Since a 2010 article in Building Design & Construction magazine, and particularly in my 2013 book, The World's Greenest Buildings: Promise vs Performance in Sustainable Design, I have argued that we need green building rating systems that focus on absolute performance, not relative improvement or relative efficiency, to meet carbon emissions reduction goals. After all, nature doesn’t care much about our relative improvement in reducing CO2 emissions, it just responds to absolute amounts of atmospheric CO2, and over the past decade these levels rose annually by 2 ppmv (parts per million by volume). Within 10 to 30 years, depending on trends in emissions, we could reach a tipping point at which global climate change becomes in essence irreversible.
In this blog post, I argue that the problem with a relative-score approach is obviously that it doesn’t evaluate buildings against the ideal: net zero energy use. It would be more valuable to set a goal for energy use for each building type (and size) so net zero energy would be included as a possibility, and then to use a rating scale such as that originally proposed in 2009 by energy expert Charles Eley with his Zero Energy Performance Index, zEPI, shown below.
Eley explains the origin of the zEPI concept in this way:
The concept of zEPI sort of sprung from frustration. Everyone was making claims: “I'm x percent better than code.” My first question was always “Which code?” and the second question was “Are you counting all the energy in the building, or just the energy that’s regulated by the standard?” So those two questions triggered my thinking about zEPI. The idea was to come up with a stable metric that would not change over time. There are two ends of the zEPI scale. One end is a zero net energy building, which is a pure concept. For the other end of the scale we decided to set it at the same level as the median building represented in the 2003 national CBECS database, which aligns with the median energy performance of a typical building at the turn of the millennium. (Author's note: I prefer to use the more up-to-date 2012 CBECS database, but that's a point for experts to argue - in both cases, zero is the goal.)
Any green building rating system that assesses the energy performance of buildings should want to use a stable metric that doesn’t change over time. Eley believes that this concept has technical validity for all buildings and also great intuitive appeal:
The beauty of zEPI is that the concept is exactly the same for new buildings or existing buildings. It’s essentially the ratio of a building’s energy use to the energy use of a similar turn-of-the-millennium building in a similar climate, with a similar use. So for a new building you would determine energy use on the basis of modeling because there’s no history, and for an existing building you would calculate the ratio based on utility bills or other empirical data.