I do not think the NZ public have been served well by the NZ Building code which only sets minimum standards and is either silent or confused when it comes to insulation or internal moisture migration. Updates to the NZ building code have been more political than scientific. It’s a shame. Some of the recent updates in 2026, we have been doing for over 20 years. It’s not rocket science.
So what do we do in our projects…
We double layer the ceiling/ roof insulation. One layer runs between the ceiling battens. The other runs between the structure in the opposing direction.
This is not just about getting more insulation in the roof space. This about any gaps that occur trapping air between opposing layers and not allowing drafts. Moisture migration is via what we call “sheet ventilation”. Air can pass under iron or membrane roof as a sheet of air, from the soffit to the ridge. ‘Vent products’ embody our current thinking on this. We have been doing this for over 20 years and first introduced on the Pounamu Apartments in Queenstown. I had to fly to Wellington to gain a dispensation as logic at the time was to seal the roof cavity, whereas I preferred it to breath and release heat built up from outside and moisture within. This contrasts with warm roof designs which are quite different and am more inclined to utilize to feature areas where I want to expose the roof or ceiling structure.
An effective trick we have utilized is adding thermal mass to the roof. Traditionally this may have been clay tiles but in our Oliver house in Queenstown we spread a gravel layer over a membrane roof. This absorbed the heat in the day and dissipates at night meaning the insulation under has less work to do, maintaining a more consistent interior temperature. It also protects the membrane and looks great if visible. Ideal for the spiking temperatures of Queenstown and Otago area especially.
For the same reason thermal mass to the outside is an advantage as long as you don’t have to sacrifice insulation. Having thermal mass to inside is critical to regulate temperatures. Even more so in apartment or terrace house design. Placement can be critical to block and absorb western light to allow it to dissipate well into the evenings. Examples of this are again in the Oliver house where the stone fire is internalized with large western glazing adjacent. It’s not all about minimising air flow and using mechanical moisture extraction.
In the Residence du Parc apartments, Queenstown, we had concrete filled polyblock construction to the exterior and a heat recovery system per apartment. This has slow extraction from the bathrooms bypassing fresh incoming air via a radiator, that preheats this and delivers it to the living and bed rooms. This creates a slow internal circulation of fresh, dry, preheated air. The outcome is you can have fully oxygenated interior environment with very low running costs, halving the plant you require for heating.
For exterior walls we commonly use 150mm timber stud construction with friction fit polyester insulation. This has a 50 year guarantee from slumping. That’s critical. 45mm battens with a second layer of insulation would have the same advantages as the double roof insulation mentioned above. Cavity systems outside of this utilize a 40mm batten generally for improved ventilation. Moisture migration is again critical and generally preferred over mechanical extraction. But horses for courses.
Perimeter insulation is not optional in our view. It’s another reason the 150mm stud option has such an advantage. It allows the stud to overhang the slab by 50mm allowing the same for insulation. With in slab air to water heating systems preferred, full slab insulation is a no brainer.
Insulation through your windows and wall needs to be in a continuous alignment. Face fixing windows meant the inside aluminium face was technically at outside temperature. Recessing windows aligns insulation with thermal separation line. We have actually been doing this for 25 years. And villas for a 100 years prior to that, and for good reason. We created our own flashing system, cavity batten and cladding details to optimise this with the view that if the outer layer fails, the next layer will still disperse water to the outside. Therefore have longevity and hence the target of the 200 year house. All of this is to create a more pleasant interior environment, a healthy home, lower running cost reducing energy use and improving whole of life costs in the process.