A number of years ago, my uncle and aunt were planning a building project, and asked me for thoughts on making it “as Green as possible.” I find this to be a common starting point. As I walked them through the long list of the ways a building might or might not be “Green,” I was reminded that the measuring stick which I was using to make suggestions was not just “Green,” but “Sustainable.”

I would offer that “Sustainable”, in contrast to “Green”, implies a specific state of balance, with an end point. “Green” is a characteristic, a color of course, of which you might have more or less. Usually “Green” is meant as “Better”, and immediately the question arises: how much better is better enough?

I don’t believe that human impact is bad. I believe that human impacts are necessary – important, even – for the system of life on Earth, and that they only need to be in balance with the rest of the system. “Sustainable” to me means “in balance” and as a term to organize our efforts around it offers an endpoint, with a reason why: without balance the system collapses, and so do we.

So, in retrospect I think a more useful way to start the conversation with my aunt and uncle might have been:

A building is a complex system, with many choices to evaluate. Those choices have multifaceted and counterbalancing environmental impacts. When weighing those choices a useful guide to keep in mind – an ideal to work towards – is “Sustainable”. “Sustainable” is the point of being “Green.” I cannot say exactly where “Green” becomes “Sustainable” – I don’t believe anyone can. But I am comfortable that some things require no more than common-sense thinking to know as true: that a building which is heated through its windows by sunlight uses less energy than the most high-tech boiler. That a building–mounted solar panel saves the “line loss” of electricity transported from a distant solar farm, but a building lit by daylight saves the impact of the panel’s manufacture as well. That traveling to your building on public transportation saves much of the energy used to drive there. That oil and marble are finite resources, plastic is forever a man-made introduction to the biosphere, wood will decompose and steel can be recycled… And on. Even if what it takes to get to “Sustainable” is not fully known, or even know-able, the choices which are closer to that goal are easy to see.

I find it to be a useful way to think about “Green” building.