There are a couple of impact levels that you're talking about. You're talking specifically about the operational level of these buildings, including what's in them. There is also the avoided carbon that's associated with the choice of materials. There are two parts, and when they're put together, you end up with a life-cycle analysis, a complete one that includes embodied and operational energy.
What's interesting right now is that you need a database of elements to support those types of decisions to move that forward. That database is called a life-cycle inventory. That is now under consideration at the National Research Council, and hopefully it is going to be properly funded. When you have that in place, you can make a decision relative to all those types of materials in the future. It's not just wood or steel.