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The confusion at the heart of the building code

·993 words
Author
Felix Andrews
Table of Contents
What are the goals of housing energy efficiency regulation? How does the current system deliver on those goals?

The concept of energy efficiency is pretty vague. It probably took shape after the 1973 oil crisis: we need to reduce our total energy usage, so as to be less dependent on oil imports. Then it morphed into something much broader…

Definitely something to do with trees and lightbulbs

Definitely something to do with trees and lightbulbs

Today, the following are commonly cited as goals of housing energy efficiency regulation:

  • reduce carbon emissions;
  • reduce peak load on the electricity grid, avoiding costly upgrades;
  • reduce running costs;
  • improve comfort and health.

Other benefits flow from these, such as mental health and productivity; the ability to cope with extreme weather events; and improved durability of a structure that remains free of mould.

Housing energy efficiency regulation in Australia targets average annual energy use per square metre. Unlike the goals listed above, average annual energy use in itself is not a meaningful goal. Of course, if we reduced energy use down to zero, we would meet all of the goals (ignoring embodied carbon for now). In fact, near-zero energy use can be achieved, for example with Passivhaus construction. But Australian housing is very far from that.

How does the building code deliver on specific, meaningful goals?

Reduce carbon emissions?
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If the goal was to reduce carbon emissions, the building code would regulate embodied carbon. Embodied carbon refers to emissions from constructing a building, including from materials like concrete and steel. According to the head of UNSW’s School of Built Environment,

“for any new building constructed in Australia today, we expect at least half of its total carbon footprint over its life will be embodied carbon, possibly even more."

And NatHERS, I argue, is biased towards concrete. Others agree: architect Matt Delroy-Carr says

“The rating system heavily favours masonry construction … The more concrete and bricks you put into the project, the higher the star rating, but inevitably, the higher the carbon footprint."

If the goal was to reduce carbon emissions, electricity use in sunny daytime hours would count far less than on winter nights. One third of all households and businesses in Australia have solar panels and this continues to grow. Even for households without solar panels, the emissions intensity of the electricity grid is less in daylight hours.

Generation in the National Electricity Market in the 24 hours from 16:00 22 May 2024 (source)

Generation in the National Electricity Market in the 24 hours from 16:00 22 May 2024 (source)

Specifically, this means that energy used for cooling is much less important than energy used for heating. Because the times when cooling is most needed are the times of peak solar generation. But currently, we simply add together total heating and cooling loads.

Furthermore, it makes no sense to use summer-generated solar excess (the easy problem) to offset heating demand on cloudy winter days or nights (the hard problem). Can we please stop talking about this as a “net zero house”?

Reduce peak load?
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Load at peak demand times is difficult to model in practice as it depends on occupant behaviour and specific thermostat settings and timing.

Reduce running costs?
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There is some precedent here: for existing houses the voluntary Residential Efficiency Scorecard gives a running cost star rating. Partly this is based on the efficiency of specific heating and cooling appliances. But also critical are air leaks and poorly installed insulation — major causes of energy waste in practice.

Arguably, the relevant measure for all the above goals is energy use per dwelling or per occupant, not energy use per square metre as is currently the case.

Improve comfort and health?
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If the goal was to improve thermal comfort, again we would recognise crucial factors like uncontrolled air leakage; completeness of insulation install; internal window coverings. This is from a cool climate perspective.

Perhaps a quantified measure of comfort like PMV could be used, as it often is for commercial buildings in Australia. This incorporates the Mean Radiant Temperature effects of any cold surfaces. Certainly the effect of cold concrete slab floors should be better accounted for.

On the health side, mechanical ventilation is important for air quality. And mould risk should be checked; crucially this requires that a realistic model of thermal bridging be used. Which is not currently the case.

What about Whole-of-Home?
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The new 2022 building code includes a Whole-of-Home (WoH) component, which estimates and regulates the energy required to run specific heating, cooling, hot water, and some other appliances. This seems to address some of the issues above:

Energy use is measured in terms of the societal cost of energy. This includes the following:
  • energy costs to the building user
  • costs to the environment in terms of greenhouse gas emissions
  • time-of-use impact on energy networks.

Sounds great! But: this does not apply in any way to the main NatHERS star rating (“NatHERS thermal”). The WoH component is an additional step. From what I have seen so far, the WoH component is relatively easy to comply with by specifying solar panels or efficient heating, cooling, and hot water systems. It may stop people from installing wasteful appliances. But in general I don’t think that it changes the way that houses are designed or built. I would be happy to be proven wrong.

Conclusion
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Housing energy efficiency regulation in Australia targets average annual energy use, per square metre. But that does not align well with meaningful goals like reducing carbon emissions or improving thermal comfort. We need more effective policies.

One way forward might be Anthony Wratten’s proposal to combine a Whole-of-Home rating with a measure of comfort/health, removing the star rating based on annual energy use. This would also need to be combined with a limit on embodied carbon.

In the absence of specific regulation, building designers can and should work towards meaningful goals anyway. For example, Powerhaus Engineering’s app can show modelled energy demand matched against solar generation.

Finally, the building code probably won’t achieve its goals without as-built verification.