If you want a green building, make it out of wood
The third little pig might have staved off disaster, but the second little pig was the greenest, according to the USDA. The agency looked at dozens of studies comparing wood to concrete and steel and declared wood the winner when it comes to emissions. Attempts to use materials other than wood in construction yield, on average, 2.1 tons more greenhouse gases per ton of material.
Of course, the USDA is the same organization that basically uses our children as convenient dispose-alls for subsidized corn, and it's not like they don't have a horse in the race when it comes to harvesting timber.
That said, it kind of makes sense: every time we build with lumber, we're sequestering the carbon in that lumber for as long as the house stands. Not so big a help when our country is suffering from an oversupply of the wrong kind of timber-frame construction, but at least we know what to build our shantytowns out of when it comes to that.