Life-Cycle Assessment of Buildings

Canadian Architect defines Life-Cycle Assessment (LCA) as “…a composite measure of sustainability which draws upon many simpler measures”. These simpler measures include both the embodied energy and operating energy. LCA is an important tool for evaluating and comparing a building and its components in terms of a “cradle-to-grave” analysis. This analysis means tracking the environmental impact of raw material extraction, material transportation and manufacturing, building construction, building operation, building demolition, and building/material reuse/recycle.

"LCA is an important tool for evaluating and comparing a building and its components in terms of a 'cradle-to-grave' analysis."

Some interesting and useful results arise from a life-cycle comparison of the operating energy for typical buildings verses the embodied energy. Cole and Kernan examined the life-cycle energy use for a typical Canadian office building with no underground parking, averaged over wood, steel, and concrete structures. The results have been plotted for the typical office building located in Toronto, assuming typical building envelope construction and equipment efficiencies. Figure 11 illustrates the average life-cycle energy use for this typical office building located in Toronto over a 50-year lifespan.

Figure 11: Energy Use Over a 50-Year Life-Cycle for a Typical Office Building with No Underground Parking, Averaged Over Wood, Steel, and Concrete Structures in Toronto (Cole & Kernan, 1996).

It was found that the cumulative operational energy far surpasses the embodied energy in quantity over a 50-year life-cycle. In Figure 11, the operational energy is zero. However, at time zero there is an initial input of embodied energy due to the construction of the building. Around year 3, the operational energy exceeds the embodied energy and continues to increase at a far greater rate. At year 50, Figure 11 shows the operational energy accounts for approximately 88% of the total energy consumption for a typical office building in Toronto. Compared to only 12% for the total embodied energy at year 50 (5% initial embodied energy + 7% recurring embodied energy), it’s evident that relative to the operational energy, the embodied energy in a typical office building is a relatively small component of the total energy consumption at the end of a 50-year lifespan. Therefore, the greatest strides in reducing the energy use in a typical building today are made by reducing the operational energy, not the embodied energy of the building materials.

"...the embodied energy in a typical office building is a relatively small component of the total energy consumption at the end of a 50-year lifespan."

Cole and Kernan’s study can be extended to examine the difference in total life-cycle energy use between wood, steel, and concrete structural systems for the typical Canadian office building. Figure 12 illustrates that after 50 years for the case of a typical office building located in Vancouver, the total life-cycle energy use (embodied energy + operating energy) for a wood, steel, or concrete building only differs by less than 2%. The same is true if the building is located in Toronto, which has a harsher climate. Clearly, the typical building is almost entirely dominated by the operational energy, making the choice of structural system based on material type irrelevant from the standpoint of life-cycle energy use. However, as the efficiency of buildings is improved in the future and the operational energy decreases dramatically, the choice of material for the structural system will become more relevant. Until then, arguing that one material for the structural system is superior over another in terms of life-cycle energy use is simply not true according to this research.

Figure 12: Total Energy Use over a 50-Year Life-Cycle for a Typical Office Building with No Underground Parking in Vancouver and Toronto (Cole & Kernan, 1996).