NatHERS assessments and what they mean for your build

A NatHERS assessment produces a star rating for a house, and that rating is generated by modelling the building as a thermal system rather than by checking a list of products. The model takes the house as a whole — orientation, floor plan, insulation, glazing, sealing, shading, services — and predicts how much energy it will need to stay comfortable across a full year of weather. The output is a number, but what sits behind it is a set of construction assumptions.

That is the part that matters on a job. A rating is not a certificate you file and forget. It describes an envelope that has to be built a particular way, and the trades who build it are rarely the people who read the assessment. When the rating and the finished house diverge, it is almost always because the assumptions in the model were never translated into sequencing, specifications and inspection points on site.

This article is about that translation. It covers what the assessment actually commits you to, where the built outcome most commonly drifts from the modelled one, and what to check before linings go on so the problem is caught while it is still cheap.

What a NatHERS assessment actually models

NatHERS is the Nationwide House Energy Rating Scheme, and it produces a rating out of ten stars for a dwelling based on its design and specifications. The rating comes from accredited software that simulates the house, and the simulation is not a simple sum of insulation values. It works room by room, hour by hour, across the year, and it accounts for how heat moves through the building fabric and how it is lost through air movement.

The practical consequence is that the rating responds to the whole assembly, not to individual selections. Change one thing — a window frame, a wall construction, the position of a wall — and the model re-runs. Two houses with the same insulation and the same glass can rate differently because of orientation, shading or how the floor plan separates heated from unheated space.

For a builder, the useful way to read an assessment is as a construction specification with reasons attached. Every assumption in it is something a trade has to deliver, and every one of those assumptions is a place where the built house can drift from the rated one.

Why thermal bridging is the most common failure point

The model assumes the insulated envelope is continuous. In reality, an envelope is a series of junctions where one trade stops and another starts, and each of those junctions is an opportunity for the insulation line to be broken.

Thermal bridging is what happens when a material that conducts heat well passes through the insulated layer. Structural steel, concrete slab edges, timber framing at corners and lintels all do this. The modelling accounts for some bridging as a known characteristic of the construction type, but it does not account for the bridging created on site by workmanship — insulation compressed into a gap, batts pulled short at a top plate, a section skipped because a cable run was in the way.

This is where the rating most often fails in practice, and it fails invisibly. The house still looks finished. The problem only surfaces later, as a room that is uncomfortable in a particular season, or as a complaint that comes back to the builder long after practical completion. By then the wall is lined and the remedy is expensive.

The fix is sequencing and supervision rather than product. Insulation has to go in before linings, it has to be inspected before it is covered, and the inspection has to happen at the junctions, not in the middle of a wall where everything looks fine.

Glazing: the whole window, not just the glass

Windows are usually the largest single source of heat gain and heat loss in a house, and the rating is sensitive to them. The important detail is that the assessment is modelling an assembly: the glass and the frame together. A high-performance glass unit in a poorly performing frame does not deliver the performance the model assumed, and neither does a good frame with substituted glass.

Australia has a separate rating scheme for windows, the Window Energy Rating Scheme, which rates whole window products for heating and cooling performance. WERS ratings are what allow a window to be compared on a like-for-like basis, and they are the reference point when a supplier offers an alternative to what was specified. If a substitution is proposed, the question is not whether the new window is “similar” but whether it carries a comparable whole-window rating.

On site, the glazing items to control are straightforward but easy to lose. Confirm the frame and glass actually delivered match the assembly named in the assessment. Confirm the window is sealed to the opening on all four sides, because an unsealed perimeter gap undermines the assembly regardless of how good the unit is. And confirm that shading devices — eaves, screens, external blinds — are installed as designed, because the model has credited them with blocking summer sun.

Draught sealing is a construction item, not a finishing trade

Air leakage is modelled. The simulation assumes a certain level of airtightness, and that assumption is met by sealing, not by insulation. This is the part of the rating most often left to whoever happens to be on site last, which is usually a painter or a cabinetmaker who has no reason to know the rating depends on it.

Sealing has to be treated as a defined scope item with an inspection point. The places that matter are the ones that are hard to reach later: the perimeter of window and door frames, penetrations through the envelope for services, junctions between different construction types, exhaust fans and downlights in ceilings, and the interface between the house and an attached garage or unheated space.

If sealing is left until after linings, most of these junctions are already closed up and inaccessible. The work then either does not happen or happens badly, and the rating quietly degrades.

Services selections feed the rating too

NatHERS assesses the house, and the house includes the services that use energy. Hot water is a significant load in most dwellings, and the assessment reflects the type of system specified. The Australian Government’s guidance on heat pump water heaters sets out how these systems work and what affects their efficiency in practice, including climate and placement.

That matters on site because hot water is often selected late, on price, after the design is fixed. A change from the specified system to a different type or capacity changes the modelled outcome, and on some projects it changes whether the certificate still describes the house being built. Where a system needs space, ventilation or a particular aspect, those are set-out decisions, not selections you can make at the end of the job.

Lighting and other fixed services are similar. They are part of the assessment, and substituting them without checking whether the rating still holds is the same category of error as substituting glazing. The rule is consistent: if the assessment named it, changing it is a design change, and design changes get checked.

Insulation performance depends on installation, not just R-value

Insulation is specified by its rated thermal resistance, but the installed performance is what the house actually gets. Government guidance on insulation is explicit that performance depends on correct installation: batts that are compressed, folded around obstructions, or left with gaps at edges and junctions do not deliver the value they are labelled with.

Three installation failures recur. The first is compression, usually caused by forcing insulation into a cavity that is too shallow or around services that were run through the middle of it. The second is gaps at the edges — the top plate, the corners, the perimeter of the ceiling — where cutting is fiddly and time is short. The third is interruption by services, where downlights, fans and pipework break the insulation line and nothing is done to make it good.

None of these are visible after lining. All of them are visible before, which is why the inspection point matters more than any specification note on a drawing.

Worked example: a builder in western Sydney

A builder in western Sydney is running a single-storey dwelling with a NatHERS certificate that specifies ceiling insulation, wall insulation, a particular glazing assembly and a heat pump hot water system, with the unit located externally on the southern side of the house.

The frame goes up, the electrician runs cable through the ceiling space, the plumber roughs in, and the insulation subbie arrives on a Friday with a tight programme. Batts go in around the cabling, compressed in a couple of spots where the ceiling space is congested, and the perimeter of the ceiling is left short because cutting around the top plate is slow work. The plasterer starts Monday.

Nobody has compared the installed work to the certificate. The house is finished, handed over, and the rating on paper says one thing while the built envelope does another. If the discrepancy is ever raised — at a later energy assessment, on a resale, or through a complaint about comfort — the builder is the one who has to answer for it, and the remedy involves removing linings.

The alternative costs a fraction of that. Before the plasterer is booked, the supervisor walks the ceiling and wall cavities against the certificate: insulation value and coverage, continuity at junctions, penetrations sealed or boxed, frame and glass matching the specified assembly, and the hot water location confirmed as designed. The check takes less time than a single call-back.

Where the certificate sits in your compliance chain

In NSW, the energy performance of a new dwelling is dealt with through BASIX, which is the state’s sustainability index for residential development. NatHERS is the rating tool used to demonstrate the thermal comfort component. The two documents work together: BASIX sets the commitments for the project, and the NatHERS rating is the evidence behind the thermal comfort commitment.

For a builder, this means the assessment is not a standalone document. It sits in a chain that runs from the development approval through to the occupation certificate, and the commitments made in that chain are what you are building to. If you are new to how that chain works for licensed builders in this state, the PointCPD guide to how NSW builder CPD works explains the framework and where technical topics like energy compliance sit within it.

It also connects directly to your supervision obligations. Verifying that installed work matches the specified design is a supervision function, not a paperwork function, and it is the same duty that applies to structural and waterproofing elements. There is a related article on NSW builder supervision duties that sets out how far that duty extends.

What to put in the file before you start

Most of the risk in a rated build is removed before anyone breaks ground, by turning the certificate into site documents. That means a marked-up set of drawings showing the insulated envelope and every junction where it has to be continuous, a schedule of the specified glazing assemblies including frame type, and a sealing scope that names who does what and when.

It also means a hold point in the programme. No linings until the envelope inspection is signed off. That single sequencing rule prevents the most expensive category of failure on a rated job, because it forces the check to happen while a remedy is still a tube of sealant and a replacement batt rather than a strip-out.

Finally, treat any substitution as a design change. If a supplier offers a different window, a different insulation product or a different hot water system, the question is whether the assessment still describes what you are building. If you cannot answer that from the documents in front of you, check the current requirement with the certifier or the assessor before the product is ordered, not after it is installed.

Builders who want to go further on energy compliance, supervision and documentation can find relevant courses in the PointCPD course catalogue, or start from the PointCPD home page for NSW builder CPD courses. The topic also sits alongside site safety documentation, such as the approach covered in this article on a SWMS template for residential builders in NSW, because both come down to the same discipline: defining the scope, sequencing the work, and inspecting before it is covered up.

Questions builders ask

• Compliance
• Energy efficiency
• Insulation

Published 25 June 2026. General information for builders, not legal, financial or engineering advice for a specific job. Check current requirements with the relevant regulator or standard before relying on them.