The Fuzzy Math of Embodied Energy

When it comes to reducing energy use in buildings, there are basically two camps. The proponents of "operational energy analysis" argue, as John Straube does on buildingscience.com, that "scientific life-cycle energy analyses have repeatedly found that the energy used in the operation and maintenance of buildings dwarf the so called 'embodied' energy of the materials." The opposing argument is that in order to truly understand a building's environmental impact, one must consider the full range of energy consumed across all phases of production, maintenance, and eventual disposal of building materials. This systemic approach advocates for a "life-cycle assessment" of the energy "embodied" in building materials. A range of digital tools and strategies help architects simulate the operational energy use of the buildings they design and calculate the embodied energy of materials and construction methods. These tools share an underlying assumption that making good architecture requires quantifying not only site conditions, but also most aspects of the building design.

I was recently involved in the process of designing a new building for the Center for Creative Ecology on Kibbutz Lotan. The plan was to use both thermal simulation and embodied energy data iteratively, throughout the design process, to try to minimize the energy footprint of the completed building. Despite the good intentions of the design team, a number of problems arose in practice, some of which are indicative of the inherent problem of calculating the energy embodied in building materials.

First of all, it is theoretically impossible to compute an actual, absolute value for an object's embodied energy -- the sum total of all the energy that went into producing that object and transporting it to where it is right now. Embodied energy can only be calculated relative to a limiting "system boundary," the placement of which is somewhat subjective (a fact that is often ignored). In practice, calculating embodied energy usually involves adding up the total volume of a particular material used and multiplying that by the material's "embodied energy constant." (A Wikipedia search will quickly reveal the "EE values" of various common materials.) However, these "constants" are empirically derived averages, calculated via opaque methods. Given the hyper-specific factors impacting embodied energy (how many miles was the material transported, what transportation method was used, etc.) the very idea of a universal constant is paradoxical.

The inherent fuzziness of the calculation methods raises questions about accuracy: can we realistically expect more precision than simply an order of magnitude? If not, do these calculations tell us anything that we didn't already know? For even without data, it's fairly intuitive that mud bricks will have a lower embodied energy than steal beams.