Timber & Steel Framing
The most common type of structural wall assembly in New Zealand is via timber or steel frames. As timber in New Zealand is a widely and renewable resource it is common to use it. However, with pine being used for construction (due to its fast regrowth) potential safety measures were taken within the building code to give buildings a long and worry-free life. The building code clause B2 Durability states that structural components of the building, such as joists, bearers, and studs are required to last the life of the building, but no less than 50 years (2).
Timber frame VS. Steel frame:
So what are the differences in steel and timber frame? From our research, it appears that timber frames are as steel frames take more work for installation. Steel is more durable, fire-resistant up to very high temperatures, resistant against termites, very strong and fully recyclable. Though the recycling process will require energy to be spent in melting the steel. Also, the recycling process often happens overseas, adding further CO2 emissions. Steel has less waste created during the manufacture as steel can be more precisely manufactured (3). Steel however requires more energy in its production and mining. Steel is worldwide the most recycled material and can be recycled again and again.
There is an ongoing debate between timber and steel frame as to which of the two is more efficient. Steel is more conductive than timber, meaning that it conducts in the summer heat and in winter cold from the outside to the inside. Wood is a natural material that expands in summer and shrinks in winter, potentially leaving small gaps around the insulation that let cold air through. We at Circular Project have not found any evidence suggesting a significant difference in either direction.
Types of wood used and their treatment:
Wood is a natural and renewable resource, which is widely available and sustainably grown in most of New Zealand. Most types of timber need to be treated to guarantee the 50-year durability requirement for structural components.
Structural timber for housing frames comes in New Zealand under the so called 1.2 treatment. These treatments are:
Boron treated Douglas fir or radiata pine
Azoles as a water-based emulsion treatment of radiata pine or Douglas fir
Triadimefon+cyproconazole treated radiata pine LVL
LOSPtreatments (TBTO, TBTN, IPBC) are not permitted for framing
CuN LOSP has been removed from H1.2 and is not permitted for framing(2)
More information on the treatment requirements for New Zealand Buildings please visit the weathertight website (4)
Most common is radiata pine timber treated with Boron. This treatment does have ecological side effects. The fact that the New Zealand Ministry of Health issued the requirement for the boron treated timber to be put into landfill to avoid contamination of plant, soil, animal and human life speaks for about how healthy for us and the environment treated boron frames are (5).
(2) New Zealand Ministry of Business, Innovation and Employment, 'Acceptable Solutions and Verification Methods For New Zealand Building Code Clause B2, Durability, 2nd Edition, 2014
(3) Hitchcock and King, 'Timber Frame vs. Steel Frame", https://www.hitchcockandking.co.uk/h-k-news/timber-frame-vs-steel-frame/ , 11.05.2019
(5) Ministry of Health, 'Health and Environmental Guidelines for Selected Timber Treatment Chemicals', June 1997