50 Pa (n50) divides the measured leakage airflow by the internal heated volume and is the figure Passivhaus and EnerPHit are set against. Air permeability (q50, in cubic metres per hour per square metre at 50 Pa) divides the same airflow by the envelope area instead, and is the figure UK Building Regulations Part L is written around. The two do not convert by a fixed factor, because volume and surface area differ from one building to the next. We report whichever the certifier or the regulation requires, and the converter lines all three up for you.
Can you test and find leaks in an existing or part-built building?
Yes. We run diagnostic blower door tests while the airtightness layer is still accessible, the point where a leak is cheap to fix. With the building held under pressure we trace each leakage path using smoke, an anemometer at suspect junctions and thermography in cold conditions, then mark it for the trade that owns it. On an existing building the same test sets a baseline and shows where an EnerPHit strategy has to concentrate, because in a deep retrofit the line is threaded through fabric that is already there. Pulse, our post-occupancy monitoring, confirms the fabric is performing once the building is in use.
Do we need an air tightness test for Passivhaus or building regulations?
Yes, on both counts. For Passivhaus and EnerPHit certification a blower door test is mandatory: the result goes into the evidence pack for the certifier, and the building has to meet the target with no exceptions. Building regulations in Ireland and the UK also require a pressure test for compliance, though the permitted figure is far looser than the Passive House standard. We test to EN ISO 9972 for both, and where a project is chasing certification we run an early diagnostic test so the final pre-completion result confirms what we already measured rather than gambling on it.
Get the line right before you build it.
Convert your n50, q50 and air permeability figures in seconds, or talk to us about the airtightness strategy for your project. Airtightness sits alongside thermal bridge analysis and the work of the Passivhaus designer, through to certification.