What a NatHERS or HERS Assessor Actually Checks On-Site

Two houses on the same street, built by the same builder in the same year, can score noticeably differently on a NatHERS or HERS assessment, and the reason usually comes down to details that aren't visible from the street: orientation relative to true north (or south, depending on hemisphere), glazing specification per window rather than an average, and how carefully the building envelope was actually sealed rather than just insulated on paper.

Orientation and solar exposure, room by room

An assessor models each habitable room's exposure to the sun across the year, not just the building's overall orientation. A living room with substantial west-facing glazing and no external shading will score very differently to an identical room facing south (in the southern hemisphere) with the same glazing area, because of how much unwanted summer heat gain or beneficial winter solar gain each orientation actually receives. This is why building orientation, decided at the design stage, has such an outsized effect on the final rating. It's genuinely difficult to fix after the slab is poured.

Construction details, not just headline specs

The assessment models actual wall, roof and floor construction, material types, insulation R-values, and how continuous that insulation actually is through the assembly, including thermal bridging where framing members interrupt an insulation layer. It's not enough to say "R4 batts in the walls." The assessor is modelling the real assembly, including junctions and penetrations, which is part of why the air-sealing quality discussed elsewhere on this site genuinely moves the needle on the final number, not just the batt rating on its own.

Glazing, window by window

Each window is assessed individually for its size, orientation, glazing type (single, double, low-E, and so on) and any shading, eaves, pergolas, external blinds, that affects how much sun actually reaches that specific window across the year. A home with excellent glazing on a poorly shaded west wall can score worse than expected if that detail isn't accounted for in the design, which is a common surprise for homeowners who assumed "we used double glazing everywhere" would automatically deliver a strong rating regardless of where those windows actually face.

Air leakage and sealing

Newer assessment protocols in both countries increasingly account for air leakage rather than assuming a generic, often optimistic, sealing standard, which is a meaningful shift, because it means the on-site air-sealing quality discussed in our envelope-sequencing piece elsewhere on this site can now genuinely move a formal rating, not just theoretical comfort. Ask your assessor directly whether air leakage was measured (via a blower door test) or assumed as part of your specific assessment, since the answer changes how much confidence to place in the final number.

Climate zone as the baseline

Every assessment is run against the specific climate data for that location. Australia uses 69 climate zones across NatHERS, the US uses climate zones defined for HERS and energy code purposes, because the same construction performs very differently in, say, Hobart versus Darwin, or Minneapolis versus Miami. This is why a rating from one location can't be assumed to transfer to a similar-looking home somewhere else. The climate file is doing real work in the calculation, down to hourly temperature and solar radiation data for that specific location.

What the assessor documents on-site versus off it

A lot of the process happens well before anyone visits the property, plans, specifications and material schedules feeding a desktop model. But a genuine site visit, checking that what was actually built matches what was specified, catching a substituted insulation product or a window that ended up a different size to the plan, is what separates a rigorous assessment from a purely desktop one. Ask specifically whether your assessment included a site inspection or was based on plans alone, particularly for an as-built or existing-home rating rather than a new-build design assessment.

What homeowners misunderstand about the number

A star rating or HERS index number is a modelled prediction of thermal performance relative to a benchmark, not a guarantee of your actual energy bill, which also depends on occupant behaviour, appliance choices and how the home is actually operated day to day. I'd treat the rating as a genuinely useful comparison tool between design options at the point of building or renovating, rather than a precise bill forecast. It's measuring the building's inherent thermal performance, not predicting how any specific household will live in it.

My honest take

I think the on-site and construction-document review an assessor does is more rigorous than most homeowners assume, and more rigorous than the "just get a 7-star rating" conversation usually acknowledges. The real value is in the specific measures the assessment identifies as underperforming, which is where I'd focus renovation dollars rather than chasing the headline number alone.

See our existing NatHERS vs HERS comparison for how the two systems differ at a high level, and air sealing before insulation and double vs triple glazing for two of the specific envelope elements an assessment is modelling.

The official NatHERS website lists accredited assessors and the current climate zone data for Australia, and in the US, RESNET is the body that administers HERS rater accreditation and the HERS Index methodology.

New build versus existing home assessments

A new-build assessment is done against design documents before construction, letting changes be made cheaply on paper if the modelled rating falls short of a target. An existing-home assessment works backwards from what's actually there, which is inherently less flexible since you're rating a building that's already built rather than influencing its design. If you're planning a renovation, getting an as-built assessment done before you finalise the renovation scope can identify which specific envelope elements are dragging the rating down, which is far more useful for prioritising a limited budget than a generic "insulate everything and hope" approach.

What I tell clients chasing a specific star target

If a council or lender requires a minimum rating, ask your assessor to model a couple of different upgrade combinations rather than accepting the first path that clears the bar. Sometimes a cheaper glazing upgrade plus better air sealing clears the target more affordably than the ceiling insulation upgrade a builder defaults to recommending, simply because insulation is the easiest line item for them to quote. The assessor's software can usually test a few scenarios quickly, and it's worth asking for that comparison before committing to whichever single measure gets suggested first.

Choosing an assessor

Confirm the assessor is genuinely accredited under NatHERS or as a RESNET-certified HERS rater, and ask how many assessments of a similar home type they've done recently. A rating is only as reliable as the person collecting and entering the underlying data, and I've seen enough inconsistency between assessors on similar homes to think this matters more than the modelling software itself, which is broadly standardised across accredited practitioners either way.

— Ben Sorensen, Green building