Is your new home really under 10 ACH? Why a blower door test matters for your NatHERS rating

Every new home in Australia has to meet the energy efficiency rules in the National Construction Code (NCC). For airtightness, the figure you will hear most often is 10 air changes per hour at 50 pascals (10 ACH50) — the common benchmark tied to the NCC performance threshold. NatHERS software such as HERO uses its own internal assumptions about air leakage when there is no measured result. What matters in practice is whether the finished home is actually that tight.

In our testing of new Victorian homes, many come in above 10 ACH50. That finding matches earlier CSIRO research, and it is why we recommend a blower door test before handover. Because we do both the NatHERS rating and the airtightness test, we can show what the measured result does to the rating.

This article covers the benchmark, what the research found, how the test works, when to book it, and what it means for builders, designers and owners.

What “10 ACH” means under the NCC

ACH50 stands for air changes per hour at 50 pascals. During a blower door test, a fan creates a 50 pascal pressure difference between inside and outside. ACH50 is how many times the home’s full internal air volume would be replaced in an hour at that pressure. Lower is tighter.

• A lower number means a tighter home.
• 10 ACH50 means the air in the home would be replaced ten times an hour at test pressure.
• 7 ACH50 means seven times an hour.

The NCC does not force every house through a blower door test. The performance pathway does set a permeability threshold of 10 m³/h/m² @ 50 Pa (NCC 2022 V2H6V3), which industry usually talks about alongside the 10 ACH50 benchmark. NatHERS ratings for a building permit are done from the drawings before construction, using the software’s standard assumptions about leakage — not a measured ACH50 from the finished home.

Fifty pascals is a test pressure, well above everyday conditions. It makes leaks measurable and lets results be compared between homes. It is not the same as how much fresh air the home gets on an ordinary day.

What CSIRO found when new homes were tested

In 2024 CSIRO published Sealing the comfort of our new homes, summarising Australia’s most comprehensive recent study of air infiltration in newly built dwellings — a CSIRO study Geokal contributed testing to, through ATTMA-accredited testers.

Key findings from the full report, Air Infiltration of New Dwellings in Australia (Ambrose and Maxwell, June 2024):

• 233 newly built dwellings were tested across Melbourne, Sydney, Canberra, Brisbane and Adelaide (105 apartments and 128 detached houses). Homes designed specifically for airtightness were excluded.
• The overall average air permeability was 6.86 m³/h/m² @ 50 Pa (CSIRO’s public summary rounds this to about 6.9). CSIRO describes that as roughly 50 per cent more airtight than homes tested in its 2015 study.
• Only 9.7% of dwellings in the 2024 sample exceeded the NCC performance target of 10 m³/h/m² @ 50 Pa (11.7% of Class 1 houses and 7.1% of Class 2 apartments).
• CSIRO’s earlier 2015 study of newer homes found an average of 15.4 ACH @ 50 Pa, and more than 60% of those results were greater than 10 ACH50 (Ambrose and Syme, 2015, as cited in the 2024 report).
• CSIRO’s public summary puts an ideal permeability for a new dwelling at about 5 m³/h/m² @ 50 Pa — “seal tight and ventilate right” — and notes that poor airtightness can increase energy bills by up to 20%.

The national picture has improved, but a “typical” new home is not automatically under the 10 ACH50 benchmark. In our own testing of new houses we still regularly see results above 10 ACH50. Without a test, neither the builder nor the owner knows where their home sits.

How a blower door test works

Our airtightness testing is carried out by an ATTMA accredited tester (ATTMA #13381), to AS/NZS ISO 9972:2015 (Thermal performance of buildings — Determination of air permeability of buildings — Fan pressurization method), following ATTMA Technical Standard L1. A typical test looks like this:

1. Set-up. A calibrated fan is fitted into an external doorway using an adjustable frame and an airtight panel. External windows and doors are closed, internal doors are opened, and openings such as exhaust fans and flues are prepared the way the test method requires.
2. Measure. The fan moves air out of the house, and usually into it as well, across a range of pressures. The airflow needed at each pressure is recorded.
3. Calculate. The readings give the air leakage at 50 pascals, which is divided by the home’s internal volume to give ACH50 (and by envelope area for permeability).
4. Find the leaks. While the house is under pressure, thermal imaging and smoke show where air is getting in, so the builder knows exactly what to fix.

How we use the measured result in HERO

We model NatHERS ratings in HERO, and we test airtightness, so we can connect the two. Once a home has been tested, we can re-run its rating in HERO using the measured ACH50 instead of the software’s internal leakage assumption.

If the home tests tighter than that assumption, the re-rated result can improve. If it tests leakier, the measured result can make the rating worse — which is useful information before handover, because the test also shows where the leaks are. Every design is different, so treat any star-change example as an illustration rather than a promise.

The rating for your building permit is done from the drawings before construction, so it uses the standard NatHERS approach. The measured re-rate is non-regulatory: we can issue a non-regulatory certificate showing how the measured blower door result impacts the NatHERS rating (better or worse). It does not replace the permit rating.

When to test: pre-lining and handover

The best results usually come from two tests:

• Pre-lining test. Once windows and external doors are in and the wrap or air barrier is complete, but before plasterboard goes up. Leaks are easy to find and cheap to fix while the frame is still open.
• Final test at handover. When the home is finished. This is the result that goes into the HERO re-rate, the findings report and the non-regulatory certificate.

A pre-lining test is often what turns an average result into a good one, because problems are fixed before they are hidden behind plasterboard. We agree the timing for both visits when we quote.

Beyond airtightness: As-Built Verification

Airtightness is only one part of whether a finished home matches its NatHERS and Whole of Home model. Our As-Built Verification (in the Verified package) checks the house as built against that specification:

• Windows and glazing — frame type and glazing checked against the specs, using U-value and SHGC from labels / WERS.
• Insulation — R-values and coverage of ceiling, walls and floor, with thermal imaging to spot gaps.
• Services — heating/cooling, hot water, solar PV and lighting checked against the Whole of Home inputs.
• Airtightness — blower door test to AS/NZS ISO 9972 by ATTMA accredited tester #13381.

You receive a findings report and the non-regulatory Geokal Energy Efficiency Certificate of Compliance, which shows how the as-built result compares with the modelled rating. It does not replace the permit certificate.

What it means for builders

Builders are responsible for an airtight home at handover. At the end of the job they also sign off to the surveyor — for the occupancy permit — that energy efficiency items, glazing and sealing were installed as required. They are not required to test or verify the finished home.

In our testing of new homes, that sign-off is often not matched by what we find on site. Not verifying the final product is a risk for both the builder and the owner: disputes, call-backs and unhappy clients are harder to unwind after handover than fixing issues while the builder is still on site. As-Built Verification checks glazing, insulation, services and airtightness against the modelled specification so you can verify rather than assume. A pre-lining blower door test gives your trades the chance to fix gaps while they are still easy to reach, and the final visit means you hand over with a findings report on file.

A tight, well-matched build can also show up in a non-regulatory re-rate — useful evidence of build quality when you talk to clients. See the Verified package for As-Built Verification, or our blower door testing page if you only need airtightness.

What it means for owners

Your home is your biggest investment, so it makes sense to know it was built right. A blower door test — or full As-Built Verification — tells you how the finished home compares with what was modelled, and gives you the chance to have issues fixed while the builder is still on site.

A tighter home generally has fewer draughts and lets the insulation, glazing and heating and cooling do the job they were designed for. If you are building, ask your builder or designer about As-Built Verification early, ideally when the NatHERS rating is being done, so site visits can be planned into the build.

Standalone airtightness testing

If you only need a blower door test or a draught investigation, without a NatHERS rating, our airtightness brand Building Airtightness handles that work. For a rating linked to a measured result, start with Geokal.