Although the energy associated with material extraction is included in embodied energy, material extraction has significant other environmental impacts beyond energy consumption. A few of these additional impacts include reducing biodiversity, impacting water quality, destroying habitat, affecting soil stability and many more that are all difficult to measure.2 In 2010, 73% of raw materials extracted were used in building construction alone.3 Limiting this use of raw materials is key to reducing our impact on the environment.
Knowing this need to protect the world’s natural resources, many raw material organizations have established certification programs to address how they can sustainably extract materials. The most well-known is the Forest Stewardship Council (FSC), which promotes environmentally sound forest management.
Embodied energy is the sum of the energy required to extract, process, and manufacture a material or product. In the US, 6% of all energy consumed is used to make and ship building materials, and this will continue to become increasingly important as energy usage in buildings is lowered.4 Although ideally embodied energy would encompass the energy used in product creation from cradle to cradle (a closed loop system), including raw material extraction, manufacturing, assembly, transport, maintenance and disposal or recycling, common practice currently defines embodied energy as cradle-to-gate.5 This includes only the energy used in material extraction, manufacturing, and assembly until the product leaves the factory gate.
Various materials and products require varying energy to produce. Looking at the main materials in three common structural systems, their embodied energy varies widely and should be factored in in decision making. Concrete is the lowest at 0.95 MJ/kg, then wood at 8.5 MJ/kg, and finally steel at 24.4 MJ/kg.6 However, a wood structure will weigh much less to support the same load as a concrete one would, so the total embodied energy may actually be less for a project made of wood. You can find cradle-to-gate embodied energy numbers in the ICE source.
Because the energy required to transport materials to your location is site specific, embodied energy numbers do not include transportation. However, it is still an important component to consider when material sourcing. LEED standards have recommended shipping materials from within a 500-mile radius, in order to reduce transportation-related energy and emissions.7 Various modes of transportation use different amounts of energy, with ocean and rail shipping being the least energy intensive and aircraft being the most.8 Light-weight materials, such as aluminum, require less energy to transport than heavier materials, such as concrete.