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What’s the difference between low carbon and low energy buildings?

mike rainbow Ark Resources
Ark Resources Southbank Melbourne project. Developer: Hickory Architect: Elenberg Fraser

If you ask most people what the difference is between low carbon and low energy buildings they generally give you a blank stare.

I discovered that the confusion between these two concepts really gets Mike Rainbow’s goat. This is likely because Rainbow, associate principal of Ark Resources, has spent most of his professional career designing buildings that require less energy to keep them going. 

“We used to talk in the 1990s about low-energy buildings. The concept of sustainability came to the fore later after which the term ‘low carbon’ became more prominent. Nowadays, a lot of conversations about buildings are framed in terms of the assessment of carbon performance. Yes, we talk about energy rating but is usually in terms of X number of kilograms of carbon prevented from being emitted, rather than in terms of kilowatt-hours,” Rainbow explained to me in an interview.

I asked him why he thought this matters.

“Often they are used interchangeably, but there are differences. It’s possible to have a low carbon building that uses a lot of energy, it’s just that it’s renewable energy. We’ve forgotten the importance of low-energy design.

“If you take this logic to its extreme you can create a zero carbon building – which will be a prerequisite of a Green Star 6 star certification from 2020 – but it doesn’t prevent you from having an inefficient building.

“That’s the problem. There are some backstops for those standards, but still. It’s possible to be carbon neutral with a glass box if powered by biomass boilers or other low carbon sources, so it’s not a great substitute for minimum energy performance.”

Mike-Rainbow

Mike Rainbow

I agreed that people have largely forgotten about energy efficiency.

“To an extent,” Rainbow says. “If we have equal focus on low energy design, then that is much better. But it isn’t necessarily supported by standards that allow purchasing of carbon offsets.”

We were of one mind. I told him that when I was talking to my first year architecture students no long ago, I asked them what they thought a low carbon building was. They immediately started talking about technology like solar panels. The thing is, by minimising the amount of energy building needs, the cost of whatever renewable energy supply you add to it will be lower.

“Yes. I guess one of the risks of thinking exclusively about carbon is that it takes you away from the immediacy of the building and what’s happening on the site. It makes you less aspirational with the design. What we must do is to take a fabric-first approach.”

He said it was worth noting the following “subtle perversions”:

  • NatHERS is an energy intensity rating – that what the E stands for.
  • NABERS Energy is a carbon intensity rating (indexed by state).
  • NCC Section J Energy Efficiency 2016 relates to energy efficiency.
  • NCC Section J Energy Efficiency 2019 (draft) relates to carbon efficiency referenced against an energy efficiency baseline.
  • Green Star Energy category is a complex blend of carbon and energy alternative compliance criteria.

“All buildings’ actual carbon intensities improve automatically year on year as the grid decarbonises and building stock shifts away from gas to all-electric heating. By 2050 the grid is meant to be fully renewable so all buildings, good and bad, will trend towards carbon-neutrality and become increasingly undifferentiated by their carbon intensities.”

So why count carbon if it’s energy we’re trying to save?

“The most valuable potential aspect of using carbon emissions as a metric into the future would be if we were to account for the time-varying carbon intensity of grid-power, that is, very low in the middle of the day when solar is maxing out, very high at night when baseload coal dominates.”

So carbon emissions from buildings vary according to the time of day. I’d never considered that as a useful metric.

“In both energy and carbon terms, we currently consider 1 kWh generated by PV to be identical to 1 kWh of power saved by an LED light. In real hourly emissions factor terms, the LED is better since it’s more likely operating after dark.”

So spend your money on LEDs first, then PVs. And I guess the same applies to any energy-saving measure on a cost-per-watt basis.

David Thorpe is the author of Passive Solar Architecture Pocket Reference, Solar Technology and Sustainable Home Refurbishment.

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