Quite 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 attempted to walk them through the long list of ways a building might or might not be “Green,” the shortcoming of this approach leapt out. There was no answer to “why?” And I was reminded that the measuring stick which I was applying to make suggestions was not “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”, but immediately the question arises: how much better is better enough? Hard to know. “Sustainable” is an achievable goal with an embedded purpose.

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. Every living thing consumes, and also ultimately offers itself for consumption. It seems clear enough that we are a part of that system. Our impacts need only be in in balance. For me, “Sustainable” 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 might have engaged with my aunt and uncle’s request by framing it like this:

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 the word “Sustainable”. “Sustainable” is the purpose of being “Green.” I cannot say exactly where “Green” becomes “Sustainable.” At any given moment, without an arc of time to review, 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 even a super-efficient fuel-burning 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.