In last month’s EBOSS Detailed Blog relating to NZBC-H1, “Will the Summer Overheating of Medium-Density Housing be Resolved by the 6th Edition of NZBC-H1?”, I refer to NZBC-H1/AS and NZBC-H1/VM as specifically including ‘Earth Walls’, but just with reference to establishing their R-value for thermal envelope purposes. The positive, and negative, effects of thermal mass is not addressed when considering compliance with NZBC-H1.

I have no problem with clause H1 (Energy Efficiency) of the NZ Building Code (including the 6th edition) considering just the Construction R-values (thermal resistivity) of building materials and assemblies which make-up the Thermal Envelope of a dwelling. This Energy Balance process, (the Calculation Method of NZBC-H1/AS1 which assesses the heat flow between the interior and exterior of residential buildings and vice versa), uses simple mathematical operations to calculate the Heat Losses in accordance with the two formulae, and the other provisions, in ‘section 2.1 Thermal Resistance’ of the Acceptable Solution. For compliance, the calculated Heat Loss of the Proposed Building is to be less than, or equal to, the heat loss of the Reference Building (para 2.1.2.6 of NZBC-H1/AS1).

The NZBC-H1/AS1 Energy Balance assessment of Energy Efficiency demonstrates compliance for the purposes of obtaining a Building Consent, BUT it is not a measure of the overall daily and seasonal thermal performance of the dwelling as this relates to the comfort of the occupants. It just relates to the physical heat transfer aspects, and the insulation R-values, of the thermal envelope (the exterior skin) of the building without reference to the interior. The important contribution of thermal mass, both positive and negative, to thermal performance is only considered superficially. Refer to the link below for my February 2025 EBOSS Blog.

While not ideal, given the great number of residential Building Consents issued each year, the Energy Balance approach is a sensible and pragmatic process which can be undertaken by any design office. There are simple tools available to assist with the calculations. Also some industries are offering to provide compliance information but this is based on using their products which sometimes may not be the most appropriate materials for a particular project.

On the other hand, a dynamic multi-volume hourly passive thermal simulation of a dwelling’s passive thermal performance in its specific location and orientation is a much more relevant and useful starting-point towards providing a comfortable interior environment for the occupants over time. This type of analysis goes well beyond just considering the static Construction R-values of the thermal envelope. It considers the thermal characteristics, including thermal mass, of all the materials (both interior and exterior) of the home along with the effect of the disposition of the rooms and their degree of interconnectedness. Opening doors, permanent openings and solid partitions all moderate the passive movement of warm and cool air about the building, including vertically. Also the dynamic aspects of the outdoor environment are allowed for, especially shading as discussed in my October 2024 EBOSS Blog “What Shading is Needed for Summer Cooling?”. When this simulation is undertaken before any mechanical heating or cooling is operated, the result provides a baseline of the hourly passive temperatures in every volume (rooms, garage, attics, subfloors) within the dwelling. This can then be used as a design tool, including as a room-by-room brief for the design of any proposed heating/cooling system, and its distribution, which would be appropriate for the particular occupants and dwelling.

As is fundamental to all types of thermal performance simulation software, the output temperatures are not a predictor of actual future performance as they are just a simulation, albeit to varying degrees of sophistication. This is because, along with the behaviour of each individual occupant, the current weather at any time in the future cannot be predicted. However, the hourly position of the sun throughout the year is known by AccuRateNZ so that the all important solar radiation input through the windows is a predictable variable. Of course any thermal performance analysis can be upset when the kids rush in to greet grandma and leave the back door open.

In early 2025 I wrote a series of four EBOSS Detailed Blogs related to MBIE’s call for submissions on their review of NZ Building Code clause H1 (Energy Efficiency) 5th Edition. This was later published as the 6th Edition (and is in two parts, namely NZBC-H1/AS1 and NZBC-H1/VM1) with effect from 27th November 2025.

The links to the Four Blogs are:

• February 2025: Passive Over-Heating: Does Internal Thermal Mass Make a Difference?
• March 2025: Considering the Review of NZ Building Code Clause H1 Energy Efficiency
• May 2025: The Summer Over-Heating Problem Requires a Holistic Approach
• June 2025: Modelling the Passive Thermal Performance of Dwellings

After a worldwide evaluation of passive thermal simulation software in the late 2000s, government through EECA, BRANZ, Universities and others, settled on the Australian ‘AccuRate’ software (driven by the purpose-written Chenath computational engine developed by CSIRO) as the most suitable dynamic thermal performance simulation computer program for New Zealand after which it was adapted to become ‘AccuRateNZ’. ‘AccuRate’ has been used continually by Australia for their NATHERS residential building compliance. In New Zealand there was a change of government which brought an end to the 4-day plus two day training cost subsidy, and consequently there was a drop-off in its use. Also at the time AccuRateNZ was too comprehensive for the public to be willing to pay to be told how poorly their homes performed and so NZBC-H1 compliance reverted to concentrating on the simplistic Schedule and Calculation Methods of the earlier editions. I, and probably others, have continued to use AccuRateNZ as a high-level design tool, and also to show NZBC-H1/VM compliance. BRANZ continues to use AccuRateNZ for their research, including for ‘carbon use’ analysis.

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. These dwellings can be standalone, or within terrace or apartment housing. I consider that MBIE should dust off the software and support its availability to those architects and designers who wish to add this much more nuanced software which gives a greater depth of objective analysis to the design of their projects. This is especially so with the growing awareness of the summer over-heating problem within multi-unit residential developments, both terrace and apartments. I wrote of the difficulty of trying to apply simplified standard thermal performance solutions to medium-density housing in my EBOSS Blog of April 2017 “Thermal Performance Aspects of Low-rise Apartment Design.” 

I have had members of the Passive House (Passivhaus) design community comparing AccuRateNZ unfavourably with the very prescribed PHPP design process. (In the interest of openness, I need to declare that I am registered with PHPP v10 and a holder of the “Passive House Planning Package – The Energy Balance and Design Tool” manual. I also have used Homestar’s ECCHO calculator.) The negative comparison is actually quite erroneous because running an AccuRateNZ simulation afterwards actually complements PHPP outputs. PHPP describes in close detail the construction of only the thermal envelope of a dwelling (which can also be defined as an entire residential housing block). In New Zealand, PHPP uses twelve monthly average outdoor temperatures to determine the annual heating and cooling loads. It then goes on to describe the HVAC systems needed for the proposed dwelling. Customised Excel spreadsheets are used for calculations.

AccuRateNZ analyses each housing unit as a series of distinct volumes, the results for which are retained but then combined to report on the passive thermal performance of the whole dwelling – energy requirements for: heating, sensible cooling (lowering temperature), and latent cooling (dehumidifying). There is no single thermal envelope as required for the Energy Balance process, although the simulation of each room/volume considers the energy balance of each of its differing bounding building elements, both internal and external, in relation to the current calculated hourly temperature of the adjacent volume. Because AccuRateNZ simulates the thermal performance of each space each hour of the year, there are produced 8,760 temperature datapoints for each volume. This information output is available to use to adjust the planning, window type size and placement, materials, construction details, etc., to refine the design and construction details before another simulation is run.

AccuRateNZ does not determine the make-up of the various building elements; on the contrary it is up to the architect/designer/client to make all these selection as it is they who have the overall knowledge of the proposed dwelling and its construction. It is because of this that there is no problem with the construction details arising from, for example, a Passivhaus or WUFI analysis, or SIPs panels, etc., being used as the description of the components of the home being simulated. 

The front-facing AccuRateNZ software is the building designer’s text-based user-friendly input/output translator which communicates with the internal Chenath computational engine which does the hard work in the background. I have been told by a colleague who knows both programmes that AccuRateNZ tracks closely with the U.S. Department of Energy’s ‘EnergyPlus’ computer programme. EnergyPlus is the Gold Standard of dynamic thermal performance simulation software and is used around the world by engineers for many different types of buildings and complexity. A high level of skill is necessary to operate EnergyPlus. AccuRateNZ is a ‘smart simplification’ tailored to be used by non-engineers for domestic dwellings but with a similar degree of complexity of input information.

It is unfortunate that AccuRateNZ arrived too early on the scene, and to then fade away, because it is now needed as one of the design tools which can address the growing summer-overheating difficulties of new residential developments.

As I wrote in my March 2026 EBOSS Blog: Do the communities of the future want to have to deal with demolishing Energy Slums if we do not treat the over-heating problem properly?