Calculating the amount of non-renewable energy it takes to acquire, process, manufacture, transport, construct, and repair/replace a building material has been studied for several decades. According to Canadian Architect, there are two forms of embodied energy:
1) Initial Embodied Energy: “In buildings, the Initial Embodied Energy represents the non-renewable energy consumed in the acquisition of raw materials, their processing, manufacturing, transportation to site, and construction” (Canadian Architect). The Initial Embodied Energy can further be subdivided into two parts:
Direct Energy: “The energy used to transport building products to the site and then construct the building”.
Indirect Energy: “The energy used to acquire, process, and manufacture the building materials, including any transportation related to these activities”.
2) Recurring Embodied Energy: “… represents the non-renewable energy consumed to maintain, repair, restore, refurbish or replace materials, components, or systems during the life of the building”.
In its most conventional form, embodied energy is expressed as the amount of non-renewable energy that must be consumed to produce a unit of building material. Common units of embodied energy are Mega-Joules (MJ) per unit area (m2) or per unit of weight (kg or tonne). Numerous publications exist with tables of different building materials and corresponding embodied energies. From one source to the next, there can be significant variation and discrepancy in the amount of embodied energy that’s associated with certain building materials. These discrepancies reflect the complex nature of calculating embodied energy, the lack of standardized procedures for doing so, and the degree of uncertainty therein associated. For example, the amount of embodied energy in a building varies considerably with the type and quantity of building materials used, manufacturing process of materials, location of resources and site, building function, lifespan of the building, and many more factors.
"...calculating the embodied energy of a building is a multi-faceted process that’s as specific as the building itself."
Therefore, the initial embodied energy of a material manufactured in one country can differ significantly from the same building material manufactured in a different country. Whenever making a direct comparison of embodied energies between different building materials like steel, wood, and concrete, it’s critical that a consistent and appropriate set of assumptions be made to avoid any biased comparisons.
Similarly, the recurring embodied energy can vary greatly from one building to the next. The recurring embodied energy is directly related to the lifespan of the building. If more durable materials are initially selected and regular maintenance is performed, a particular building might survive longer than another, thus increasing the amount of recurring embodied energy associated with the building. Clearly, calculating the embodied energy of a building is a multi-faceted process that’s as specific as the building itself.
Despite the uncertainties associated with calculating the embodied energy of a building project, it’s useful to examine a general building case study in order to identify some predominant trends. Cole and Kernan studied the embodied energy of a typical Canadian office building constructed from three different structural systems: wood, steel, and concrete. The case study building was a 4620m2, three-storey office building located in Canada.