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NatHERS makes it hard to build small houses

·850 words
Author
Felix Andrews
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Or to put it another way, NatHERS lets big houses off the hook. In two ways. Because small houses naturally have more surface area; and energy use limits are per square metre, not per house.

Why do we limit energy use per square metre?
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NatHERS star ratings are used to define the minimum “energy efficiency” of houses in Australia. This is a limit on heating and cooling energy per square metre. It is not a limit on the energy required by the house as a whole. A study recently found that Australian homes are getting bigger and bigger, and it’s wiping out gains in energy efficiency. That is, while predicted heating and cooling energy per square metre has been going down with updates to the building code, the amount per house has been going up. Um, good work everyone?

Presumably, the reasoning for a per-square-metre limit is that bigger houses can accommodate more people. I imagine the designers of the scheme saying “It’s not fair to put the same energy budget on a 4-bedroom house as a 2-bedroom house.” However, back in reality, household sizes have gone down while house sizes have gone up.

New homes in Australia are some of the biggest in the world, averaging over 230m2. Housing in Australia is also notoriously expensive. But most people don’t know that construction in Australia is not expensive on a per square metre basis:

Average build cost (per m2) of eleven building types in USD. Source: Turner & Townsend International construction market survey 2023

Average build cost (per m2) of eleven building types in USD. Source: Turner & Townsend International construction market survey 2023

Compared to similar countries, we are building huge houses at low quality, and paying the price.

House size in NatHERS
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Here is a small house, made up of 3m x 3m rooms:

And a bigger house, the same but with larger 4.5m x 4.5m rooms:

And a tiny house, with roughly the same zone locations:

They share the same specifications:

  • Canberra climate zone with suburban exposure; north is up the page
  • Brick veneer walls, 2400mm high, with R2.5 insulation
  • Aluminium thermally-broken double glazed windows (ATB-004-01)
    • Windows are 30% of wall area
    • All fully openable
  • Two external doors
  • Pitched / Attic metal (hip) roof with R5.0 ceiling and R1.3 foil-faced roof blanket
  • Suspended timber floors with R2.5, 300mm enclosed subfloor
  • Eaves all around at wall height, projecting 600mm

They also have (almost) the same percentages and location of zone types:

  • Daytime 47% (including kitchen/living), north
  • Bedroom 32%, one third north, two thirds south
  • Unconditioned 21%, south

Here are the results:

Tiny Small Bigger
Floor area (m2) 42.8 85.8 181.6
NatHERS star rating 6.3β˜… 6.7β˜… 7.3β˜…
Heating (MJ / m2 . year) 153 145 103
Cooling (MJ / m2 . year) 56 35 21
Total energy use (MJ / year) 7083 12196 17483
Area Correction Factor 0.706 0.745 0.899

By increasing the floor area, the rating has gone up from 6.3 stars (fail) to 7.3 stars (pass). At the same time, the heating and cooling energy required by the house as a whole has increased dramatically. And of course, the amount of embodied carbon in the building has increased too.

Here are the model files for these houses.

Adjusting for house size
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Why are the per-square-metre heating and cooling loads higher in the smaller houses? Mostly because they have more external wall area compared to floor area. This is just basic geometry. I should note though that the issue is more complicated than just geometry: bigger buildings are not just expanded versions of smaller buildings.

Those involved in NatHERS and the building code are aware of this surface area issue. To compensate for it, the modelled heating and cooling loads are multiplied by an area correction factor. In a paper Area Correction Factors in AccuRate v1.1.4.1, Chen (2012) explains:

The area correction factors were obtained by simulating the energy use of an idealised house using NatHERS and changing the floor plan size from 49 m2 to 625 m2 in six steps. Assuming that this house plan should receive the same rating regardless of floor size ...
The idealized house design used in developing the area correction factors (from Isaacs (2004))

The idealized house design used in developing the area correction factors (from Isaacs (2004))

The details are glossed over, other than “After feedback from industry was considered, the area correction factors evolved into dimensionless factors in 2005”. The actual area correction factor is defined by a mathematical function of the Net Conditioned Floor Area and is different in each climate zone. In Canberra (climate zone 24) it looks like this:

Area Correction Factor as defined in NatHERS climate zone 24 (a 5th-order polynomial!)

Area Correction Factor as defined in NatHERS climate zone 24 (a 5th-order polynomial!)

In all cases, if the conditioned area is less than 50m2, it is set to 50m2. That is, no further adjustment is made for houses with less than 50m2 of conditioned floor area.

But as can be seen in the results above, the area correction as currently implemented does not seem to adjust well for the effects of house size. In effect, we are incentivising larger houses.

And in any case, as long as the building code limits are per-square-metre, there is little prospect of overall energy use (or embodied carbon) in the sector coming down.