Home Performance Gap: Why Australian Homes Fall Short of Their Energy Rating

In 2020, researchers at Curtin University published findings that should have prompted a national conversation about Australian housing quality. They found that residential buildings in Australia were performing at 20–80% below their as-designed energy rating.

Not 5% below. Not 10%. Twenty to eighty percent.

A home designed and certified to achieve 7 NatHERS stars might be operating as a 4-star building. The occupants don’t know. The certifier who issued the rating doesn’t know. The builder who constructed it almost certainly doesn’t know. The gap between what a building was designed to do and what it actually does once built has a name — the performance gap — and in Australian residential construction, it is vast.

Understanding why it exists, and what closes it, is the most important conversation the Australian building industry isn’t having.

What Home Performance Gap Actually Means

When a NatHERS assessor models a home, they are simulating its theoretical energy performance under idealised conditions. The model assumes a specific set of inputs: the geometry and orientation of the building, the specification of insulation and glazing, and a fixed infiltration rate.

That last input — infiltration — is where the model diverges most sharply from reality. NatHERS assumes an air infiltration rate of 10–12 air changes per hour at 50 Pascals (ACH50). It assumes this regardless of how the building is actually constructed, because the model has no way of knowing. Airtightness is not tested or verified at any point in the standard Australian building approval process.

What that means in practice is that every home in Australia is rated against the same infiltration assumption — whether it’s been sealed meticulously or not at all. The rating cannot reward good work or penalise poor work in this regard.

We recently tested newly completed homes in Canberra. Several were sitting above 20 ACH50 — more than twice the infiltration rate the NatHERS model had assumed. These homes would be operating at less than half their rated thermal performance as a result of this factor alone.

This is not a fringe finding. It is consistent with what building scientists have been measuring in Australian homes for years.

The Four Causes

The performance gap in Australian residential construction has four primary drivers. They are well understood. What’s less understood is how structurally difficult they are to solve within the current delivery model.

1. Airtightness

As established above, this is the dominant factor. Uncontrolled air infiltration accounts for 25–40% of total heating and cooling load in a typical Australian home. Every air change above the modelled assumption represents real energy loss and real comfort degradation. The problem is that airtightness requires every junction, penetration, and transition in the building envelope to be sealed correctly — and in site-built construction, that responsibility is split across multiple trades with no single point of accountability.

1. Thermal bridging

NatHERS does not model thermal bridging in meaningful detail. Structural elements that conduct heat through the insulation layer — steel studs, window reveals, slab edges, roof-to-wall junctions — can reduce the effective thermal performance of a wall assembly by 20–40% compared to its nominal R-value. A builder who doesn’t address thermal bridges produces a wall that performs significantly below specification, and the rating certificate doesn’t capture this.

1. Construction quality and trade sequencing

Even when a project is well specified, site conditions work against consistent quality. Weather events delay work. Trades follow each other in sequence, each one potentially compromising what the previous trade installed. An electrician cuts through a vapour membrane. A window installer leaves a gap at the sill. A plumber penetrates the top plate without sealing it. None of these are deliberate failures — they’re the predictable outcome of a fragmented delivery process where no single person owns the envelope performance.

1. The verification gap

In standard Australian construction, there is no requirement to verify that the as-built performance matches the as-designed performance. A NatHERS certificate is issued on the basis of drawings and specifications — not measurements of the completed building. The certificate is never updated to reflect what was actually built.

This is the fundamental accountability gap. The person who issues the rating and the person who builds the building operate in different systems, with no connecting verification step.

Why DfMA Changes the Equation

Design for Manufacturing and Assembly (DfMA) is the methodology that underpins the Net Zero Plus system. It is not simply a construction technique — it is a quality management philosophy applied to building delivery.

In a DfMA process, every design decision is made with manufacturing in mind from the outset. Components are designed to be fabricated in a controlled factory environment, assembled with consistent processes, and installed on site according to documented procedures. The goal is to eliminate the sources of variability that produce the performance gap.

Here is what that means in practice for airtightness specifically:

• Factory assembly: Wall panels are assembled in a controlled environment. No weather delays. No competing trades. The airtight membrane is installed, taped, and inspected by a single team before the panel leaves the factory floor.
• Window installation: High-performance windows are set into panels in the factory, not on site. The seal between frame and panel is consistent and verified before delivery.
• Electrical conduits: Pre-run in the factory within the panel structure, eliminating the need for electricians to penetrate the membrane on site.
• Sequential delivery: Panels arrive numbered and sequenced. The connections between panels — the joints that are most susceptible to air leakage in site-built construction — are sealed according to a documented procedure.

The target for a Net Zero Plus system, constructed per specification, is 1 ACH50 or below. This is between ten and twenty times better than the infiltration rate NatHERS assumes in its modelling.

Given the precision of the DfMA process, the as-built building should perform at or better than the as-designed rating. That’s the goal, and for high-performance prefabricated structures, the evidence supports it.

The Verification Step That Changes Everything

One of the most important things Net Zero Plus does is test airtightness on completed installations with a blower door. The test takes approximately two hours. It pressurises the building to 50 Pascals and measures the rate of air escaping through the envelope.

This single step closes the verification gap that exists in standard Australian construction. It converts a theoretical rating into a measured outcome. It creates accountability — for the system, for the installation, and for the performance claim.

If the result doesn’t meet the target, it tells us exactly that. Gaps can be found and addressed. The building can be tested again. The final airtightness figure becomes part of the project record.

This is what ‘performance measured, not assumed’ actually means in practice.

What This Means for Buyers, Builders and Developers

The performance gap has different implications depending on where you sit in the building process.

For homeowners and owner-builders, it means the energy rating on your drawings is not a guarantee of how your home will perform. If you want to know what your building will actually do, you need to specify airtightness as a design target, test it on completion, and choose a construction system that makes it achievable.

For builders, it means that the quality of your envelope work is currently invisible — not rewarded by the certification system and not penalised when it falls short. That’s a commercial reality today. It will not be a commercial reality indefinitely. As energy disclosure and building performance standards tighten, the builders who have been doing this properly will be differentiated from those who haven’t.

For developers, the performance gap is a risk that sits outside the balance sheet. A building that performs at half its rated performance creates unhappy occupants, higher running costs, and reputational exposure. A verified high-performance building creates the opposite: measurable outcomes that support premium positioning, council confidence, and long-term asset value.

The Takeaway

The performance gap is not a secret. The research has been published. The physics are well understood. The fix is not technically complex.

What it requires is a delivery system that treats airtightness and thermal performance as engineered outcomes rather than assumed ones — and a verification step that confirms the building performs as designed.

Until the Australian building industry normalises that step, the gap will persist. At Net Zero Plus, we’ve built it into the process from the start. Speak with a Net Zero Plus expert today.