Blower doors, airtightness & the Passive House myth
What blower‑door results mean in real life for your client’s noise, drafts and power bills – and why it’s not a Passive House just because you made it airtight.
Airtightness has become the new flex.
Everywhere you look it’s:
“Our builder tapes everything”
“We got 3 ACH50, is that good?”
“We hit 0.6, so it’s basically a Passive House now, right?”
Short answer: no, it’s not a Passive House just because you make it airtight.
Is airtightness important? Absolutely. Is it the whole story? Not even close.
We’ve tested hundreds of homes that were sold as “high performance” and “almost passive,” then whistled like flutes once the blower door went on. We’ve also tested genuinely tight homes that still felt average inside because the rest of the envelope was phoned in.
Let’s turn the numbers into something you – and your clients – can feel.
What the ACH50 numbers roughly mean in real life
The specifics will vary by climate and construction, but as a rough translation:
10+ ACH50 – “Leaky” territory
3–5 ACH50 – “Decent new build”
≤1 ACH50 – “High‑performance” territory
For the client, the number is not the point. What they care about:
Do I feel drafts?
Is it quiet inside?
Are my bills reasonable?
Your job is to connect the dots.
Airtightness is not a personality, or a certification
Airtightness does three main things well:
Reduces drafts and cold air sneaking in wherever it feels like it
Makes your insulation work like insulation, not an expensive filter
Gives you control over where and how air moves (so ventilation systems actually function as designed)
But airtightness does not:
Compensate for terrible thermal bridges
Magically fix under‑spec’d glazing or non‑existent shading
Turn a random design into a Passive House
Passive House (done properly) is a whole‑building standard – airtightness is just one part. There’s also minimum insulation, window performance, thermal bridge control, ventilation, overheating risk… the boring bits that don’t fit on a sticker.
So if a project hits 0.6 ACH50 but still has massive thermal bridges and minimal shading, call it what it is: a tight house with other issues. Good, but not magic.
Leak locations architects should care about
You don’t need to become a blower‑door tech, but you should know where leaks love to hide:
Around recessed lights and downlights
At top plates and junctions with roof structure
Around window frames where tapes and flashings “kind of, sort of” meet
At service penetrations (pipes, cables, ducts) that got drilled after the airtight layer was installed
Internal garage doors and loft hatches
Every time we run a blower‑door test, we end up with a bunch of blue tape flags at these spots. The houses that perform best aren’t necessarily the ones with the fanciest products; they’re the ones where someone actually cared about the continuity of the airtight layer.
After 15+ years of holding smoke pencils up to “airtight” envelopes and watching the smoke disappear, I’m convinced: draw the airtight layer like a continuous line, then actually build that line. Don’t let it vanish behind marketing.
How to talk about results with clients
Instead of emailing the client a test report and hoping they don’t ask what ACH50 is, try this:
“We tested your home and got about X ACH50. That puts you in the [leaky / decent new build / high‑performance] category.”
Then translate:
Drafts – “On windy days you’ll [notice / barely notice] air moving around windows and doors.”
Noise – “You’ll find outside noise is [about what you’re used to / noticeably reduced / a big step down].”
Bills – “Your heating will need to work [quite hard / moderately / surprisingly little] to keep you comfortable.”
If the number was worse than hoped, don’t panic. Be honest:
“We missed our target a bit here. The good news is we know where the main leaks are, and we can [fix some / live with it]. It’ll still be better than a typical house, but not as quiet/even as the very tightest homes we’ve worked on.”
That’s Endless‑Customers‑101: tell the truth, even when it’s slightly uncomfortable. People remember that.
When to chase 0.6 – and when not to
Going from 10 ACH50 to 5 is huge. From 5 to 3? Still very helpful. From 3 to 1? Excellent, but now you’re in the land of diminishing returns and tighter tolerance on everything else (ventilation, moisture, detailing).
If your clients:
Want serious performance
Are happy to invest in good ventilation
Have a builder who actually wants to do this properly
…then yes, aim low. 1 or below is worth chasing.
If they’re:
Doing a tight budget build
Not willing to pay for balanced ventilation
Working with a team who thinks tapes are “optional”
…maybe don’t die on the 0.6 hill. Aim for mid‑single digits, plan the airtight layer sensibly, and spend money on windows, shading and insulation too.
It’s about coherent performance, not hero numbers.
The “not a Passive House” mic drop
The line I use a lot:
“Airtightness is like having good skin. It’s important, but it doesn’t make you an Olympic athlete.”
You need the rest of the system:
Muscles = insulation and thermal bridge control
Lungs = ventilation
Training = orientation, shading, sensible design
We’ve tested houses that hit 0.6 ACH50 and still overheat every summer because they’re basically glass boxes with no shading. We’ve also seen 2–3 ACH50 houses with smart design and ventilation that feel fantastic all year.
So when someone says, “It’s basically a Passive House – look at the blower‑door number,” feel free to raise the eyebrow.
A builder told me last month, “We can tape a house like crazy if they pay us, but if they don’t put a decent ventilation system in, it just goes stuffy faster.” Exactly.
And no, it’s not a Passive House just because you taped the hell out of it.
