Embodied Energy--Just What Is It and Why Do We Care?
Almost all the attention paid to energy conservation in buildings has focused on reducing their operating energy, but embodied energy can add up to many years’ worth of operating energy in an efficient building.
Comparing Embodied Energy with Operating Energy
Almost all the attention paid to energy conservation in buildings has focused on reducing their operating energy. Given the energy hogs that many buildings were (and are), this focus is appropriate. As builders and architects succeed in making their buildings more efficient, however, the energy used to build buildings starts to look significant. This embodied energy can add up to many years’ worth of operating energy in an efficient building (see table). While taking steps to reduce operating energy is clearly the first priority, it makes sense to look for options that minimize the initial energy investment—the embodied energy—as well.
Embodied energy is a term coined to express the energy consumed in the production of a particular product or material. Some scientists use the term energy intensity to describe the embodied energy per unit (pound, kilogram, cubic foot, cubic meter, etc.) of a material. The energy used to produce the materials, together with the energy needed to assemble them, gives the embodied energy of a building component, or of a whole building. Energy intensity figures for materials, or embodied energy values for components, can provide useful information for comparing different building products, or choosing from among several different materials.
Quantifying the energy intensity of materials is not an exact science. Manufacturing procedures can vary greatly from one site to the next, or by season. Every effort to measure energy inputs requires many assumptions, and these often vary greatly among researchers. Production generally begins with one or more raw materials that have to be mined or harvested, transported to factories, and processed into usable products. The finished products then have to be transported to the building site. For some materials the processing and manufacturing stage can have several steps, with initial processing at one site, further transportation and secondary processing at another site, and so on. The flow of materials and energy for products made up of many different components (such as a window, for example) can be quite complicated.
Some researchers have worked to clarify the assumptions involved by assigning different levels or orders of energy use based on how directly the activity is connected to the manufacturing process:
- First order energy: fuel use for mining, transporting raw material, energy use at manufacturing facility.
- Second order: energy used to produce the equipment and machinery that does the work, and to transport workers to and from the site.
- Third order: general support services and social services for workers, second order energy for machinery, equipment and infrastructure.
While some embodied energy studies have attempted to include second and third order energy values in their data, most do not. These secondary energy values can be quite large taken as a whole, but once their contribution to the energy picture is spread over the full amount of material it becomes much less significant. The amount of diesel fuel burned by an excavator in a gravel quarry (first order energy), will over its useful life dwarf the energy invested in making the machine (second order energy). Other second order contributions, such as the transportation of workers, can be more significant because they occur on an ongoing basis along with the actual mining or manufacturing. The unknowns and variability from one site to the next make these figures almost impossible to quantify, however. As a result, many researchers choose to assume that such factors will be comparable from one industry to the next, and as such can be left out without compromising the relative embodied energy assessment.
The most important and difficult part of relative embodied energy assessments is making sure that all the assumptions and boundaries of the study are the same for all materials involved. When comparative work is done by one researcher or group of researchers the parameters can be established and shared. To date, however, most embodied energy research projects have focused on one particular field or industry, and comparing the results of different projects is tricky at best.
Industry associations representing wood products, plastics, and concrete have all commissioned studies in the hopes of showing the energy advantages of their materials. As might be expected, the conclusions released from those studies that have been completed vary tremendously. A study commissioned by the Society for the Plastics Industry (SPI) and performed in 1990 by Franklin Associates of Kansas City, Missouri, provides embodied energy values for cement that are three to five times higher than those suggested by independent Canadian studies from the mid-eighties. Research done by Scientific Certification Systems of Oakland, California, for the Western Wood Products Association has not been released to the public, while the concrete studies are just beginning. Until several different associations commission studies from the same researcher, using the same methods, it is impossible to compare the results reliably.
Fortunately, there is some independent research to work from, including efforts to incorporate embodied energy into the larger environmental picture. While much of the current work is from Canada and Europe, it is American researchers in the mid-1970s who laid the groundwork in terms of energy analyses of industrial processes. The work of Harry Brown (on industrial processes in general), and Richard Stein and Diane Serber (on energy use in building construction), provided the raw data which is still used as a basis for many studies. By comparing this research with industry-funded studies it is possible to make some reasonable comparisons of materials, at least those that have been studied. As more material becomes available, EBN will keep you updated on the results of those comparisons.
Brown, Harry, et al. 1985 Energy Analysis of 108 Industrial Processes. Washington, D.C.: U.S. Department of Energy.
Cole, Raymond J. and David Rousseau (both of the Environmental Research Group, School of Architecture, University of British Columbia, Vancouver, BC, Canada, V6T 1W5). “Environmental Auditing for Building Construction: Energy and Air Pollution Indices for Building Materials.” Building and Environment , pp. 23-30, 1992.
Food and Agriculture Organization of the United Nations. Energy Conservation in the Mechanical Forest Industries. Rome, Italy. 1990.
Franklin Associates, Ltd. 1991. Comparative Energy Evaluation of Plastic Products and Their Alternatives for the Building and Construction and Transportation Industries. Prairie Village, Kansas. Report prepared for The Society of the Plastics Industry.
Hannon, Bruce et al. “Energy and Labor in the Construction Sector.” Science Vol. 202, 24 November 1978, pp 837-847.
Stein, R.G. and Diane Serber. 1979. “Energy Required for Building Construction.” Chapter 10 of Energy Conservation Through Building Design. D. Watson. New York, McGraw Hill.
Scientific Certification Systems, 1611 Telegraph Ave., Suite 1111, Oakland, CA 94612; 510/832-1415.
Canadian Mortgage and Housing Corporation, 682 Montreal Rd., Ottawa, ON K1A 0P7, Canada; 613/748-2367. CMHC is developing an extensive database program, called Optimize, to assess the environmental impact of houses. Optimize runs on Excel and requires an IBM or compatible PC with high-speed processor. It is currently in final testing stages.
Published May 1, 1993