Building Materials Their Embodied Energy and Greenhouse Gas Impact

Embodied energy is defined in the U.S. DOE's Buildings Energy Data Book as the energy used during the entire life cycle of a product including the energy used for manufacturing, transporting, and disposing of the product. For example, the embodied energy in dimensional lumber includes the energy used to grow, harvest and process the trees into boards, transport the lumber to its final destination, and ultimately dispose of the wood at the end of its useful life. Embodied energy, also called life cycle assessment (LCA), is a useful tool for evaluating the relative environmental impact of various building materials because it takes production, transportation, and disposal into account, all things that can have a pronounced environmental impact but are not necessarily reflected in the price [7].

Embodied energy is associated with LCA, but it should be noted that LCA includes other environmental impacts in addition to embodied energy.

Embodied energy is sometimes defined as the non-renewable energy used during the entire life cycle of a product. Initial embodied energy in buildings is the energy used in extracting raw materials, processing materials, manufacturing, transporting, and constructing buildings. Recurring embodied energy is the energy used to maintain, repair, or replace materials during the life of a building. Cole and Kernan [19] analyzed a typical Canadian office building and found that recurring embodied energy becomes important over the life of a building. Cole and Kernan showed that when an office building is 50 years old, recurring embodied energy is about 144% of the initial embodied energy.

Lippke et al. [20] found that for a typical U.S. home, embodied energy is small but significant compared to energy used to operate the home over its lifetime. Embodied energy in materials is about 15%, and operating energy is about 85% of the total energy used over the lifetime of a typical U.S. home. Buildings with high-energy efficiency have a greater percentage of total energy embodied in materials. For example, Thormark [21] found that for energy-efficient apartment housing in Sweden, embodied energy is about 45% of total energy over a life span of 50 years. Thormark found that about 40% of embodied energy can be recovered through recycling at the end of the building's lifecycle, if buildings are designed with recycling potential in mind.

Embodied energy for a building material may vary depending on extraction processes, production methods, transportation distances, and other variables. Nevertheless, estimates of embodied energy are available for many building products, so architects, builders, purchasers, and others can make informed decisions. The U.S. Energy Research and Development Administration published an extensive report on embodied energy in 1976, and many subsequent publications have referred to the report. Research is needed to update estimates of embodied energy for new materials, building construction practices, recycling processes, etc. Researchers use various methodologies for calculating embodied energy, so comparing embodied energy between studies is difficult. Standards for determining embodied energy are needed.

Embodied energy can usually be reduced by using recycled materials. For example, embodied energy in recycled aluminum is approximately 5% of embodied energy in aluminum produced from raw material. Reusing materials, as well as reusing whole buildings, can reduce embodied energy even more than recycling in some cases. A recent EPA report [8] describes opportunities for reducing greenhouse gas emissions, thereby reducing embodied energy, during construction of buildings. Embodied energy in a building product can be reduced by minimizing energy used at any stage of production. For example, cement produced by a dry-process kiln can use about 50% less energy than cement produced in a wet-process kiln. Using durable, long-lasting materials in construction can reduce embodied energy over the life of a building. Using local materials can reduce transportation costs and thereby reduce embodied energy.

The International Energy Agency [4] estimated that worldwide efforts to improve materials and product efficiency could annually save 5 EJ (Exajoule: 1018 J) of energy and could reduce carbon emissions by 0.3 Gt CO2. Life-cycle evaluation is needed to realize potential savings, but most companies do not consider the entire life cycle. Government measures are needed to encourage life-cycle considerations. The IEA [4] also estimated that feedstock substitution could annually save 5-10 EJ of energy and could reduce carbon emissions by 0.4 Gt CO2.

Incentive options for reducing embodied energy in buildings could include "green" building rating systems such as LEED, tax incentives, carbon-trading systems, and voluntary measures. Economic incentives for reducing embodied energy grow as the cost of energy increases. The 2020 Vision Workgroup, a collaborative effort between EPA and the States, is developing recommendations for a comprehensive materials management strategy that would significantly reduce embodied energy in materials [22]. Effective policies, supported by scientific research, can encourage innovative ways to reduce embodied energy in all buildings.

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