How to Achieve 0.6 ACH: Hitting the Passivhaus Airtightness Target
0.6 air changes per hour. That’s the number that decides whether your project gets Passivhaus certification — measured with the building pressurised to 50 Pascals, the whole envelope leaking no more than 60% of its air volume per hour. For context, a typical UK new build leaks five to ten times that. Hitting 0.6 isn’t a product choice; it’s a process. Here’s the process.
1. Win it at design stage
Every air test is won or lost before anyone opens a tin. Trace the airtight line around every drawing section — it must be continuous, and someone must own every junction it crosses. Joist ends, window reveals, service penetrations, material changes: if the drawings don’t say what seals them, site guesswork will. (This is exactly why we mark up drawings free — cheaper to fix a detail on paper than through a plasterboard wall.)
2. Use a system, not a product
No single product seals a building. The projects that land well below 0.6 use the right tool at each part of the envelope:
- Bulk walls and ceilings — a liquid airtight membrane like Passive Purple, sprayed or rollered into one seamless coat. On blockwork it does the job a wet parge coat does, faster and flexibly — see our liquid membrane vs parge coating comparison.
- Frames and board joints — IM Fleece Tape, plasterable and vapour-open.
- Junctions and penetrations — brush-grade membrane worked into corners and joist ends, plus Passivhaus airtight sealant for gaps and movement joints.
3. Protect the line through the build
The airtight layer usually goes in perfectly — then follow-on trades put a hundred holes in it. Brief every trade on where the line is, make penetrations bookable rather than improvised, and seal as you go with the brush grade. The purple (or white) finish makes inspection visual: uncoated means unsealed.
4. Test before it’s too late to fix
Never let the certification test be the first test. Smoke-test suspect details as you seal, and run a pre-completion air test while the airtight layer is still accessible. Diagnostic testing during the build is how our projects arrive at the final test with nothing left to find.
What good looks like
Done this way, 0.6 stops being a ceiling and becomes a floor: 0.078 ACH on a £44m Passivhaus school — our best result on a school, recorded on a McLaughlin & Harvey project by EDP Building Sciences to ATTMA TSL4 in November 2025. That project ran on our guaranteed 0.6 ACH or lower supply-and-install service: we spec it, we install it, we stand behind the number.
Got a target you can’t afford to miss? Talk to us — or send us your drawings and we’ll trace the airtight line for free.