Tuesday, August 18, 2009

The Greenest Building is Usually the One Already Built, but Not Always

I'm really not trying to anger preservationists or tree-huggers here, I'm just analyzing a trite slogan from an engineering perspective. I'm a pragmatic preservationist.

The question is whether or not you should rehab and do a green retrofit on an existing building or replace it with a well-designed low energy new building. The correct answer can only be found after making some reasonable assumptions, cost estimates, and estimates of resale value.

You don't "waste btu's" when you tear down an old building. Those btu's were spent when the building was originally built, and there's nothing you can do to get 'em back.

You can only choose not to spend new btu's on a new building.

And btu's are just another form of dollars.

So it's always an economic problem, eg., what's the present value of retrofitting the old building vs. building an entirely new one. This analysis must be done for every building using each case's unique problems and assumptions.

In residential at least, it's fairly easy to build a new "zero energy" replacement house, but fairly hard to retrofit an old house to zero energy. The cost of the new house can be estimated pretty accurately, but trying to estimate the retrofit cost of the old house is risky. Every old building is different.

The embodied energy of the new building is just part of the down payment on a really good investment. The value of that energy is in the cost of construction. The landfill space required for the old building is also given a dollar value in the cost of demolition. The embodied energy of the old building is a "sunk cost" and doesn't factor in the analysis.

The main thing that doesn't have a dollar value in this analysis is the generation of CO2. Lawmakers have begun trying to put a dollar value on that. Eventually this "carbon tax" will be pretty accurate, and will favor the old building.

Most estimates put the embodied energy of a new building at 5-15% of the lifetime energy usage of the building. That means the operating energy usage of the building is about ten times more important than the original energy content.

Therefore, it may be a catchy phrase, but it's very unscientific to generalize: "The Greenest Building is the One Already Built". Be sure to say "usually".


  1. Presumably the embodied energy of a good new building will be a higher fraction because the good new building will use a lot less energy over its lifetime...

  2. You're right but the point is moot. The 5-10% is an average for new buildings.

    If you build a new low energy building, then the percentage might be 50% or 90%. So what? That's better than living with the old energy hog building.

  3. Followed your link here for the +GBA article on "Historic Building Preservation". Not every building needs torn down but not every building is worth saving here in #StLouis. Ongoing debate as the City wrestles with this same dilemma.