In issue #193 (December 2022), of BRANZ’s bi-monthly magazine ‘BUILD’ there is an article titled “Make Way For Change”. This was re-printed in 'Building Business' issues #108 of April 2023. The article advises that; “The government has flagged significant upcoming changes in its Building for climate change programme and other workstreams”, and goes on to detail what these changes might include.

Under the heading “What’s coming” are four bullet points of topics that could be on the horizon, followed by the statement that; “The H1/AS1 schedule method of determining insulation needs will likely be withdrawn, and modelling will be required for either embodied carbon or energy use or potentially both”, and goes on to further state that; “Computer modelling for things like energy use and carbon footprint will eventually be the norm for new buildings, including houses.”

Here I would like to address the supposed need to do away with the Schedule Method for showing compliance with NZ Building Code - Clause H1. Of course it would be nice to objectively model in detail the thermal performance of each standalone house, (let alone each dwelling in terrace and apartment developments, which are all homes), before it is built, but is that even remotely practical or necessary if the statement “..will eventually be the norm..” is to be achieved? To not treat each dwelling individually — i.e. group houses by type under an umbrella analysis — how is that not essentially the Schedule Method?

In my opinion it is essential to have more accurate and objective tools for the passive thermal design of our dwellings, as I discuss in my blog of June 2018 ‘Can Thermal Performance Be Measured Objectively?’, but will this be achieved by removing the Schedule Method to rely only on thermal simulation modelling, even for simple standard buildings?

For each era of building the design, the majority of New Zealand houses are generically similar with essentially similar passive thermal performances (before heating and cooling is introduced). This is one of the primary reasons why ECCA’s HERS (Home Energy Rating Scheme) was put in abeyance when the rating score for most older homes was down to as low of two stars out of a possible of ten. Who wants to pay good money to be told that? Even so, all the promotion for healthy homes over the past decade arise directly from the extensive research undertaken in the 2000s to find suitable thermal simulation software tailored for the housing market. The most suitable software, AccuRate, was found in Australia, where it is still in use, and adapted here for our local conditions. There was comprehensive preparation for the release to the design professionals and the public, as well as the training of Assessors. Government/EECA made a very large investment in HERS which is probably sitting on a shelf somewhere waiting to be dusted-off.

Any comprehensive passive thermal performance analysis of each specific dwelling will need to be completed, and adjustments made to the design, BEFORE construction begins. While a theoretical passive performance can easily be obtained there is no guarantee that this will be achieved for the built structure. Also how, for example, is the future growth of a shading tree or the erection of a three unit three storey suburban infill block on the sunny boundaries to be later incorporated into the original analysis, for the analysis to remain valid? If this can’t be accommodated, what is the point of a compulsory comprehensive objective analysis for each home which, by its very nature, must remain static? Of course the simple answer is to re-analyse the new thermal performance at each change as a Building Consent Variation and hope there is still compliance. What is to happen if it doesn’t?

I cannot understand why the scheduled method must be “withdrawn”. I have no problem accepting that the current settings for the present Scheduled Method may not be adequate but that is easily solved by continually adjusting the settings to sensible and rational levels. These settings can be conservative to cater for the current broad climate zones and where necessary honed to the degree required for specific climate anomalies. If only modelling results are acceptable, then the analysis must be site specific if it is to be of any relevance. Because of geography, a site in a deep valley in Remuera adjacent to a site on the ridge above, or sites on the east and west side of the Mt Victoria ridge in Wellington have totally different passive environmental thermal performances. Basic site selection is a major determinant of the need to add heating and cooling appliances for a dwelling to maintain comfort levels for the occupants. I discuss this in my blog of February 2015 – 'Site: It is Never Too Early to Begin'.

Have the proponents for withdrawing the Schedule Method, and just using computer modelling to show compliance with NZBC-H1, been given proper and comprehensive consideration as to how the computer modelling is to be carried out for each Building Consent Application? Given the number of Consents approved each year, where are sufficient competent and experienced, (so Councils do not need to check the analysis), consultants to be found and available to undertake the work? This is a fundamental aspect of the change which must be resolved first. There is no point in the scheme if the practical realities result in the modelling being overly simplified and applied to broad classes of dwellings. Is not this a version of what we have at present? When developing amendments to NZBC-H1, doesn’t BRANZ undertake high-level detailed computer simulation analysis and then generalise the results to produced the minimums required to be met by the current Schedule Method? Why can’t this continue?

After my being aware of and studying for decades the contribution the environment makes to the passive thermal performance of buildings, it wasn’t until the late 2000s that it all gelled when I attended a seminar explaining a proposed scheme in England to rate the thermal performance of all houses before being sold. A very laudable intention, but apparently the primary impediment to the scheme was a totally inadequate supply of suitably qualified assessors to meet the demand for such objective ratings.

When a complex process such as thermal modelling is diluted to make it more easily used, with less effort, it quickly becomes just ‘smoke and mirrors’ and of little practical and objective benefit. With the structural performance aspects of the Building Code and NZ Standards, as applied to domestic buildings, there is no problem with being able to use the equivalent of H1’s Schedule Method. I’m sure there is no proposal to require all homes to have their structural components analysed by structural engineers rather than just looking-up standardised tables which have been compiled by competent professional engineers.

Hopefully the above will generate a wide discussion. I encourage all readers to give thought to the withdrawal of the Schedule Method from NZBC-H1 before it becomes a reality.