What Is Airtightness — and Why It Matters More Than Your Energy Rating

Most people buying or building a home in Australia have never heard the word ‘airtightness’ used in a meaningful way. Their builder hasn’t mentioned it. Their certifier hasn’t tested it. And their energy rating doesn’t reflect it.

That’s a problem — because airtightness is one of the largest single drivers of real-world building performance. Understanding airtightness in Australian homes is essential if we want to understand why some homes perform very differently from what their energy rating suggests.

What Airtightness Actually Is

Airtightness describes how much uncontrolled air passes through a building’s envelope — its walls, roof, floor, windows, and doors — under pressure. A leaky building allows warm or cool air to escape and outside air to infiltrate, continuously undermining whatever your heating and cooling system is trying to achieve.

This is why airtightness in Australian homes matters. Airtightness is not about preventing ventilation or making a home feel sealed off from fresh air. It is about controlling where air enters and leaves the building, rather than allowing it to move through uncontrolled gaps and cracks.

It’s measured using a blower door test. A calibrated fan is mounted in an external doorway, the building is pressurised or depressurised to 50 Pascals, and the flow rate of air leaking through the envelope is recorded. The result is expressed either as air changes per hour at 50 Pascals (ACH50) or air permeability in m³/h/m² of envelope area.

A drafty older Australian home might sit at 15–20 ACH50. A well-built new home might reach 5–7. A high-performance panelised home built to Passive House principles targets 1 ACH50 or below.

These measurements give us something that drawings and energy ratings cannot: an actual result for the airtightness in Australian homes being tested.

What the Latest Research Shows

In 2024, CSIRO published the largest airtightness study of new Australian dwellings ever conducted — 233 newly built homes tested across Melbourne, Sydney, Brisbane, Canberra, and Adelaide.

The results showed genuine improvement. Average air permeability in new homes has roughly halved since a 2015 equivalent study: now around 6.9 m³/h/m², compared to the equivalent of 15.4 ACH in 2015. Around 90% of homes tested now meet the minimum performance threshold introduced in NCC 2022.

But the same study found something troubling: there is no statistically meaningful correlation between a home’s NatHERS star rating and its measured airtightness. A home with an 8.9-star NatHERS rating in the study returned an airtightness result above the average for its dwelling type. The star rating didn’t predict the performance.

This highlights an important limitation in how airtightness in Australian homes is currently understood and assessed. A high energy rating does not necessarily mean a building has been physically tested for air leakage.

The CSIRO also found that 26% of new homes are now so tight they trigger requirements for continuous mechanical ventilation under the NCC — but none of those homes had ventilation installed, because nobody tested them and nobody knew.

As airtightness in Australian homes improves, ventilation needs to become part of the conversation too. A tighter building needs deliberate, controlled ventilation rather than relying on accidental air leakage.

Why Airtightness Isn’t Verified

In standard Australian construction, airtightness testing is not required at completion. A NatHERS energy rating is based on drawings and specifications — not measurements of the finished building. The certificate is issued before construction begins and is never updated based on what was actually built.

This creates a fundamental accountability gap. The thermal performance of your home depends in part on airtightness. But airtightness is never checked.

For anyone researching airtightness in Australian homes, this distinction is important: a building can be designed to perform well without necessarily achieving that performance once construction is complete.

The CSIRO 2024 report specifically recommends that mandatory airtightness testing become part of the Australian completion process — consistent with standards that already exist in the UK, Germany, and much of Europe. That recommendation has not yet been adopted in Australia.

Until that changes, homeowners and builders who want to know the actual performance of a building need to test it themselves.

What Causes Poor Airtightness

Every junction, penetration, and transition in a building’s envelope is a potential air leakage point. CSIRO documented several that appear consistently:

• Bath fans: a significant air leakage pathway in 63% of the homes tested. An exhaust fan that isn’t properly sealed when not running becomes a direct connection between inside and outside.

• Door seals: poor or absent in 65% of houses — including the gap at the base of external doors and the seal between frame and wall.

• Sliding glass doors: showed leakage issues in 63% of apartments and 48% of houses.

These examples demonstrate why airtightness in Australian homes cannot be achieved simply by specifying better insulation. Insulation slows heat transfer through materials; airtightness controls unintended air movement through gaps in the building envelope.

None of these require sophisticated building science to fix. What they require is a deliberate process in which airtightness is treated as a design outcome rather than an assumption — and someone who owns that outcome end to end.

How Factory Prefabrication Addresses It

The core challenge with site-built airtightness is that it requires many different trades to make many different decisions correctly, in sequence, under variable site conditions. An electrician who penetrates a membrane without sealing it. A plumber who doesn’t flash a roof penetration. A carpenter who doesn’t tape an OSB joint. Any one of these decisions can undo careful work elsewhere.

This is one reason factory prefabrication can offer a different approach to airtightness in Australian homes.

In a factory prefabrication process, the airtightness layer — the membrane — is installed, taped, and inspected in a controlled environment by a single team before the panel leaves the factory. Windows are set into the panel frame before delivery, eliminating site-installed window seals as a variable. Electrical conduits are pre-run within the panel, eliminating the need for trades to penetrate the membrane on site.

By moving critical parts of the airtightness process into a controlled factory environment, the number of variables that need to be managed on site can be reduced.

The target for a Net Zero Plus panelised system, installed per specification, is 1 ACH50 or below — and it is verified with a blower door test on completion.

For airtightness in Australian homes, that verification step is particularly important because it provides a measured result rather than relying on assumptions about how well the building envelope was constructed.

That verification step is what converts a theoretical performance claim into a measured outcome. Until mandatory testing exists in Australia, commissioning a blower door test remains the only reliable way to know what your building actually achieves.

The difference is simple: performance measured, not assumed.

As the standard of airtightness in Australian homes continues to improve, testing will become increasingly important. The better we understand the relationship between the designed building and the building that is actually constructed, the better we can deliver homes that perform as intended.

Performance measured. Not assumed.

Net Zero Plus designs and supplies high-performance panelised wall and roof systems for residential and medium-density projects across Australia. All systems are engineered to achieve verified airtightness. Visit netzeroplus.com.au to speak with an expert.