One of the major uses for energy is building construction and operation. According to the Environmental Resource Guide, produced by the American Institute of Architects, more than 30% of the energy consumed in the United States goes to making and maintaining buildings. This includes both operating energy--the energy required for space heating and cooling, lighting, refrigeration, water heating and other building functions--and energy embodied in the physical structure.

Most people are familiar with the concept of improving the energy efficiency of buildings by reducing the operating energy they use--especially if they've been reading Home Energy. It's a common claim that energy-efficiency measures can reduce the operating energy of an individual building by 60% or more. Comparatively little attention has been focused, however, on recognizing or reducing the embodied energy of structures.

Embodied energy, or "embedded energy," is an assessment that includes the energy required to extract raw materials from nature, plus the energy used in primary and secondary manufacturing activities to provide a finished product. There is embodied energy in any processed product, from a drinking cup to a car. In embodied energy terms, buildings represent a huge, relatively long-duration energy investment.

Every building is a complex combination of many processed materials, each of which contributes to the building's total embodied energy. The energy required to extract and process the raw material for an individual component, as well the energy used to transport the finished product to the job site and install it, all become part of the embodied energy cost of the completed structure. Furthermore, energy involved in maintaining an individual building component, and finally removing it and recycling it or otherwise disposing of it at the end of its useful life, can all be part of the embodied energy equation for a particular building material, depending on how the embodied energy is quantified.

As the operating energy required for buildings declines, the embodied energy they represent becomes a more significant percentage of the total energy buildings use. In coming years more efforts will probably be directed toward measuring and reducing the amount of embodied energy in buildings.

An Inexact Science

The quantification of embodied energy for any particular material is an inexact science, requiring a "long view" look at the entire manufacturing and utilization process, and filled with a large number of potentially significant variables. Consequently, the complexity of embodied energy calculations is frustrating even for scientists, and it is easy for the individual homeowner, builder, designer or government specifier to become discouraged at the difficulty of obtaining accurate figures. Fortunately, precise figures are not absolutely necessary for informed decisions on which building materials to use in order to lower the embodied energy in a structure. Builders need only recognize the potential differences in relative embodied energy to make wise material and system choices.

Part of the challenge of assessing and making decisions based on embodied energy is the lack of current data. The "definitive" U.S. study on embodied energy was produced under the auspices of the Energy Research and Development Administration and dates from December 1976. Many of the statistics it includes are of 1967 vintage, and most current papers and references on embodied energy still cite data drawn from this old study. While some of the data may still be relevant, the tremendous advances in processing technology and recycling during the past 20 years limit the applicability of this information. Tools, transportation, and installation methods have changed, and most significantly, some building materials in widespread use today didn't even exist at the time the report came out. Fortunately, some researchers in other countries haven't let embodied energy research lag as much as the United States has.

The Canadian Mortgage and Housing Corporation, and SAR Engineering, have developed a computer program called "Optimize" that is designed to estimate the embodied energy, lifecycle energy, and environmental impact of a house. Figures from the creators of the Optimize program provide the estimate that for a "standard house in Toronto with a 40-year life," the total embodied energy is 2,352 Gj (One gigajoule is equivalent to about 1 million Btu) The total operating energy over 40 years is 9,060 Gj, which results in expected operating energy of approximately 226 Gj per year. This means that a typical house will exist and operate for ten years before the total operating energy starts to outstrip the embodied energy contained in the building components. The embodied energy of a house is not static, either. Very few houses go through an entire 40-year life span without undergoing remodeling projects that involve tearing out old materials and installing new ones, further increasing the embodied energy contained within a house.

Estimates by Ray Cole of the University of British Columbia's School of Architecture also compare embodied energy with operating energy. Cole's figures relate to a 3,750 ft² ranch-style home, constructed in either conventional or energy-efficient style.¹ The energy-efficient version of this house includes R-42 ceilings, 226 walls, additional glazing on the south elevation, and added thermal mass. As in the other Canadian study, Cole's figures reveal an embodied energy for both versions of the house that is equal to several years' worth of heating energy, which is the major component of home operating energy in Canada (see Table 1). According to Cole's data, it follows that the more operating-energy efficient the house is, the larger percentage embodied energy will be of the structure's total energy.

Another study, conducted by Andrew Buchanan and Brian Honey of the University of Canterbury in New Zealand (which has a similar range of climates to California) concluded that the energy required to manufacture a house is of a similar order of magnitude to the energy required to heat the house over a 25-year life (see Table 2). That study drew upon research conducted at New Zealand's Energy Research and Development Committee in 1983, which among other things estimated "energy coefficients" for various building materials (see Table 3).

Comparing Apples and Oranges

Even with computer programs and sample data, embodied energy of entire buildings--or even of individual materials--is difficult to quantify, since production and installation of building components is a lengthy and complex process involving many variables. No measurement standards exist, either. In practice, each individual researcher studying embodied energy has a different methodology for quantifying the embodied energy of materials. Some calculations fail to take disposal costs of a material into account, and little is known about the long-term disposal costs for many materials, especially as landfill siting becomes more difficult and the cost of containing leachate from existing landfills rises. Some measurements don't include transportation, or consider only part of the extraction energy involved in producing a material.

Some researchers differentiate between the embodied energy of the house "as built" and "life-cycle embodied energy" that includes the maintenance, repair, demolition and disposal of the structure. (This life-cycle embodied energy is not the equivalent of life-cycle analysis, which includes environmental costs such as resource depletion and pollution as a way of internalizing the costs of present externalities. Life-cycle embodied energy only measures energy use, not energy costs or other environmental costs.)

Comparing the various figures provided on different materials is akin to comparing the proverbial apples and oranges. And as though the figures for materials weren't confusing enough, calculations of embodied energy will be different for each job site. Factors such as distance of the site from manufacturer, distance from railhead, and even the distance that tradespeople must travel to and from the site during construction, are all part of the embodied energy equation. Furthermore, the type of fuel used in processing and transporting materials can affect the amount of embodied energy contained in the final product. Two products of identical appearance may have different embodied energies, depending on where and how they were made, and where and how they will finally be used.

Given these complexities, builders bent on reducing energy consumption should learn as much as possible about materials options, and consider the probable relative embodied energies of these materials. Fortunately, a few general rules of thumb apply:

The embodied energy in recycled building materials is generally less than that contained in new materials. Recycling provides easily obtainable manufacturing feedstock. There is very low extraction energy associated with recycled materials. Although manufacturing with recycled feedstocks can involve transporting, cleaning, and sorting the recycled materials, this often requires far less energy than manufacturing from a virgin resource that must be extracted and refined before use (see Table 4.

While these figures will vary with the quality and quantity of recycled feedstock, as well as with the efficiency of the processing equipment used, they show that using recycled materials as raw material for manufacturing can result in substantial energy savings for at least a few frequently used materials. Figures provided by Resource Conservation Consultants in Portland, Oregon, cite energy savings of 88%-95% for use of recycled copper, and 70% for recycled rubber, among significant savings for other materials.

Reusing materials, or even reusing entire buildings by retrofitting them, reduces the total amount of embodied energy even more than using recycled materials. Although reusing materials often requires intensive cleaning, and frequently entails repair, it represents a means of attaining substantial embodied energy savings. Builders can save embodied energy by incorporating as many salvaged and reused building components as practical.

Meanwhile, the embodied energy of a manufactured material can be lowered by reducing the energy required at any stage of production. For instance, the energy of lumber is greatly reduced by air drying it instead of kiln-drying it. Even if the lumber is harvested, milled, and transported by the same means, lumber that is air-dried has only about one-third as much embodied energy as kiln-dried lumber, measured in millions of Btu per thousand square feet.²

Choose durable, long-lived building materials. Durable materials, especially those with low maintenance requirements, tend to have a lower embodied energy than disposable or short-lived materials. Although less-durable materials may not involve as much energy in their manufacture, the need for frequent replacement, combined with the need to dispose of the product following removal, results in a higher total embodied energy over the life of the structure.

Use indigenous, or local, materials. Besides lower transportation energy costs, indigenous materials usually involve less processing energy than conventional construction materials. Using materials such as local stone for patios involves less embodied energy than using concrete or treated wood for patios and decks. Some builders build entire structures of indigenous materials, such as adobe, straw bales, straw-clay mixes, or rammed earth (see "From the Southwest, Unconventional Insulations," HE Mar/Apr '93 p.11).

Common Sense Decisions

After all the rules of thumb have been applied, the best hope of reducing embodied energy in buildings comes down to the reasoned actions of responsible individuals. Designers, builders, homeowners, manufacturers and policy makers can act to reduce the level of embodied energy in building materials in a number of ways.

First, encourage improved efficiency in manufacturing, transportation and installation. Second, look for additional and updated research on the embodied energy of building materials. Only with more current and standard information can materials be compared with one another to determine the most energy-efficient alternative. Beware of comparing results from studies that may have different parameters for measuring embodied energy. Finally, even before final evaluations of embodied energy are available for all materials, identify building products with relatively lower embodied energy.

As operating energy is reduced through energy efficiency measures, embodied energy comes to represent an increasingly significant percentage of the total energy consumed by a structure. Only by addressing both components of energy usage--the operating and the embodied--can we comprehensively address the vast amount of energy consumed by buildings.

Notes

1. This information first appeared in Environmental Building News. See "Embodied Energy--Just What is it and Why Do We Care" by Nadav Malin in the May/June 1993 issue. Environmental Building News R.R. 1 Box 161, Brattleboro, VT 05301. Tel: (802)257-7300; Fax: (802)257-7304.

2. See "Assessing Sheathing Options," in the Sept/Oct 1992 issue of Environmental Building News.

Further Reading

Environmental Choices for Home Builders and Renovators, published by The Canadian Home Builders' Association, Canada Mortgage and Housing Corporation and Ontario New Home Warranty Corporation. Ottawa, Ontario, January 1994. Contact: Canadian Homebuilders Association, 150 Laurier Ave West, Suite 200, Otowa, Ontario, Canada, K1P 5J4 Tel: (613)230-3060.

Environmental Resource Guide, The American Institute of Architects, 1735 New York Ave NW, Washington D.C. 20006. Tel: (202)626-7300; Fax: (202)626-7421.

"Energy and Carbon Dioxide Implications of Building Construction," by Andrew H. Buchanan and Brian G. Honey, Energy and Buildings, 20 1994. Elsevier Science Inc, 655 Avenue of the Americas, New York NY, 10010. Tel: (212)633-3764; Fax:(212)633-3764.

"Environmental Impact of Energy Conservation in Buildings--Real Case Studies," by Peter Suter, ETH Zuerich, Energy and Environment Division, Lawrence Berkeley Laboratory, 1 Cyclotron Road, Berkeley, CA 94720. Tel: (510)486-7438; Fax: (510)486-6996.

"The Estimation of Energy Consumption and CO₂ Emission Due to Housing Construction," by Michiya Suzuki, Technology Division, Shimizu Corporation, Tokyo, Japan; Tatsuo Oka, Faculty of Engineering, Utsunomiya University, Tochigi Prefecture, Japan; and Kiyoshi Okada, Faculty of Engineering, Utsunomiya University, Tochigi Prefecture, Japan.