Australia has a star rating system, administered by the Federal Government, known as NatHERS. Stars sound very clear and positive. However, there is a bit of confusion about the star ratings produced by NatHERS.

Importantly, NatHERS assumes something that is virtually never tested.

The NatHERS scheme has the long title “Nationwide House Energy Rating Scheme” and its website is administered by the Department of Industry, Innovation and Science.

Many people will know that 6 stars is the minimum requirement for new houses, and that 10 stars is amazing. In fact, the government website says that “occupants of a 10 star rated home are unlikely to need any artificial cooling or heating.”

I’m not sure about that. In any event, 10 stars is supposed to be better than 6 stars.

NatHERS has a software program that takes into account a number of things that will affect the thermal performance of a building (how comfortable it is inside and how much energy is used to achieve that comfort).

It is a fairly rudimentary program compared to some other international software packages.  It produces a very short report.

It also assumes things that are not tested by law.

One of the very important assumptions that is made in this scheme is how airtight the building is.

Airtightness is important, together with other elements of good design, so that you can keep a stable temperature inside your home without excessive heating or cooling systems. It’s important so that uncomfortably hot air or cold air are kept out.

While some people are frightened by the term “airtightness” (because they think it will be claustrophobic, or will encourage mould growth), airtightness is crucial for building performance along with proper systems for ventilation including openable windows.

For human comfort and health, ventilation needs to be controlled, temperatures need to be controlled, and fresh air needs to be maintained – all at the same time.

This means that it’s just not good enough in many parts of Australia to have buildings that are wind tunnels.

In Melbourne, for example, we need to keep warm for much of the year. This means we need to keep the cold out of our buildings for well over half of the year. In the same way that we as humans go outside with socks, shoes, and coats to protect us from the cold wind, an airtight seal helps to protect our buildings from the cold outside our building envelope.

It is actually the same concept when it’s hot outside. A well-designed, well-sealed building can keep people cooler inside and keep the heat out as well.

I was shocked when I first saw the level of detail that goes into building physics calculations of a certified passive house. Some 30 pages of spreadsheets are produced by the International program called the “PHPP” (passive house planning package).

NatHERS has a more basic calculation method than the PHPP.  This is evident by the report it produces.

Accordingly, one just can’t compare a sophisticated modelling tool like the PHPP with a much more primitive one.

So don’t be impressed if a passive house comes out as 9 stars on NatHERS, because a very poorly performing building can also be given 9 stars on such a basic tool. It is like doing a primary school test for English vs a university level exam. It is easy for a university student to get 10/10 for a grade 4 English test.  The primary school test just doesn’t assess higher level standards.

Leaving aside complexity differences, there is one crucial difference between a NatHERS assessment and a PHPP one: airtightness.

The thermal efficiency results produced by the PHPP modelling program depend on one crucial final test of any compliant passive house building: it has to be very well sealed. It has to pass a stringent pressure test called a “blower door test.”

Airtightness is something that can be tested quickly and easily on a physical building: you do a test, you get a result. Yes, you can do slightly different types of tests, but they will give you some result of how much air is leaking from the building.

A recent CSIRO report highlighted this issue by giving us real assessments of the airtightness of buildings in Australia: the Housing Energy Efficiency Inspections Report from December 2015. According to this report, 15 air changes per hour at a pressure of 50 pascals is assumed by NatHERS for its rating calculations. For short, this is described as 15 ACH@50Pa, or simply 15.

This report mainly reviewed a small number of houses around the country which were less than three years old – so they would have to be 6 star houses as a minimum, because that is our current mandatory standard.

The testing of the Melbourne houses is not really a comparable result because the houses were up to 10 years old and a 4 and 5 star standard.

Also, the number tested overall was quite small, and based on volunteers, which would skew the figures in terms of national averages. The writers make the comment that some of the volunteers had bespoke, high quality builds, making them perhaps more likely to be well-sealed.

To contrast the energy efficiency modelled by NatHERS with that modelled by the International Passive House Institute, it is interesting to look at the different airtightness approaches.

The Passive House Standard requires a 0.6.

NatHERS assumes 15.

That means the NatHERS house assumption is 25 times as leaky as the Passive House Standard requirement.

Put another way, the Certified Passive House is over 25 times better sealed than that assumed by NatHERS.

What did the Australian homes who volunteered for the CSIRO report achieve on this sort of test?

Averages reported were: Canberra 14, Hobart 8, Perth 25, Sydney 21, Adelaide 9, Brisbane 11, Melbourne 20. All houses averaged 15.4.

But averages (and medians) can be misleading. Many of the houses tested blew more than 15. The graphs in the report are not highly detailed, but it seems as though over more than half of the houses blew higher than 15.

A key point that can be made is that your energy rating assumes an airtightness result after construction that is quite likely not to be correct, or perhaps as likely as not to be incorrect. You won’t know.

So when you say “I bought a 6 star house” what does it really mean? If your building leaks, it is not the star rating predicted.

And how do you get a good result on your next building? The CSIRO report says at page 11 “General build quality and attention to detail seem to be significant factors…Houses with uPVC window frames recorded much lower air change rates than most other houses.”

But how do you know your building is airtight?

There’s only one way to know, and that’s to get it tested.

The report recommends that there should be “specific air tightness requirements in the NCC,” but that ensuring compliance could involve “a random selection of newly built houses” being tested. Further, “NatHERS could allow high performing houses to receive higher star ratings by incorporating certified air pressure results into NatHERS calculations. Currently, houses that have achieved good air infiltration results get no star rating benefit from this.”

Next time you commission a new house, you might like to consider instructing the builder to get a blower door test done. Even better, ask for a specific result to be achieved. Why stop at 15? Ask for 10, five, or maybe even the passivhaus standard of 0.6.

That is likely to make your house better, assuming that all other design elements like ventilation, orientation, windows, insulation and thermal bridges have also been given some attention, as they are given attention for the Passive House Standard.

And on the other hand, if certified Passive Houses had all their benefits calculated properly on NatHERS, including the blower door test result, could we be looking at 15 or 20 star houses?

Now that would be literally “off the scale.”

  • Hi Fiona,

    Thanks for your excellent article. As a developer of Manufactured Building Systems (including prefab modular buildings) I have long lamented the inadequacies of NatHERS, specifically because it does not truly reflect air tightness> For example, I can create highly energy efficient buildings that have little thermal mass. They don't need it, but our system requires it. I say to my clients "just imagine an Esky". It is a highly insulated airtight box. Add to that controlled temperature air movement and very quickly the concept is grasped. Hopefully our regulators will wake up soon, because this understanding can lead to highly energy efficient buildings with much lower thermal mass and much lower embodies energy. They can also be much quicker to build, which is another energy saving.

    • if you are unhappy with the current building standards, I encourage you to submit your Proposal for Change to the NCC. See their website for the proforma or write to NatHERS.
      they are supposed to take notice well informed public submissions..

    • Thank you Andrew for your feedback. It sounds like you are doing excellent work. Thermal mass is one of Australia's myths and legends I agree. I used to "believe" in it until I started asking for calculations as to how much thermal mass is required, what if you don't face North, etc. You are right about its limitations. The Passive House Standard doesn't focus on thermal mass which is really interesting when you come from the Australian "passive solar" rhetoric. Good point about the benefits of fast building too.
      In terms of people understanding, when our first Superpod was built the tradespeople suddenly "believed". A heat wave outside? The building not even commissioned yet massively cooler inside? The results speak for themselves.

  • Dear Fiona,
    A good article. The PassivHaus standard is a great international benchmark and many Australians are exploring its adoption. However, many researchers have called into question the adoption of a very-cold climate system within temperate climates. Even in Tasmania, windows can be left open all year round in a well designed 7 to 9.5 star house. But a few pointers – You can have a very efficient home that is not to the PassivHaus standard. A key point to remember is that the very low ACH rate specified in the PH standard requires a mechanical ventilation system that runs 24/7 to remove stale air and provide fresh air. These systems require regular tuning and maintenance, which in many post occupancy studies has been found to be not occurring, significantly impacting on human health and building energy use. The NCC does include requirements for draft proofing and building sealing, which also includes references to Australian Standards. The problem here is Compliance. A primary objective of the current government is for less regulation, so Compliance is going to be an even greater long-term challenge.
    BUT I do agree whole heartedly that many Australian homes need to be 'tighter" and simple things like NO VENTED DOWNLIGHTS, no vented bathroom heat lamps, no cavity sliding doors, no fire-boxes and well sealed doors between conditioned and unconditioned/outdoors are a must.

    • Thanks Mark for your feedback.

      I agree with you that overregulation is a problem in this country. As a lawyer who practises in the regulatory world, I would suggest that one of the issues we face is regulating the wrong things and ignoring the more important things.
      Your concerns about the Passive House Standard as only being relevant to a very cold climate have been well addressed internationally. .You might like Elrond Burrell's blog/article where he interviewed me inter alia on the benefits of passive house in Australia. Just step inside our Superpod during a heat wave and you will immediately feel the benefits of a passivhaus without active airconditioning.
      Yes I agree that you can have all sorts of levels of "good design" in a house. Some houses are better than others. In my view a wholistic system that is well calculated and well documented for any climate, like the Passive House Standard, is hard to beat. I will look at other designed systems and compare them to that. "Passive solar" principles are not that rigorous.
      Your concerns about maintenance of an air ventilation system can't be viewed in a vacuum. You have to take into account everything wholistically. Standard heating and cooling units require maintenance, and we all accept that. I would much rather have a passive house with a simple heat recovery unit and virtually no active heating, than a non-passive drafty mouldy house with hot dry air blasting all winter.
      I don't see much point in Standards if there is no compliance. That's what my article and the CSIRO report is about. Compliance equals reality, and that is what we all live with. We don't live with hypothetical Standards. We live with physical buildings.

  • Thanks Fiona, an interesting article. As someone who has been involved in the air pressure testing of a large number of commercial type buildings, I would suggest that the problems are even greater in this sector – not least because these buildings often require year round conditioning.
    I suspect there is now sufficient evidence in Australia and from around the world to justify mandating leakage rates and pressure testing, though with the new 3 year BCA/NCC cycle I suspect it will take a while.
    Who will lead the charge?

  • HI Joan – I can't make this comment post as a reply. Thanks for your suggestion. Fiona

  • Dear all. Just a small but important "flag" from an ex-director of the Swiss Minergie certification body for low-E and Passivhaus buildings: Virtually all of the "a-little-bit-more-air-tight" buildings we implemented throughout the 1980ies and 1990ies turned out to suffer from mould challenges (with associated respiratory problems by the occupants) – hence these "better" buildings became unhealthy buildings. So please, EITHER stay with terribly leaky buildings OR ELSE go to seriously air-tight buildings (ACH of less than 1.5 at 50 Pa) – everything else in between ended up with mould. PS: Today's distributed, ductless Energy Recovery Ventilation (ERV) systems from Europe such as the LUNOS line of products are rather appropriate, super-efficient and low-maintenance – no worries.

    • This is a great comment Andreas and very pertinent, as the Australian building industry is only just beginning to understand all the areas of buiding science such as condensation, mould, moisture etc. I also am concerned that this push for air-tightness is being made without due regard to those issues, and a half-assed approach will bring more problems than it solves.

    • Hi Andreas
      There's quite a bit on the net about mould, and I have written about it on sourceable. I disagree that the solution to mould is EITHER leaky or seriously air-tight. You need a properly designed building. Leaky buildings can be mouldy and are in this country. Seriously air-tight buildings can be mouldy if they are full of thermal bridges.

      You need a proper wholistic design and system, like the Passive House Standard design. Which includes air ventilation units, as you note.


  • Thank you for a most informative article -where do we go from here – there is so much that needs addressing but nothing seems to change -is it possible to incorporate change into the NatHERS system to incorporate these observations
    -at least we have started to delve into the science behind NatHERS to develop a clearer understanding which is excellent -thanks again

    • Hi Mike
      Perhaps one thing we can do is be more open about the assumptions behind modelling systems. If those assumptions are incorrect, and never tested, then our predicted results will not be correct. The more we talk about the impact of building design elements the better! Maybe then – if people are better educated – they will start to build better too.

  • Lets get this in perspective and accept what the concept behind NatHERS is intended to provide. Don't get me wrong as an accredited NatHERS assessor I am well aware of some of the deficiencies in the NatHERS software. However we do need to start somewhere in setting a line in the sand where we need new buildings and renovations to be, in order to improve energy efficiencies. NatHERS is a cost effective tool that helps achieve that goal. Sure we could look at introducing blower tests on all new dwellings and renovations and provide compressive thermal reports but the cost of this exercise would just be unaffordable for a standard new home or renovation.

    • Thanks Ian Fry – My point is largely that we should be dealing with the truth, not untested incorrect assumptions, whether on NatHERS or other similar programs.

      For example, you could have star ratings based on different airtightness results, including more real results, and the star ratings should change accordingly.

  • While there are many merits to the PassivHaus certification, your article seems to suggest that it is superior to NatHERS simply because it has more stringent parameters around which airtightness is measured and tested. While NatHERS is far from a perfect rating scheme, I am of the opinion this is not an appropriate comparison to make. There are a number of factors that can affect the thermal performance of a building – to look at just one of these in isolation is a flawed methodology for critique.

    • I agree sid 2.0 you should not look at one item in isolation.
      I have written in sourceable about a range of different items for building performance.
      This article is about airtightness. But see my second last paragraph.

  • Fiona, a great article and I'm glad that you're highlighting the issue.

  • Fiona you seem to assume that the underlying NatHERS engine is a “primitive” and “rudimentary” tool based simply on the size of the summary reports that some of the NatHERS certified tools are able to produce. This is simply incorrect. In reality, the underlying Chenath engine that all NatHERS software use is regarded internationally as one of the most sophisticated thermal modelling engines for buildings. It has been developed by CSIRO over many decades and indeed in some areas (like ground slab modelling) it is used as the international benchmark. A full NatHERS report for a typical house will easily run to 30 pages and in addition you will be provided with 30 minutely temperature data for every zone, for every day of the year. For a house with 10 zones this is 175,200 data points!
    Also, you seem to misunderstand how the ten star rating system works. NatHERS rates a house design on its ability to keep within predefined comfort bands with predefined conditions and calculates how much additional energy is required to keep the house within those comfort bands. The less additional energy required the higher the star rating. A 10 star rating means the house needs zero or close to zero (depending on the climate zone) additional heating or cooling energy. So, yes a 10 star house is not likely to need a heating or cooling system. Achieving 10 stars is no easy task and in some climate zones of Australia is virtually impossible. Even 9 stars is a tough ask and saying that a very poorly performing building could achieve 9 stars is nonsense. A 15 or 20 star house is an impossibility as you cannot have negative energy consumption. Its like saying the fuel efficiency of a car is minus 10 litres/100kms!

    • Thanks Michael for your clarification.
      I was referring to the kind of report that produces a 6 star (or above) rating for a residential house. My report on my latest house was about 2 pages. I know the data that went into it. That's the kind of report that the CSIRO is talking about when they discuss star ratings for housing in the research paper I referred to. I seriously doubt that the 30 page report you talk about is what is required for our star rating system – seeing that the government (building surveyor) seems to rely on a 2 page report. Also, you are surely aware of the level of data required to input into the program to get a 6+ star rating – it's not much at all. As for your comments about the ten star rating system, I do believe I understand it. As my article says, airtightness is assumed not tested. That's the point of my article (and the CSIRO report). The star rating system is like primary school testing vs university testing. I know you can't get a 20 star rating on NatHERS because the tool doesn't measure the sophisticated improvements that can be made in buildings – it doesn't measure the improvements that can be made in a sealed building that is properly designed. That, also, is the point of my article.

  • Fiona, as many others have mentioned, a great article for opening discussion on a very important industry topic.
    I question your repeated mentions of uPVC windows, which is growing in popularity due to price:thermal performance and heavy marketing, while ignoring environmental, air quality, durability, or aesthetic measures. For more information on uPVC, skip the manufacturer's spin, for example:
    Greenpeace report:
    UK ecological group:
    US/Slovenian University study:
    — timber appears here to be best performing product and way ahead environmentally when compared to uPVC or Aluminium.

    • Thanks Karlos for your information. i will read these with interest. Funnily enough I just came back from Germany where I did tours of Upvc factories. I was very impressed with the environmental credentials. But a big issue with timber in Australia as well as overseas is the cost. If you want a passivhaus compliant window you will pay a lot more for timber than Upvc. People won't spend the money. Which means, if they want timber because of so-called "environmental credentials", they may well buy a cheaper poor performing window, and then be losing a lot more energy in the decades the building operates. Not to mention maintenance costs to the environment. A whole life cycle analysis, also taking into account what people will actually spend, can be a very useful thing to do. We have made a film which will be launched online soon. The carpenter who worked on our first Superpod was struck by the long life credentials and energy efficiency of the Upvc compared with the timber windows he typically instals which rot and require replacement in, say, 10 years. So you have to then look at the cost to the environment of at least 2 timber windows vs one Upvc window – and the cost of energy too.

  • In Australia the "documentation" can be produced. This doesn't equate to everyday practice on a building site. Change of actual building practices needs to take place for these "unchecked" report recommendations to be a built reality. One way is through education, the other legislation. Time will tell…

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