H1 energy modelling is inaccurate and creating perverse outcomes How soon can we all move to predictive energy models?

17 March 2024 by Jason Quinn

Sometimes energy modelling is a waste of time undertaken solely for the purposes of compliance—a bureaucratic box-ticking exercise. 

What? I’m the guy that argues that the single most important, and indeed defining, principle of Passive House is energy modelling at design stage. I’ve even said that it’s the ONLY principle that is completely essential in every Passive House project.

I believe both these things because (1) there is energy modelling that is predictive (the kind that Passive House designers do). Specifically, it predicts how much energy will be required to keep a building at a specific temperature given certain boundary conditions. The software Passive House designers use, PHPP, has been proven by multiple studies to be accurate and useful. If built in accordance with the plans and specifications, and operated according to intentions, a building’s energy efficiency and thermal performance in reality will match up with the model’s predictions*.

Then we have (2) the energy modelling that happens on Building Code-minimum projects in New Zealand for the purpose of complying with H1 energy requirements. The key difference is that is not predictive. It obliquely says so right in H1/VMI, in D.1.2.5 “The results of the thermal modelling should not be construed as a guarantee of the actual energy use of the building.”

NZS4214:2006 edges toward a clearer disclaimer in 1.4  Exclusions:

The Brits, to their credit, spell out the distinction very clearly (RICS Whole life carbon assessment for the built environment 2nd edition):

(Their Part L is similar to H1 in that it’s for the purpose of Code compliance.)

The industry in general needs to understand the difference, not just the Passive House geeks. I have a lot of sympathy for building owners that have to pay for energy modelling in order to demonstrate compliance, but which beyond that is of nil benefit. However this situation is going to change: MBIE’s Building for Climate Change programme has indicated that predictive thermal modelling will be a future requirement. 

Currently, building scientists understand the distinction between energy modelling that is predictive and energy modelling which is not, but others do not, even people in the industry that need to know. An H1 energy model for a large office building say, may cost $20-30K. I understand why a building owner might be very unhappy that the results do not align at all with how their building actually operates.

Why is H1 energy modelling so inaccurate?

The problem with the H1 energy modelling is that it intentionally excludes important thermal elements that impact performance in order to simplify the modelling. It exists in order to compare the building design to a reference building. You could call it comparative energy modelling, as opposed to predictive energy modelling. 

H1 energy modelling isn’t required to be accurate—that is, to accord with reality. It just has to be done in a certain way. Then the box can be ticked and building can proceed, even if in reality the building does not meet what was intended to be minimum Building Code requirements. (This is discussed also in this article, about timber fractions in walls.)

The simplified model has led to bad habits and some truly awful design details. It’s creating perverse and unintended consequences as designers seek to save upfront costs for their clients (even as it might cost them big down the track). Making the reference building worse is one typical way the requirements can be gamed. I’ve seen examples of theoretical reference buildings that the laws of physics would prevent them from being actually constructed. I recall one reference building that had glazing so badly performing that it was actually impossible to find such a product for sale. That’s really quite creative manipulation. 

Consider also the drawings for a new school that crossed my desk recently. The walls were insulated R2+ up to the ceiling tiles; then there was a 500-1000mm gap with absolutely no insulation until the R6.6 roof insulation began. How could something this absurd come into being? Because that external wall area above the ceiling tiles is excluded by some interpretations of H1. For the record, I do not agree with those interpretations… 

This detail means the building will not perform anything like the H1 energy model. There’s also a reasonable likelihood of moisture problems. Thankfully for the small children it is designed to house, it’s still in design stage and if my very frank advice is heeded, it will not be built like this. No school, especially a newly built one, should be damp and mouldy.

Horrible practices like this became standard practice at least in part because NZS4218 previously used internal measurements and neglected losses at junctions. Withdrawing NZS4218 hasn’t stopped this sort of drawing showing up in my email; its awful legacy lives on. H1 still allows junctions without any insulation that create significant thermal bridges. It overlooks steel beams as long as they are tucked into corner junctions that are excluded from the model. It also ignores ventilation losses and window insulation thermal losses— while improvements to airtightness cannot be included. Fundamentally H1 modelling is designed only to show compliance with the Building Code. And the limitations of the simplified model and the ways that it is being gamed means it is not really even fit to actually demonstrate that.

The sooner the Building Code stops incentivising people to do dumb things that result in buildings not fit for purpose, the better. I’d like to see timeframes on when we can expect MBIE to require predictive energy modelling. That might be unrealistic in the current political environment, but we need to get on with upskilling the industry. Architects and designers need greater understanding about building performance. That’s one of the reasons that Sustainable Engineering Ltd has moved into delivering training programmes; our consulting and certifying work really shows us where the knowledge gaps are. The H1 improvements have given people a nudge already. Now what we need is a transition plan away from modelling to demonstrate compliance to modelling that actually predicts how buildings perform. For many, it will be an a-ha! moment, to realise that is possible to design for known performance. And with that, the unintended consequences prompted by the Building Code will go away.



*It can still go wonky, which comes down to people: say an occupant likes the indoor temperature to be way hotter or cooler than the model assumed. Some readers will have heard my story about the lady in NE USA (think deep snow-in-winter type climate) who left her Passive House home’s back door open all day for her dog to trot in and out at will, marvelling at how low her heating bill stayed.)