NatHERS star ratings are deemed by the public and parts of the building industry to be a reliable way to put an energy rating on a house and make it comparable to other houses. I think the saying goes, “the best camera is one you’ve got in your hand”, and in a similar vein, star ratings are a good system purely because it is established, relatively easy to understand by the public and is “better than nothing”. However, star ratings are far from a perfect system. Having worked with NatHERS ratings for many years, let me tell you what I know about them.
- There are four different software programs under the NatHERS banner (First Rate 5, Accurate, BERS Pro and now Hero) and in my experience, they do not all produce the same star rating for the same house.
- Air tightness / air infiltration has a big impact on thermal comfort, yet can not yet be assessed or altered in any NatHERS software
- Insulation loss around penetrations in the ceiling are factored in (e.g. downlights and exhaust fans), however insulation loss around the frame is not considered.
- Thermal bridging at the frame is not considered, meaning steel and timber are equal in the simulation, however, steel has a significantly higher conductance value meaning that heat and cool is transferred into the conditioned space much more easily, resulting in a higher heating and cooling requirement. The A/C has to work harder to combat this additional temperature transfer.
- Some assessors do not submit the ratings to First Rate 5, therefore they do not produce a certificate or submit their work meaning they can’t be audited.
- There is a perception that a house rated at 6 stars is “energy efficient” which is not correct. There is a scale and one “energy efficient” house will perform very differently to another “energy efficient” house. This performance depends very much on who designed the house and who built it. Star ratings are out of 10 stars, and the current minimum requirement is 6 stars out of the available 10 stars. Having said that, a 6 star house today it is more energy efficient than a dwelling built 10 years ago, however it is the worst house you can build under the star rating system and will still require significant mechanical heating and cooling.
- There is commonly a difference between what is designed by the builder or architect, what is assessed in NatHERS and what gets built on site, known in the industry as “the performance gap” and the trend is generally downwards as the projects gets further along in the design/construct process.
- Most importantly, and most frustratingly, the outcome of a NatHERS star rating very much depends on who is sitting at the computer. Assessors have different technical understanding, modelling skill and level of integrity. This means that the assessors with looser morals and handshake deals with their clients can quite easily manipulate star ratings to be better than they should be. If, when you are building your new home, you want to get it right, make sure you have the right team on your side from the start and that includes an energy assessor with integrity and good technical skills.
Although star ratings are the most commonly recognised method of assessing energy, they are largely inaccurate, inconsistent and unpoliced and this leads to lower than expected energy efficiency in Australia’s housing stock. The National Construction Code is attempting to address this in the 2022 revision by tightening up DTS requirements and boosting the minimum star rating to a proposed 7 stars out of 10 instead of the current 6 stars, however it is my thoughts that the same weaknesses in the software will exist, the same loop holes in the energy efficiency compliance system will remain and there will still be no mandated audits of the as-built product.
The NatHERS tools are somewhat useful for thermal simulations, but they are very limited. Energy efficiency and building health goes far beyond a star rating.
Give us a call if you want to know what I mean.