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Steel is the most recycled material on Earth. Timber is the ultimate renewable resource. So finding the best option for framing your environment-friendly home may not be a clear-cut decision.
Engineers use a concept called 'embodied energy' to describe the sum total of all the energy that went into manufacturing a material or product, beginning with raw materials and including processing and manufacturing.
Embodied energy varies from product to product, but the trend is quite clear: timber framing uses less energy in its manufacture than does steel.
Kiln-dried softwood - the type used in framing - takes about 3.4 megajoules per kilogram (MJ/kg) to manufacture. Galvanised steel, on the other hand, gobbles up around 34 MJ/kg.
The difference between the two materials shrinks a bit when you consider them in terms of real walls, rather than independent materials.
A square metre of a wall consisting of a timber frame, brick veneer, and plasterboard lining uses between 496 and 561 MJ per square metre, whereas the same wall with steel framing takes between 604 and 705 MJ for the same area.
The gap narrows because the timber wall is actually heavier per square metre than the steel wall.
Embodied energy doesn't say anything about where the energy came from, though, and steel and timber manufacture affect the environment in different ways.
According to Tim Grant, of RMIT University in Melbourne, energy for most manufacturing in Australia, including steel, comes from coal - a dirty, fossil energy source.
With timber, at least at some stages of manufacture, lumber mills are run with steam power derived from burning timber offcuts, instead of coal.
Over a 100-year lifespan, the CSIRO estimates that a home uses between 4500 and 6500 gigajoules (GJ) of energy for heating, cooling, lighting and other activities.
Nearly 15 per cent of a home's energy goes towards heating and cooling, and steel - which is around 400 times more conductive to heat than is timber - requires more insulation to prevent heat loss to the ground and air.
At the end of a house's life, steel reigns supreme. According to Grant, 75 to 85 per cent of steel from construction is recycled in Australia today.
Thirty to forty per cent of the metal in new steel frames is recycled content, and the Australian Greenhouse Office reports that recycling steel reduces its embodied energy by 70 to 80 per cent.
While virgin steel uses about 34 MJ/kg to manufacture, recycled steel uses only about 8.9 MJ/kg. That's still higher than timber, but not by as much. And more to the point, recycled steel doesn't take up space in landfill.
Timber is a slouch in comparison. Recycling timber is a labour-intensive, inefficient process, further complicated by the adhesives that are replacing nails and screws in construction.
Additionally, care must be taken when disposing of or reusing timber that's been treated chemically with preservatives and insecticides.
For Graham Treloar, a lifecycle expert from the University of Melbourne, waiting until a house is demolished to glean the benefit of steel's recyclabilty doesn't make sense, compared to timber's low initial energy.
"I reckon we've got 25 to 50 years to solve the greenhouse issue," he says. "If you can use timber to massively reduce embodied energy, and the small cost of that is some chemicals for treatment, that's a risk worth taking."
The big picture
As with any product, when choosing a frame for your house, one of the biggest steps you can take towards minimising the environmental impact is to buy local.
The embodied energy figures noted are averages, and will vary substantially depending on whether a product that is manufactured in Sydney is erected there, or shipped by road to a home site in Perth.
If you live close to timber mills and sustainable logging operations, timber is probably the best option. On the other hand, in areas where timber plantations are scarce, it may make more sense to choose a recycled steel framing product.