Why doesn't my new home perform the way it was rated?

The home that gets built is rarely the home that got rated.

A new house in Adelaide arrives with a number on it — a star rating, an energy figure, a set of drawings that promise a certain level of comfort and a certain power bill. Then people move in, and the home is colder in July, hotter in February, and dearer to run than the paperwork said it would be.

That difference has a name. The industry calls it the performance gap, and understanding it is the difference between a home that performs as promised and one that only promised to.

Why doesn’t my new home perform the way it was rated?

Because the rating is a prediction, and almost nobody checks it against the finished building.

An energy rating is calculated from the design — the plans, the materials, the orientation — fed through modelling software that assumes the home will be built and lived in a certain way. It is a forecast made before a single brick is laid. The forecast is only as good as its assumptions, and the assumptions are rarely tested once the house is real.

So the gaps creep in. Air leaks through junctions the model assumed were sealed. Insulation gets compressed, cut short around a pipe, or left with a void behind a downlight. Heat sneaks out through thermal bridges — the slab edge, the steel lintel, the window reveal — that the rating barely accounts for. None of it shows on the certificate. All of it shows on the power bill.

What a NatHERS rating actually checks

A NatHERS star rating measures the design, not the house.

NatHERS — the Nationwide House Energy Rating Scheme — rates a home by running its plans through approved thermal-modelling software. According to NatHERS, the software uses standardised assumptions about occupancy and operation so that ratings are comparable and repeatable. It deliberately models a typical household, not your household, and a typically built home, not your specific build quality.

The most consequential of those assumptions is air-tightness. NatHERS modelling assumes a home will leak at around 10 air changes an hour at 50 pascals. The trouble is that real homes don’t. A CSIRO study of 129 relatively new Australian dwellings, published through NatHERS in 2024, found the average new home leaked at 15.4 air changes an hour — and around 60% of them were leakier than the 10 the model assumed.

That is the performance gap in one statistic. The tool assumes a level of airtightness the finished home routinely fails to reach, and nothing in the process catches it. A NatHERS certificate is a useful design comparison. It is not a measurement of the house you end up living in.

The gap is real, and it’s been measured

The performance gap isn’t an Australian quirk; it’s been documented internationally for years.

UK research into the building-energy performance gap has found that new homes can emit, on average, around 2.5 times the carbon their design stage predicted — a body of work summarised across the academic literature on the subject. The causes recur everywhere: optimistic modelling inputs, air and insulation barriers installed imperfectly on site, and occupants who use the home differently from the model. Some of that is unavoidable behaviour. Most of it is fabric — the physical envelope not matching the drawing.

The lesson is not that modelling is useless. It is that a model left unverified drifts, and the drift is almost always in the wrong direction.

Air leakage is where most of the gap lives

Of all the reasons a home underperforms its rating, air leakage is the largest and the most fixable.

An uncontrolled gap in a home’s skin leaks conditioned air out and unconditioned air in, and it carries moisture into the wall while it does it. We’ve written separately about why air leaks cause condensation inside your walls — the durability side of the same problem. On the performance side, leakage is simply the biggest variable the rating assumes away.

It’s also invisible. You cannot see airtightness on a plan, and you cannot see it walking through a finished home. The only way to know whether a home is as tight as it needs to be is to measure it — which is exactly what the rating process never does.

The fix is verification, not a better model

You can only manage what you measure. A home performs as designed when someone checks that it was built as designed.

This is the structural difference between an energy rating and a Passive House. A rating predicts; Passive House verifies. The Passive House Institute sets a hard limit of no more than 0.6 air changes an hour at 50 pascals, and that figure isn’t modelled — it’s confirmed on the actual building with an on-site blower-door test, in both pressurised and depressurised states.

A blower-door test seals a calibrated fan into an external doorway, pressurises the house, and measures how much air escapes through the envelope. It turns airtightness from a hopeful assumption into a number the builder either hits or doesn’t. We run it at shell completion — early enough to find and fix leaks while the structure is still open — and again before handover, as proof. The 0.6 target sits against the 15.4 average CSIRO found in new homes; the distance between those two numbers is most of the performance gap, closed by measurement.

Verification doesn’t stop at the blower door. It runs through the whole sequence: insulation inspected before it’s covered, thermal bridges designed out and checked, the energy model reconciled against what was actually built. The point is consistent — a high-performance home is a built result, not a drawn intention.

What this means for an Adelaide build

A rating is a starting line, not a finish line. The figure on the certificate is a forecast of how the home should behave; whether it does behave that way depends on what happens on site, and on whether anyone measures it.

A short list, if you want a home that performs the way it was sold to you:

• Treat the star rating as a design tool, not a guarantee. It rates the plan, not the build.
• Ask whether the home will be blower-door tested — and when. A test at lock-up can still fix what it finds; a test at handover only reports it.
• Spend on the envelope before the gadgets. Airtightness, continuous insulation and thermal-bridge-free detailing are where the gap opens or closes.
• Engage a builder who measures. The cheapest way to avoid a performance gap is to make verification part of the contract, not an afterthought.

On the custom homes we build across West Lakes, Henley Beach, Grange and Lockleys, the envelope is tested rather than assumed — for the same reason we start every project from Passive House rather than treating it as an upgrade. A home is a fifty-year-plus proposition. It should be built to a measured standard, not a modelled hope.