Lots of folks are touting a new study by The National Trust for Historic Preservation claiming that re-habbing existing buildings is far more green than building new energy efficient buildings — but it’s not at all clear this is accurate.
The study itself is an impressive effort, and I laud their rigorous attempt to dig into this question. But they might be wrong, even though reports are flying around talking about about why you need to rehab instead of build new energy efficient buildings.
My concern is three-fold:
- A study done at University of Michigan found the opposite result. This study compared the LCA (life cycle analysis) of new construction single family residences, one energy efficient and the not. This found that the ongoing energy use of the buildings outweighed the carbon impact of the materials used by at least 4 to 1, and that was only over a 50 year lifespan (the new study uses a 75 year span). The results clearly indicated that energy use mattered more than the material use for the LCA.
- The National Trust’s own study shows that in a variety of contexts (depending on building type and climate) 10-12 years is all it takes for the energy use of the building to “catch up” with the embodied energy of the materials. When a building’s anticipated lifespan is 50-75 years, then this confirms the University of Michigan study.
- Finally, the new study compares rehabbed buildings to new energy efficient buildings and assumes that the new buildings are a notable 30% more efficient. Depending on the age, type of building and climate, I think this number may be misleadingly low and therefore harms the conclusion of the study. Residential construction in Michigan, for example, had such horrible energy codes until 2008, that a new energy efficient home (especially one that’s built green with a low HERS or ENERGYSTAR index) could easily be 40% more efficient and possibly more.
The new results may be accurate, but there’s plenty of reasons to wait before claiming that new truly energy efficient construction is worse than rehab.
Btw, some disclaimers:
- I have a strong interest and dedication to rehabbing buildings and historic preservation in my personal life and career — which is all the more reason why I want to be clear where the benefits and trade-offs really are.
- despite being right by the University of Michigan, I have no connection to the authors or department which produced the report.