I would echo that. The obvious answer we always give is that energy efficiency is the first fuel. That should be your first investment, the lowest-cost fuel type.
Even when we talk about some of our contractors in our industry promoting geothermal, their first focus—it isn't always this, but it should be—is to seal up the envelope. Draft-proof the building or the home, insulate it to the maximum possible, and then invest in this replacement lower-cost heating technology. You start with the envelope and then work towards the mechanical system, but clearly there are limits, and those are the costs. The answer to your question depends on the vintage of the housing stock and a whole lot of other variables. In an older home—