How to Pass a Blower Door Test
A good result is built, not tested for. Here's what actually determines your ACH50 — and how to get ahead of it.
There’s a difference between preparing for a blower door test and actually passing one. Preparation is about the day of the test — closed windows, turned-off exhaust fans. Passing is decided weeks or months earlier, in how the building’s air barrier was designed, sequenced and sealed during construction. This guide is about that earlier, more important part.
12 min read
Key takeaways
- A blower door result is a consequence of decisions made during design and construction — not something you can influence much on test day itself.
- Set a specific numeric target before construction starts, not after a disappointing result comes in.
- Think of the air barrier as one continuous surface around the whole building — a chain that's only as good as its weakest, unsealed link.
- A pre-lining test, before plasterboard goes up, is the single highest-leverage thing you can do to actually pass.
- A handful of recurring details — top plates, window reveals, service penetrations, downlights, exhaust ducting — account for most failed targets.
Jump to a section
1. Passing is decided early, not on the day
By the time the tester arrives for a final test, the result is largely locked in. Everything that determines a blower door result — how carefully the frame, membrane, windows and services were sealed — happened during construction, often months earlier and behind plasterboard that’s now impossible to see through. If you want a guide to what to do on test day itself, see preparing for a blower door test. This guide is about the much larger lever: what happens before that day.
2. Set your target before construction starts
You can’t aim for a number nobody has agreed on. Confirm which target applies to your project before the frame goes up:
- Passive House certification: ≤0.6 ACH50, fixed and non-negotiable.
- NCC 2022 ventilation trigger: 5 ACH50 — testing at or below this requires mechanical ventilation meeting a minimum calculated airflow rate.
- MVHR performance: ≤3 ACH50 is the widely recommended minimum for heat recovery ventilation to work as designed.
- Builder or contract specification: whatever number is written into your contract — check the exact wording, as “best endeavours” and “shall achieve” are very different commitments.
For the full breakdown of how targets work and what happens if you miss one, see Can you fail a blower door test?
3. Think of it as one continuous air barrier
The single most useful mental model for passing a blower door test: your building has one continuous air barrier that must wrap the entire conditioned volume — floor, walls, roof, every window and door, every service penetration — without a break. It only takes one unsealed gap to let air move freely, no matter how well everything else was done. For the full explanation of what this barrier consists of, see What makes a house airtight?
4. Sequencing and trade coordination
Airtightness usually fails not because any one trade did bad work, but because no single trade was told they owned it. The air barrier gets touched by framing, insulation, electrical, plumbing, window installation and plastering — often in that order, with each trade covering up the last one’s work. The fix is simple in principle: nominate someone (usually the builder or site supervisor) to own airtightness explicitly, brief every relevant trade on the specific details before they’re buried, and check the work before it disappears behind linings.
HiPer Haus field note
On the projects that hit tight targets most reliably — including certified Passive House builds we’ve supplied and commissioned MVHR for in South Australia — airtightness was treated as a named responsibility from the start, not an assumption that “someone” would handle it.
5. Test pre-lining, not just at the end
If there’s one action that most reliably improves your odds of passing, it’s this: book a blower door test before plasterboard goes up, while the air barrier is still exposed. A pre-lining test tells you exactly where leaks are while they cost almost nothing to fix — tape, sealant, a gasket. Waiting for the final test to find the same leaks means finding them behind finished surfaces, where remediation is far more disruptive and expensive.
Want leaks found while they’re still cheap to fix?
Book a pre-lining test →6. The details that decide the result
A relatively short list of recurring details accounts for most of the difference between a home that passes comfortably and one that misses its target:
Top plates & wall/ceiling junction
Tape or seal before the ceiling is lined — this junction runs the full perimeter of the building and is easy to miss.
Window and door reveals
Membrane must be continuously taped around the full frame, not just at the sides.
Service penetrations
Every cable, pipe and duct through the air barrier needs a grommet, sealant or purpose-made sleeve — plan for this before services are roughed in.
Downlights and exhaust fans
Specify IC-rated, gasketed downlights and properly sealed exhaust ducting at design stage, not as an afterthought.
Rangehood and evaporative cooling ducts
Large ducts to outside air need a well-fitted damper and sealed, insulated ducting — see this in detail in air sealing during construction.
Bottom plates & subfloor
Seal where the frame meets the slab or floor structure before flooring goes down.
For the full builder-focused checklist with photos, see 10 Air Sealing Details That Are Easy During Construction (But Expensive to Fix Later), and for real leak examples from South Australian tests, see Common Air Leaks Found During Blower Door Testing.
7. Budgeting for sealing
Sealing cost depends almost entirely on timing. Pre-lining, most sealing is tape, sealant, gaskets and an hour or two of a trade’s time — inexpensive, and easy to fold into normal construction sequencing. After linings are up, the same leak may require reopening a finished wall or ceiling, which costs considerably more in both materials and disruption. Budgeting a small allowance for pre-lining sealing work is far cheaper than budgeting for remediation after the fact.
8. What’s still worth getting right on test day
None of the above is an excuse to skip preparation on the day itself — an unlatched window or a running exhaust fan can still mask a genuinely good result or add a false leak to the report. See our full preparation checklist for what to check immediately before the tester arrives.
Frequently asked questions
What's the single biggest factor in passing a blower door test?
Having a continuous, unbroken air barrier planned before construction starts, and a pre-lining test to check it before finishes conceal any gaps.
Is this the same as preparing for a blower door test?
No. Preparing for a test covers what to do on the day itself — closing windows, turning off exhaust fans. This guide covers what determines the actual result: how the building was designed and sealed during construction.
Can I pass without a pre-lining test?
You can, but it's a considerably higher-risk strategy. Without a pre-lining test, any leaks are only found after linings are up, when fixing them is far more disruptive and expensive.
What ACH50 should I be targeting?
It depends on your project. Passive House certification requires ≤0.6 ACH50. NCC 2022's ventilation trigger sits at 5 ACH50. Many high-performance builders target ≤3 ACH50 so MVHR performs as designed. Confirm your specific target before construction begins.
Which trades are usually responsible for airtightness?
In practice, no single trade owns it unless a builder assigns responsibility. The air barrier is touched by the frame, insulation, plasterboard, window installation, electrical and plumbing trades — coordination between them is what makes or breaks the result.
How much does sealing typically cost?
It varies hugely by what's found and when. Pre-lining sealing (tape, sealant, gaskets on an exposed air barrier) is inexpensive labour and material. Remediation after linings are up can require reopening finished surfaces, which costs considerably more.
Do I need a building science background to get this right?
No, but someone on the project needs to own airtightness as an explicit goal — usually the builder or site supervisor — and communicate the specific details to trades before they're buried.
What if my pre-lining test result isn't where I want it?
That's the point of testing early — it's a diagnostic checkpoint, not a final verdict. Leaks found pre-lining are sealed while still accessible, then verified at final test.
Related articles
Building now and want to get ahead of your target?
Book a pre-lining test with HiPer Haus while leaks are still cheap to fix, or get in touch to talk through your project’s airtightness target.
Written by
Jonathen HindryFounder of HiPer Haus. 25+ year plumber turned Certified Passive House Tradesperson — blower door testing, MVHR design and heat pump hot water across Adelaide and South Australia.
