At the time of writing (as winter bites) it is being reported on the radio that studies by BRANZ and others show that the temperatures in New Zealand’s homes are too cold, especially children’s bedrooms, for our long-term health. It is suggested that this is exacerbated at present by the high cost of heating, and also by the need for more thermal insulation. Even so, the studies did find that our homes have become warmer over time. Comment from the Green Building Council said that the higher standards being applied to our newer dwellings have given more consistency of temperature across the day for them but more work is needed. Did the “…becoming warmer over time.” come about because of increased numbers of modern homes being built, rather than by improvement to the worst performing dwellings?

Yes, mandating thermal insulation and double glazing to our older housing stock would improve the statistics, but families live in their individual house, not the ‘average house’. It could be argued over the coffee cups that perhaps any improving should be directed to the poorest performing houses first as an alternative to condemning them — a mammoth logistical and political project, and where would the money come from? Of course, a genuinely adequate supply of housing across the board would allow market forces to dispose of the worst-case cold homes.

In the fourth paragraph of my EBOSS blog of May 2026 Thermal Performance: Medium-Density Residential Units, I refer to a New Zealand Green Building Council and Auckland Council’s joint letter of 21 November 2025 to the Building and Construction Minister Hon. Chris Penk, as reported in NZGBC’s media release of 24 November 2025 Councils Call for Action on Sweltering New Homes. This was published around six months ago in late November last year, ie., in summer.

How come there are complaints that our housing is too cold but also that it is too hot when we have a single New Zealand Building Code, Clause H1 (Energy Efficiency) which is applied equally to all Residential Housing? (Yes, older existing dwellings would have complied with earlier standards, but there were still across the board standards common to all houses built at that time.) Is there something wrong with the Building Code? Does the solution lie elsewhere, rather than applying the simple case of going along with the quick-fix of mechanical heating and cooling, with its inevitable financial and environmental costs? 

At the moment NZBC-H1 divides the country into six climate zones. To be more nuanced would very rapidly complicate compliance to the point where it would be unmanageable for those trying to demonstrate compliance. Even within small districts New Zealand’s micro-climates are so variable, especially when it comes to the thermal performance of individual dwellings. It must always be remembered that NZBC-H1 is a baseline, not a target. Allowing there are many other requirements to be considered, there is no sanction if NZBC-H1 is exceeded.

NZBC-H1/AS1 is the Acceptable Method of showing compliance of the thermal envelope with the Building Code by using the Calculation Method. NZBC-H1/VM1 is the Verification Method of demonstrating compliance of the thermal envelope by the Modelling Method. This requires that the sum of the calculated annual heating load and the annual cooling load of the Proposed Building does not exceed that of the threshold Reference Building. The two buildings are to be assessed as like-for-like at a general level, but with the Building Code specifying specific variable items such as the windows. Refer to my EBOSS blog of March 2026 Will the Summer overheating of Medium-Density Housing be Resolved by the 6th Edition of NZBC-H1?

There are two basic approaches to modelling the thermal performance, and hence the comfort level, of a built dwelling. (It is important to note that, as with all types of modelling, the temperature figures produced are NOT a prediction of actual performance to be experience in the real-world building.) 
One is to model the thermal characteristics of the Thermal Envelope of the building so as to determine the balance of the outward and inward heat flow through its specific construction.

The other is to take the particular materials and constructions of the whole dwelling (both exterior and interior elements and their interrelationships along with controlled and permanent internal openings) plus the exterior environmental influences, so as to then simulate dynamically the thermal performance of the home before any heating or cooling is activated. The gathered temperature data is an objective baseline from which the dwelling design can be tweaked as desired, and also it can be effectively used as a detailed Brief for the heating/cooling/ventilation systems providers. 

Refer to my EBOSS Blog of April 2026, Thermal Performance: Comparing Energy Balance and Dynamic Simulation which sets out the differences between dealing only with the energy flow through the thermal envelope on a monthly basis (all that is needed for NZBC-H1 compliance), compared with the real-time (hourly) simulation of the passive thermal behaviour of the interior spaces of the home before mechanical heating and cooling systems are factored in as correction features.

BRANZ’s ‘Level’ Bulletin of October 2022, 'Passive Design – Thermal Simulation' briefly introduces the advantages and benefits of using dynamic multi-volume hourly passive thermal simulation to objectively evaluate dwellings and makes particular reference to AccuRateNZ. AccuRateNZ was brought to New Zealand by EECA in the late 2000s for their HERS (Home Energy Rating Scheme). I refer the reader to my various EBOSS Blogs on the origin and usefulness of this software which arrived much too early for the public to accept. Even so, the research and recommendations of that time have informed our perception of the thermal performance of our homes ever since. BRANZ still uses AccuRateNZ (as I and others also do) to inform objective research into our housing. Since the beginning of the 20teens I have been using the software primarily to simulate the hourly interior thermal performance of standalone homes.

AccuRateNZ was referred to by name in the New Zealand Standard NZS 4218:2009 (Thermal Insulation – Housing and Small Buildings). Up until the fourth Edition of the Homestar Technical Manual, AccuRateNZ could be used by HomeStar Assessors to demonstrate compliance with the EHC-1 Space Heating Credit.

It is my opinion that if Government wants to seriously tackle the heating, cooling and ventilation problems with the thermal performance of our existing and new housing stock then they should give proper consideration to bringing back AccuRateNZ from EECA/MBIE’s dusty backroom shelf. With the various bells and whistles Australia has been adding to AccuRate and its Chenath computational engine since the turn of the 2000s, the software could be released to the architectural and mechanical design communities. The major firms, residential developers and group housing companies would then be able to develop generic and specific construction details, building elements and procedures to help minimise the difficulties of the heating and cooling problems that are still with us, and growing.