Serious flaws remain embedded in the NZ Building Code with regard to modelling thermal performance. This is despite the recent, welcome, improvements. Performance-based building codes use one of two basic approaches. They can set absolute or relative energy performance targets. Unfortunately the NZ Code is based on the relative approach.
MBIE’s Building for Climate Change programme wants the industry to focus on reducing the amount of operational and embodied carbon in our buildings. To meet the climate change reduction programme the government has signed up to, the construction sector has to make significant absolute energy reductions. So let’s use the modelling methods that help us achieve that.
Demonstrating Code-compliant thermal performance by way of comparison to a reference building (NZBC H1 Verification Methods) delivers utterly flawed outcomes and the reference building approach needs to be removed. The industry (and the climate) would be much better served by using an absolute energy metric.
Let’s step through the issue with the reference building comparison*. How do you show the most improvement in thermal performance between your building and the reference building? By designing something hopelessly energy-inefficient. If the building’s form factor is high (complex shape, lots of corners), there’s a lot of glazing and lots of glass on the cold side of the house, those choices will be mirrored in the fictitious reference building. It’s easy to increase the R-value of some or all components and get a big percentage number to show your design’s relatively superior performance. What remains opaque is the almost certainly terrible absolute energy performance.
This graph is from the paper referenced below. It shows the Thermal Energy Demand Intensity which in Passive House is the 15 kWh/m2/year permitted for heating: this is the black dashed line labelled NZE-r (Net Zero Energy Ready). The four cases shown all have the same useful floor area (TFA) but different shapes. Using a reference building approach, there is a very large variation in the building’s energy use. The worse the reference, the better your planned building looks. Look how the expensive custom home with 50% savings (case 4) performs worse than the worst-case reference building with a simple form factor (case 1).
On the flip side, if the form factor is lower, the building footprint is relatively simple and sensible choices have been made in relation to the percentage and placement of glazed areas, it will be much harder to demonstrate an improvement on the reference building. However, the absolute energy performance will be far, far better than our first example.
The comparison to a reference building is based on the wrong measurement. It’s not relative thermal performance that matters, it’s absolute performance.
There’s another knock-on problem too: business-as-usual industry players are used to energy models that don’t have a relationship with reality and so therefore dismiss evidence of the energy savings that are possible if they improved building design.
This reference building distortion is not unique to us. It’s a problem in the US, Canada and Australia too. The Europeans do sometimes use representative building (confusingly, they call this a reference building), but their building codes require statistical linkage of this representative building to actual performance averages. Thus they avoid the distortions we’re saddled with.
An article has just appeared in the Energy and Buildings journal on this very problem, written by Canadian experts. You probably won’t have access to the full article, but the abstract is worth reading. Most pertinent:
It is shown that the RBA [reference building approach] has serious flaws. Most importantly, by creating a sliding scale, the RBA does not deliver net-zero energy performance, while incentivizing inefficient designs and poor energy modeling practices. Despite the regional focus of the data, the conclusions are applicable to the RBA in general. Based on the results, it is recommended that the use of the RBA in building energy codes and standards be discontinued. [emphasis mine]
Also worth amplifying is the authors’ observation about the ongoing cost of thermally inefficient buildings: “Decarbonizing low-performing buildings requires an unrealistically large supply of renewable energy, with costly generation and transmission infrastructure to support buildings as well as other sectors such as transportation.”
Sepehr Foroushani, Rob Bernhardt, Mark Bernhardt, “On the use of the reference building approach in modern building energy codes”, Energy and Buildings, Volume 256, 2022
* Most people reading this are aware that reference building method is used when one or more of the components don’t meet the schedule set out in the Code and it’s necessary to prove that overall the building’s thermal performance is still at least as good as the Code minimum. It is truly ironic that Passive House designers are forced to justify the use of superior Passive House-rated components and so sometimes resort to this method to gain building consent.