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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.