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Confessions of a NatHERS assessor

In addition to being an architect and consultant that specialises in eco-effective building design, I am also a NatHERS accredited assessor. NatHERS refers to the Nationwide House Energy Rating Scheme, which is used throughout Australia to rate the thermal performance of residential buildings. NatHERS software is used to calculate the heating and cooling energy loads used by a proposed building, scoring between 0 stars (requires a lot of energy for heating and cooling) and 10 stars (no artificial heating or cooling required). Most homes in Australia require a minimum rating of 6.0 stars to comply with the energy efficiency requirements of the National Construction Code.

Something many people don’t realise: there is actually no requirement to be “accredited” in order to be an assessor. Theoretically anyone can use NatHERS software to perform an energy rating on a home, assuming they can work out how to use the software, understanding apply the relevant principles and protocols for conducting the rating. However, to be an accredited assessor requires a number of steps to be fulfilled including:

  • Successfully completing a Certificate IV in NatHERS Assessment with a Registered Training Organisation;
  • being a member of an Assessor Accrediting Organisation;
  • agreeing to a professional code of conduct;
  • holding professional indemnity insurance;
  • undertaking minimum Continuing Professional Development activities; and
  • submitting the house energy ratings completed so that they can be audited.

It’s this last requirement that I think most accredited assessors dread the most. The one and only time I have been audited previously was back in 2010 (yes, I passed). I would like to think I complete my house energy ratings diligently, and comply with all the technical requirements of the NatHERS protocol. However, there’s nothing quite like having my work scrutinised by one my peers to make me wonder if I am as diligent as I would like to believe!

So I submitted the requested house energy rating and project information to be audited. For those of you that have never been audited before, a grade of 80% or higher is required to pass an audit.

I got 56%.

Yep. How embarrassing.

The nine page report I received back provided a comprehensive list of all the technical errors I had made in my house energy rating. The following summarising paragraph covers the important bits:

This rating for a complex small house was poorly done and… is not to an acceptable standard due to the number of errors and omissions found. Errors were found with floor height above ground, zone partitioning, zoning, floor areas, zone volumes, floor type, floor coverings, roof type, roof construction, air gap emissivities to roofs and walls, roof colour, wall type, wall dimensions, eave offsets, shading screen dimensions, window dimensions, window opening style, window openability, window head height, window system type and window group type. Several wing walls were omitted. Several relevant plan pages were not provided. The floor type was not clear from the plan pages provided. No insulation requirements or building sealing requirements were shown on the plans. Insulation Contact downlights were noted in the NATHERS Certificate but not specified anywhere on the plans.

Ouch. Yes, I really did wince when I finished reading the report. Having achieved such a poor result, a teleconference was then scheduled with the auditor and Accreditor Assessor Organisation to discuss the results and required remedial action.

Now I’ve been told by reliable sources that this is the point where some individuals just say “stuff it”, forgo their accreditation and then continue to conduct house energy ratings as non-accredited assessors. Alternatively, some attend the teleconference, are argumentative or don’t understand what they need to do to rectify the rating, and THEN become non-accredited assessors. Neither is a particularly good outcome for anyone, but I must admit these options briefly crossed my mind.

However upon reflection, it eventually dawned on me that this was actually a valuable learning experience. While my audit results were disappointing, I realised this was an opportunity for me to learn from one of my more experienced peers. Their feedback would help me to improve the way I conduct house energy ratings. So prior to the teleconference, I systematically worked through each of the issues raised in the report, and kept a running spreadsheet of how each correction affected the calculated star rating. You can view a spreadsheet of the corrections I made here Through this process, and in addition to the feedback received in the report and during the teleconference, I realised there were a number of areas for significant improvement:

  • Some zones types had been modelled incorrectly;
  • the construction of some external walls had been modelled incorrectly;
  • I had been using the wrong type of reflective air gap for the intended performance;
  • I had omitted some vertical screens and wing walls;
  • I had incorrectly inputted the data for some horizontal screens; and
  • the drawings, notation and documentation I supplied did not correspond with data used in the rating.

The last of these was perhaps the most valuable thing to realise. I generally complete my house energy ratings early on in the design process, typically just before a planning or development application is submitted. I believe this is the optimum stage at which to check how a proposed design is likely to perform, as it then means inexpensive or simple design changes can be made before being submitted for approval. You can read more about this approach here.

However, sometimes variations and changes do still occur after a development approval has been received and while the building licence and construction drawings are being prepared. For example, in this project the external wall construction for the main living area changed and some window openings were consolidated, which meant that the original house energy rating I had conducted needed to be updated. My construction drawings were now inconsistent with the rating, which could have created confusion for the builder with their costing and/or during construction.

This was a most crucial lesson for me to learn – the houses we rate will only have good thermal performance if they are built according to the assumptions made and information inputted into the rating. This information needs to be clearly communicated to the owners, architects, building designers and builders who will be involved with the project long after the rating has been stamped and certified.

Somewhat ironically, after completing all of the corrections identified in the report, while the heating and cooling loads changed with each update, the final rating for the house remained the same – 8.1 stars. This came as something of a relief, as I believe this demonstrates that my understanding of how to design a thermally efficient house is fundamentally sound. However, I also appreciate that for a rating scheme such as NatHERS to be successful, it is vitally important that the way the software is used and how data entered is as consistent as possible.

While it can be confronting and a bit humiliating to have your work examined and found to be unacceptable, I remain a strong advocate of the auditing and accreditation process. I genuinely believe these processes play an important part in helping to improve the energy efficiency of homes in Australia; that’s ultimately what I am striving for as an eco-effective architect. While we might prefer to receive positive comments and compliments that make us feel good about ourselves, it’s often when we receive critical (but constructive) feedback that we really get the opportunity to grow, develop and improve. Even if you don’t get audited anytime soon, I encourage you to adopt this attitude in other parts of your professional and personal life. In the words of American psychologist David Scharch:



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