Building Material, Thermal Performance

Scanning your Project like a NatHERS Assessor

Conducting a NatHERS assessment involves a lot of careful review on the Assessor’s part to make sure the project is accurately modelled by the rating software. The wrong inputs can have a detrimental impact on the rating certificate, and there is a considerable amount of data entry involved as they parse through design and drawing details.

Accredited Assessors are also subject to auditing by their Assessor Accrediting Organisations (AAOs) for quality assurance purposes. This means there is a certain standard of evidence required for assessments, which can involve a level of back-and-forth with designers to make sure the plans accurately reflect what’s modelled.

When you engage an Assessor, they will ask you for certain documentation and information in order to conduct the assessment. The Assessor should have asked you, and you’ve (hopefully) provided them with:

  • Site plans
  • Floor plans
  • Sections
  • Elevations
  • Lighting/electrical plans

Other drawings or documents may also be requested depending on the project and level of detail covered by the above. In most cases though, if you can cover this list then you’re ready to get started.

Your Assessor will be combing over these in detail as they pull all the information they’ll need to conduct their assessment of the project. It is not uncommon to either have missing information that needs to be added to the plans, or that there is a conflict in the information that needs to be resolved.

But perhaps you want to be mindful of the lens that the Assessor is looking through, so that you can preempt some of their questions or come better prepared for the next project. We’ve put together a quick list of items of what your Assessor is looking for and why:


Assessors have a strict protocol to follow when assigning zones to various rooms. The way the zone is modelled (between ‘day-time’, ‘night-time’, ‘conditioned’, ‘unconditioned’, ‘kitchen/living’, etc.) will inform the software how to model occupancy behaviour and the subsequent energy intensity of heating or cooling those spaces.

Smaller projects with fewer zones tend to be straightforward to model. Larger projects with more zones, multi-storey projects or projects with many purpose-specific or unique rooms will make the modelling more difficult as your Assessor is trying to correctly assign the zone type in accordance with the NatHERS requirements.

Building construction

Wall and floor constructions naturally will have a bearing on the thermal performance of the project. Assessors will review the plans to ensure they have the correct construction type for each building element in the rating tool.

Common constructions such as brick veneer or weatherboarding are easy to model. Selecting a very particular brand or unique product can make the Assessor’s job more difficult when there is a lack of information on that construction profile (and the Assessor will need to research this in order to produce the assessment). Having a variety of construction types on a single project can also lengthen the optimisation stage.

Windows and doors

Your Assessor is painstakingly reviewing each and every one of them – from their dimensions to their position, their type and operability. Windows allow for ventilation and heat loss, and consequently have a significant impact on the assessment result.

You may be asked by your Assessor to confirm window operability (e.g. awning, bi-fold, casement), construction type (glazing and frame material) or clarify any conflict in dimension where observed.

A high window-to-floor ratio will be problematic if you’re not prepared to consider double glazing or higher thermal performance windows.

Ceiling penetrations

For the purposes of a NatHERS assessment, your Assessor is interested in knowing the gap in the ceiling insulation from recessed lights and fan fixtures. These penetrations will reduce the average R-value in the roof space and increase uncontrolled air infiltration between the roof space and the zone below. Ceiling fans also provide some thermal comfort for occupants through improved airflow.

You can help your Assessor by having downlights and fans clearly marked and specified so that they can be modelled accurately.

Shading and obstructions

Naturally, the thermal performance of a dwelling will be impacted by elements outside of the building envelope as well as inside. When your Assessor is reviewing details on the site plans, they’re checking for neighbouring buildings, boundary fencelines and other shading devices both natural or artificial that could have an impact on how the dwelling performs.

Rating software limitations mean that the Assessor has to make a thoughtful decision on which particular obstructions will have the most impact on the project. Knowing the neighbour’s dwelling height, eaveline and fenceline height will help your Assessor model the project accurately.

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