Timber versus steel – which is the most sustainable?

Damian Clarke

timber building
Photo by Toby Wong on Unsplash

In the same week that New Zealand’s NMIT Arts & Media Building was being publicised as the first multi-storey seismic design building constructed from timber, the US Steel Framing Industry Association came out fighting with Steel industry responds to claims of wood’s sustainability.

There is no denying the impressiveness of the NMIT Arts & Media Building – its soaring spaces and exposed structural elements bring to mind a cathedral, and it has a “warmth and aesthetic appeal quite different from equivalent steel or concrete construction methods” according to architect Andrew Irving of Irving Smith Jack Architects.

Irving adds that the strength, warmth and versatility of timber allowed him to design noisy, silent, large and small spaces. This variety is important in a building that will house both loud activities and those that require sound proofing.

Steel is not, necessarily, the bogeyman

To be fair, the SFIA doesn’t argue with any of the many qualities of timber as a construction material; its focus is on the reuse and recycling elements of the waste hierarchy. Cold-rolled steel framing is manufactured with a proportion of recycled material, the SFIA argues. Steels can re-used on site – such as in relocating a partition wall – or dismantled and stored for later use. At the end of its useful life, steel framing can be recycled.

Bluescope Steel in its 2019-20 sustainability report includes a section on “the future of steel” which claims steel “is well placed to lead in a circular economy”.

Timber is not always a hero

Timber, the SFIA claims, while a carbon sink while in use, becomes a waste product at the end of its life. Subsequent decomposition or incineration release timber’s embodied carbon back to the atmosphere.

The SFIA’s claims aren’t hollow – they are backed by Yale research about the externalities of timber construction versus steel, and extended by new low-carbon-emission steel production techniques developed at MIT.

The SFIA argues that the carbon sunk into steel’s initial production is amortised against a far longer life than timber, with less waste at the end.

Does it really matter?

In Norway – Powerhouse Telemark – raises the question of how important this argument really is. Telemark, an 11-storey building in Porsgrunn, will generate 256,000 kWh a year from its roof and south eastern wall, and will generate more energy over its lifetime than was used to build and operate it – taking the pressure off the embodied carbon measurement.

Powerhouse Telemark is the fourth Powerhouse designed by architects Snøhetta. The others are the Powerhouses Brattørkaia, Drøbak Montessori school and Kjørbo.

Powerhouse Kjorbo is also believed to be the first renovated energy-positive building in the world, having started life as a 1980s office building. The renovation redefines the idea of a building’s end of life. If we can renovate buildings to become net zero or even energy positive, there is less pressure to choose recyclable construction materials as there will be less waste. Leaving timber in place, of course, also keeps its embodied carbon there.

New Green Star calculation tool says embodied carbon is only part of the picture

In Australia all new buildings must be now be net zero before they can achieve a 6 Green Star rating, and the requirement will apply to 5-Star buildings from 2023, 4-star from 2026 and any building hoping to achieve any Green Star rating from 2030.

To achieve Net Zero, a building must:

  • Be free of fossil fuels
  • Consume 100 per cent renewal energy
  • Be extensively efficient
  • Not use gas
  • Be built with low upfront emissions
  • Use offsets (or its own generation) to offset any carbon that it emits in its lifetime

Once these criteria have been achieved, a construction can earn its six Green Stars by satisfying seven of these eight criteria:

  • Responsible design, procurement and construction
  • Resilience
  • Supporting and facilitating resident health
  • Enhancing the environment
  • Supporting the construction of safe, inclusive and interconnected spaces
  • Facilitating solutions that support affirmative social outcomes
  • Promoting an active connection between human beings and nature
  • Demonstrating leadership, expressing a vision for business or increasing the building industry’s ability to renovate

Based on these criteria, whether you front-load the carbon release by using steel, or back-load it by using timber, the true benefit comes from offsetting the lifetime carbon emissions of the building.

The hope for the future here is the Cultana Solar Farm, currently gearing up to start construction north of Whyalla, where Sanjeev Gupta hopes to achieves his vision of steel made with power from renewables. That’s when the carbon mathematics are likely to change again.