I found this interesting snippet today in an article about architecture in the NZ Listener (January 28 – February 3 2006, Vol 202 No 3429):
“Timber requires one megajoule per kilogram to process from the log state to a milled section. It takes altogether five megajoules to go to its finished state, such as door, architraves, beautifully finished. Steel takes 43 megajoules per kilogram, aluminium takes 142 megajoules per kilogram.”
Of course this may be a simplistic analysis, since it does not account for the energy involved in maintaining structures made from each of these materials.