Natural building materials and the National Construction Code (NCC)

There has long been tension between the lived experience and knowledge of those who have lived in and built with traditional materials and the way these simple materials have been rated and designated in the National Construction Code in Australia, and across many parts of the world.

Overall, there has been a piece-meal approach to earth building styles, dating back to the original mud-brick builders working in Eltham in the 1940s looking to gain permission for their particular type of earth dwellings that were not load-bearing, and designed to work in that particular climate. The interaction between those who wish to use natural materials, and the building authorities and local council are largely decided on a case-by-case basis, and often set by local precedent.  Oftentimes a single-project performance solution is required to demonstrate the material is or will be suitable for the site, location and climate it is being built in.  This is the case for newer materials such as hemp blocks, and designs such as air-crete domes.

Today, earth-wall construction must comply with having a minimum wall thickness of 200 mm though generally a 300mm rammed earth wall will not meet National Construction Code requirements for external wall insulation. Most modern Australian rammed earth walls do not require additional waterproofing except in very exposed conditions. With thick enough walls, mud bricks on the other hand have been proven to create load-bearing structures up to several storeys high. Vaults and domes in mud brick prove that it can be used for many applications other than vertical walls. It may be used as infill in a timber frame building or for load-bearing walls.

Likewise, straw bale walls require footings with a similar load-carrying capacity to that required for a masonry wall, although a straw wall is generally lighter. To facilitate compliance with the National Construction Code, footings are usually concrete strips or slabs. Although it is possible to build strong and effective single-storey straw bale structures, it is often easier to ensure National Construction Code compliance and predictable engineering outcomes if the walls are constructed as infill elements between loadbearing frames.

Hempcrete is a relatively low-strength material compared with concrete and other masonry products. Therefore, the building code requires it be formed in such a way as to make it ‘monolithic’ – solid and full thickness wherever possible. Thin ‘leaves’ should be avoided, as these are prone to cracking. Thus, hemp homes typically use an embedded timber frame to carry the major loads of roofs and upper floors as required by the National Construction Code (NCC) as it will comply with Australian Standard AS1684 Residential timber framed construction. This is due to the requirement that all vertical, lateral and racking loads should be carried by the frame and not the hempcrete. The hempcrete then performs the functions of insulation and vapour management, and (unless off-form) provides a substrate for the external rainskin and internal linings.

The Australian Building Codes Board (ABCB) has been asked by the former Building Ministers’ Forum (BMF) to update the energy efficiency provisions for new residential buildings in the 2022 edition of the National Construction Code (NCC).

While Australia has made considerable progress in the energy performance of residential buildings, there is still opportunity to implement actions that could further reduce the energy consumption of the sector, and this includes incorporating low-carbon intensive materials that can be found on site. Estimates show that Australian housing is around 40 per cent less efficient than equivalent standards in comparable countries.

The recent report into the NCC had submissions from the Earth Building Association of Australia who argued that NatHERS Regulation Mode modelling cannot assess passive low energy buildings, and which has been shown by CSIRO as unreliable in predicting cooling loads. They also argued that there is a lack of understanding of the principles of passive solar design and that the assumption that meeting minimum envelope thermal performance is the only or best way to deliver low energy buildings is false. The EBAA also argued that Embodied energy in high energy materials (e.g. concrete) and hungry energy processes (e.g. air conditioning) must be considered as part of the problem and can be reduced through passive solar design principles.

While the NCC updates are still being developed, there is space for a national movement to continue to push for increasing presence of natural materials, and passive solar design principles, to be more tightly woven into the modelling outcomes that has dictated so much of the housing we build must comply with an outdated framework that preferences pre-fabrication over local, low-carbon and recyclable natural materials.

Join the movement, be part of the solution.

References

YourHome 

NCC Changes and submissions

EBAA publications