Your New Home Has a 7.1-Star Energy Rating. Has Anyone Actually Tested It?

A 7.1-star NatHERS energy rating sounds impressive. The Forrest Passive House in Spotswood has one. It also has a certified blower door result of 0.41 ACH50 and a heating demand of 13.1 kWh per square metre per year. Those are not the same thing, and understanding the difference could change every decision you make about your next home.

A NatHERS energy rating models a building's predicted performance in software. A certified Passive House proves performance through physical testing after construction. NatHERS does not test airtightness, does not verify insulation installation, and does not require anyone to check whether the building performs the way the model said it would. Passive House certification does all three.

The Forrest Passive House achieved a certified heating demand of 13.1 kWh/(m²a) and a blower door result of 0.41 ACH50 — results that were physically measured on the finished building, not modelled in software.

What does a NatHERS star rating actually measure?

NatHERS, the Nationwide House Energy Rating Scheme, uses AccuRate or similar software to estimate the annual heating and cooling energy loads of a building based on its design inputs. Feed in the insulation specification, the window sizes and orientations, the floor plan, and the software produces a predicted energy demand and a star rating.

It is a compliance tool. It was designed to set a minimum standard for new homes, not to verify that individual buildings perform as designed. The rating is produced at design stage. The building has not been built yet. Nobody tests the finished product.

Why does the assumed airtightness in NatHERS matter so much?

NatHERS does not test airtightness. It assumes a leakage rate. For a 7-star rated home, the modelled infiltration assumption is around 0.5 to 1.0 air changes per hour at natural pressure conditions.

The average Melbourne home measures 19 ACH50 under pressurised test conditions (CSIRO / Australian Building Codes Board, 2016). At natural pressure, that translates to roughly 1.0 to 1.9 air changes per hour. The NatHERS model assumes the home performs better than the Melbourne average without ever checking. If the finished home actually measures 15 ACH50 or 19 ACH50, the star rating does not change. The certificate has already been issued.

The Forrest Passive House measured 0.41 ACH50 under pressurised test conditions. The Melbourne average is 19 ACH50. These are not comparable buildings, and a NatHERS star rating cannot tell you which one you are living in.

What else does NatHERS not verify?

NatHERS does not verify insulation installation quality. If the insulation specified in the design is compressed during installation, missing in sections, or incorrectly detailed around ceiling penetrations and wall junctions, the star rating does not change. The model assumed the insulation was installed correctly. Nobody checked.

NatHERS also does not model thermal bridges with the precision that PHPP does. Every timber stud in a framed wall is a thermal bridge. PHPP accounts for this by modelling the wall as a composite of insulation and timber conductivity, producing a blended U-value that reflects real construction. NatHERS simplifies this. The difference accumulates across the full envelope area.

What does PHPP do differently?

PHPP, the Passive House Planning Package, models the building assembly by assembly, thermal bridge by thermal bridge, window by window, orientation by orientation. Every glazing unit is entered with its specific U-value and g-value. Every wall assembly is modelled with the conductivity of the insulation, the framing fraction, and any composite layers accounted for. The result is a predicted energy demand that reflects the actual specified construction in significantly more detail than NatHERS.

Then the building is physically tested. The blower door result is not an assumption. It is a measured fact. If the airtightness does not meet the Passive House threshold, the building does not get certified, and the builder has to find and fix the leaks before certification is issued.

So what does a 7.1-star NatHERS rating actually tell you?

It tells you that the design, modelled in software using simplified assumptions, predicts that the building will perform well relative to the NCC minimum baseline. In the case of the Forrest Passive House, the 7.1-star NatHERS rating reflects genuinely good design. The building was also certified as a Passive House Classic, so the design quality is real.

But the NatHERS rating alone tells you nothing about whether the finished building was constructed correctly, whether the airtightness is anywhere near the modelled assumption, or whether the insulation was installed the way the drawings intended. Only a blower door test and post-construction PHPP verification can tell you that.

You could design a building that achieves 7.1 stars on paper, build it with compressed batts, gaps around the windows, and no airtightness layer to speak of, and hand over a 7.1-star certificate on completion day. Nobody would know. The certificate has already been issued. The test was never done.

What does the Forrest Passive House tell us about the gap between the two systems?

The Forrest Passive House has both a 7.1-star NatHERS rating and a certified Passive House Classic result. The NatHERS rating reflects the design. The Passive House certification reflects the finished building. Both describe the same house. Only one required a physical test of the actual structure after construction.

The heating demand of 13.1 kWh/(m²a) was not predicted by the NatHERS software and then assumed. It was modelled in PHPP before construction, refined during construction as actual component specifications were confirmed, and then verified against a measured blower door result of 0.41 ACH50 after construction. The certifier checked the PHPP inputs against the actual materials installed. The model and the test had to agree.

That is a fundamentally different level of accountability to a star rating issued at design stage.

What should you be asking your builder before you sign a contract?

Ask how many of their finished homes have had a blower door test done. Ask what the results were. Ask whether they can show you the post-construction PHPP file for their last certified project. Ask what the airtightness threshold is for a certified Passive House and what their best result has been.

If they cannot answer those questions, they have never built to a verified standard. They have built to a modelled assumption and handed it over hoping the reality matched the prediction.

The Forrest Passive House was tested. The result was 0.41 ACH50. The certificate was issued because the building earned it, not because the design predicted it.

You are about to spend more than a million dollars on a home. Is a software prediction of performance good enough, or do you want a measured result?