New H1 calc methods more compatible with Passive House The Building Code catches up

19 March 2023 by Jason Quinn

New Zealand Passive House designers are already deeply familiar with the energy efficiency calculation methods specified in European standards. There will be little learning curve for us as New Zealand adjusts to the new version of H1, the energy efficiency section of the NZ Building Code. The core standards now mostly reference European standards. This is a really good outcome. Passive House professionals have been introducing these standards into New Zealand industry for a decade because they are more accurate and deliver better outcomes. 

Before I walk through the changes and their implications, I want to make this point. Please don’t ignore engineering standards. They illustrate what is considered ‘good enough’ according to the very experienced professionals who volunteer thousands of hours of time. I acknowledge some bias can get baked in. Standards get voted on, so not every committee member will agree but this is the best process we have. 

The alternative is even more expense and delay. Think if you had to hire a structural engineer to design a simple timber-framed garage! You don’t, because timber framing standard NZS3604 exists. Be glad it does. It overbuilds that garage a little bit, creating some margin for error in exchange for not having to hire an expensive consultant for every single job.

Calculations must be done in accord with the current New Zealand standard and you and your client need to know that’s the case. We’ve come across some suppliers selling imported products where the performance data reflected American standards. This is not acceptable. Beware unwittingly including components not calculated according to H1 approved standards. You risk wasting time and money at the consenting stage if your design does not demonstrate compliance with H1 and your client can afford neither. Buy the standards, read and understand the standards and use them.

Window/door performance

These are now calculated in the NZ Building Code using the ISO10077-1 -2 Standard “Thermal performance of windows, doors and shutters—Calculation of thermal transmittance”, the same standard Passive House designers already use. The glass performance is now common too, referencing the EN673 Standard “Glass in building – Determination of thermal transmittance (U value) – Calculation method” and the EN410 Standard “Glass in building. Determination of luminous and solar characteristics of glazing”. 

There’s one difference: Passive House design includes the thermal bridge for window and door installation, whereas the Building Code ignores this. 

If you have tables of window and glass performance values for the purpose of Passive House modelling, you can now use these to show compliance with H1 also. I’ll discuss this in more detail next month.

Slab-on-ground performance

This is calculated using the same kind of finite element model as used for Passive House to determine the slab edge PSI value. However, the NZBC R-value used for compliance is different to the way PHPP calculates thermal performance.

This means you cannot use PHPP or slab R-values suitable for Passive House to calculate the NZBC R-values. You need to follow a different section of the engineering standard, ISO13370 Standard “Thermal performance of buildings — Heat transfer via the ground — Calculation methods” and ISO 10211 Standard “Thermal bridges in building construction — Heat flows and surface temperatures — Detailed calculations” to build a two-dimensional or three-dimensional finite element model to calculate the slab-on-ground steady state thermal performance as the slab R-value or use tables from manufacturers (or in H1/AS1 Appendix F from BRANZ).

[Image 3D validation model]

What about using ISO6946 instead of NZS4214 ‘Methods of determining the total thermal resistance of parts of buildings’? I’d argue that H1 permits this but your consenting authority may take a dissenting view. It is clear to me that you can’t use R-values from NZS4214 in computer models: it says that exact thing in the Standard. That means you can’t use them for the Verification Method, despite most everyone doing that. This has to get fixed, the question is how long it will take. My bet is ISO6946 will eventually be confirmed as an acceptable method of calculation. The best way for MBIE to resolve this would be deleting NZS4214 from H1 and replacing it with ISO6946.


Moisture and ventilation? 

The NZ Building Code is still not there yet. Australia has tweaked ANSI/ASHRAE Standard 160-2021 ‘Criteria for Moisture-Control Design Analysis in Buildings’ and issued it as DA07 Criteria for Moisture Control Design Analysis. I think New Zealand will end up adopting something similar to evaluate construction systems. 


For individual projects, I strongly recommend the Passive House fRSI system, as it’s a lot simpler and catches the core physics well. We’ve recently completed a cooperative study on this thanks to some additional funding from the NZGBC. Once we’ve sorted through the recommendations and implications we’ll release the findings.