For the control of the construction of our built environment, the various clauses of the Building Act, and its associated regulations, set out what is to be achieved, but the methods of achievement are not stated there. These are described in the New Zealand Building Code (NZBC), and take the form of Acceptable Solutions and Verification Methods. For each type of building construction and the various aspects of that construction, there are industry-standard ways of carrying out the work, ie. common and accepted methods. For non-standard situations, a specific design process is necessary which is shown to comply through the prescribed Verification Method.

In the case of NZBC – clause H1 (Energy Efficiency), the Acceptable Solutions primarily consist of references to New Zealand Standards and the various listed minimum R-values (rather than including specific construction detail drawings as used in NZBC – E2 External Moisture). The alternative Verification Method does not specify particular R-values but instead requires a demonstration that the constructed building will have a Building Performance Index not exceeding 1.55, as required by the building regulations. Compliance is shown by thermal simulation modelling of the heating and cooling loads for the entire building in its particular geographic location and the surrounding environment. (It is not necessary to go further into the details of the verification process, and the various building types).

Over the years there have been calls for the New Zealand Building Code to be updated so as to improve the thermal performance of our homes. In reality, improving the NZBC will not improve the performance of our housing stock because virtually all of them have already been built and so are outside of the influence of the NZBC (unless they are being altered). Yes, I appreciate that people are talking of updating the requirements for new constructions, and it is appropriate for this to be undertaken.

The complaints are usually around NZBC requirements being too low, however, there is no reason why this should be a problem considering that the NZBC specifies Minimums NOT Maximums. There is no reason why better performing housing cannot be built. Through my NZBC-H1 compliance analyses, I find that most new housing is actually constructed significantly above the minimums. In the early days I was being asked, for developer projects, to calculate how many double-glazed windows could be deleted from those required by the Schedule Method (A false economy when the re-sale value is considered). This is no different to a structural engineer being asked to calculate a specific (smaller) lintel to use instead of that dictated by NZS 3604.

The usual calls for change we hear are from the glazing and insulation industries. As I have expressed in earlier blogs, the thermal performance of a house is determined by the performance of every component and their complex interactions (see my December 2014 blog post, “Insulation, Thermal Mass and Glazing: The Juggling Game"), and so change which is not holistic is only of limited value.

I understand that BRANZ is continually looking at how NZBC – H1 can be improved, but it is not a simple matter. Fundamentally all that needs to happen is to reduce the Building Performance Index value in the Building Regulations. While this is not a problem for demonstrating compliance when the Verification Method is being used, there is a tremendous amount of work to be done, and an agreement reached with industry and the market before new Acceptable Solutions can be published.

A suggestion I hear is that triple glazing should be mandatory. Maybe this would be appropriate for a ski lodge but not for most houses. It must be remembered that the NZBC does not outlaw triple-glazing so, by all means, use it in appropriate situations. The same goes for increased thermal insulation. In most cases, a greater R-value for this does not make any significant difference unless the other influences, including site orientation, and fenestration design, are also considered.

There is another approach to improving the thermal performance of housing rather than by just ‘changing the existing Building Code’. That is to think more innovatively about the items and aspects to be covered by NZBC-H1 (Energy Efficiency). An example would be to consider the heat losses from the window elements of the exterior wall. Triple-glazing is an option, but also the size and orientation of the opening, as well as curtaining (see my June 2019 blog post, "The Insulating Effect of the Humble Curtain Revisited"). As an example, I consider that it is correct not to include curtains in the NZBC requirements as they are not permanent features of building construction, and the type and style are so varied and personal to the occupants. But the NZBC could require a curtain track to be installed without specifying what form it should take, leaving the design totally over to the designer; be it a single surface-mounted track, pelmet, fully recessed, etc, etc.

Updating the New Zealand Building Code – Clause H1 is not a simple matter, especially when considering the need to cater for the major changes that have occurred in the form of our dwellings over the last decade, and continuing into the future (e.g. the growth of terrace housing and apartments).

Again, what needs to be remembered is that the NZBC is a minimum NOT a maximum.