IAONZ Submission Summary DZ 3604:2026

NZS 3604 is one of New Zealand’s most widely used building standards. While its primary purpose is structural, the way timber framing is designed has a direct impact on how effectively insulation can be installed and how much of the building envelope is occupied by timber rather than insulation.

IAONZ supports the direction of the proposed revision, particularly the measures intended to facilitate improved thermal performance.

Importantly, IAONZ also supports the inclusion of NZS 4246 within the draft standard.

NZS 4246 provides the recognised guidance for installing bulk thermal insulation in residential buildings. Its inclusion in the revised NZS 3604 is important because the two standards address different parts of the same performance outcome.

NZS 3604 determines the structure, framing geometry and junctions into which insulation must ultimately be installed. NZS 4246 addresses how that insulation should be installed to achieve its intended performance.

IAONZ’s submission therefore seeks to reinforce that connection and ensure the relationship between the two standards is clear at the key interfaces where framing decisions directly affect insulation installation.

From specified R-value to installed performance

A central theme of the IAONZ submission is that nominal product R-value does not, on its own, determine the thermal performance of a completed building. The performance ultimately achieved depends on the complete system.

Framing content, thermal bridges, junctions, gaps, compression, inaccessible cavities, service penetrations and areas where insulation thickness is restricted can all affect the performance of the finished thermal envelope.  This is why IAONZ believes structural design and insulation installation need to be considered together.

The revised NZS 3604 provides an opportunity to make good insulation installation easier to achieve by design, rather than relying on installers to overcome difficult framing details later in the construction process.

Slab and foundation interfaces

IAONZ supports the proposed provisions that make slab-edge insulation easier to incorporate into conventional construction.

Our submission also asks for greater recognition of the thermal consequences where insulation is interrupted by perimeter foundations, footings and slab thickenings. Structural integrity must remain paramount. However, where verified solutions can safely provide greater thermal continuity, the standard should facilitate their use and recognise the impact of thermal discontinuities on overall floor performance.

Better wall corners and junctions

IAONZ strongly supports the proposed wall-junction details that allow insulation to continue through external corners and wall intersections.

Traditional multi-stud junctions can create significant areas of timber and, in some cases, concealed spaces that become difficult or impossible to insulate properly once framing is completed. Our submission recommends that thermally improved details be treated as the preferred approach wherever structurally appropriate.  This is a practical example of how NZS 3604 can help improve thermal performance without changing the insulation product itself.

Reducing unnecessary framing

Timber framing is a thermal bridge through an insulated wall.  The greater the proportion of framing within the wall, the less area remains available for insulation.

IAONZ has therefore encouraged the revised standard to avoid unnecessary full-depth framing where structural requirements can be satisfied using alternative arrangements.  This is not about compromising structural performance. It is about ensuring framing is not added unnecessarily where doing so reduces whole-wall thermal performance.

Service cavities

IAONZ supports the introduction of service-cavity provisions.

A properly designed service cavity can allow electrical and plumbing services to be routed without repeatedly disturbing, compressing or penetrating the main insulation layer.  It can also assist with maintaining continuity of an intended air-control layer.

IAONZ believes the service-cavity provisions are an important opportunity to make higher-performing timber-framed construction easier to achieve using normal construction methods.

Lintels and mid-floor junctions

The proposed use of thinner lintel arrangements that create space for insulation at otherwise timber-dominated junctions is also supported.  The benefit, however, depends on whether the available space can actually be insulated effectively.

IAONZ has therefore asked that the associated details provide a clear and practical insulation path and avoid excessive cutting, compression or inaccessible voids.

Roof, ceiling and top-plate junctions

The junction between an external wall and the roof or ceiling remains another important thermal interface.  Conventional eaves and top-plate arrangements can restrict insulation thickness close to the perimeter of the building, reducing the effective thermal performance of the ceiling.

IAONZ has recommended that the revised standard provide guidance that enables the specified insulation thickness to be maintained as close as practicable to the external wall line while still meeting structural, ventilation, moisture and cladding requirements.

Again, this is an area where the connection with NZS 4246 is important.  The insulation installer can only install the required insulation effectively if the structure provides sufficient space to do so.

NZS 3604 and NZS 4246 working together

IAONZ requests the recognition of NZS 4246 in the draft NZS 3604.

Our submission seeks to build on that by encouraging clear references at the principal insulation interfaces, including slab edges, wall junctions, service cavities, lintels and ceiling perimeters.  

The objective is simple.  Where NZS 3604 determines the physical space into which insulation must be installed, the framing detail should enable the insulation requirements of NZS 4246 to be achieved in practice.

This connection between the two standards is particularly important as New Zealand moves toward higher levels of thermal performance.

Designing for what is actually built

IAONZ’s wider message is that insulation needs to be considered as part of a complete building-performance system.

A specified R-value is important, but it is only the starting point.

The framing, junctions, installation quality, thermal bridges and continuity of insulation all influence the performance ultimately delivered by the completed building.

The revision of NZS 3604 provides an important opportunity to better connect structural design, insulation installation and actual building performance.

IAONZ supports the direction being taken and welcomes the inclusion of NZS 4246 within the draft.

Our submission is intended to strengthen that connection and help ensure that the framing systems we design allow the insulation we specify to actually perform as intended.