Why method matters: Temporal, spatial and physical variations in LCA and their impact on choice of structural system

Abstract

Life Cycle Assessment (LCA) is increasingly used as an early-stage design-decision tool to support choices of structural system. However LCA modellers must first make numerous methodological decisions, and the resultant wide variations in approach are often inadequately described by the modellers. This paper identifies, and quantifies, the three major areas of methodological variation. These are: temporal differences in the stages considered; spatial differences in the material boundaries; and physical disparities in the data coefficients. The effects are then demonstrated through a case study of a student residential building in Cambridge. The cross-laminated timber (CLT) structure is compared with concrete frame, steel frame and load-bearing masonry, considering the influence that varying the temporal boundaries, the data coefficients, and the spatial boundaries has on the choice. While for this building CLT is confirmed as the lowest impact material, the paper demonstrates that varying the methodological choices can change the results by an alarming factor of 10 or even more. The findings confirm the need for the utmost clarity and transparency with all LCA calculations. Making wider industry or policy decisions based on LCA results should be undertaken with extreme caution.

Widening understanding of low embodied impact buildings: Results and recommendations from 80 multi-national quantitative and qualitative case studies

“…Impacts vary between 50-87 kg CO2-eq/m2 corresponding to between 5-22% of the whole life embodied impacts of the buildings, which is somewhat lower than for other life cycle stages. However as Moncaster et al (2018) point out the figures are frequently based on limited evidence. The effect of adding demolition activities and transport to suitable processing sites (stages C1-2) would also increase the figures and percentages for the end of life stage.…”

Embodied carbon of concrete in buildings, Part 1: analysis of published EPD

“…One of the key challenges for calculating embodied impacts from buildings is the choice of carbon coefficient for materials at the early design stage before they have been fully specified (Moncaster et al 2018). For concrete, the potential variations are almost infinite, dependent on the type and proportions of cement and aggregates, the addition of admixtures and plasticisers, and on the specific manufacturers' plants, processes and fuels.…”

“…A review by Pomponi and Moncaster (2018) found that this variation has led to a wide range of energy and carbon coefficients being assumed by academic researchers, in developing case studies of buildings. It has also led to considerable uncertainty for designers as to which is the appropriate value to use for the embodied carbon of concrete at the early design stage of a building, and the potential for extremely differing answers, as demonstrated by Moncaster et al (2018).…”

More Timber in Construction: Unanswered Questions and Future Challenges

“…A curiosity associated with one of these [20] is that when extending the assessment to include biogenic carbon according to the Cherubini method [21], the carbon cost associated with the use of biomass energy outweighed the carbon benefit associated with storage, and the relative advantage of timber was actually reduced. Although concrete has lower EC than timber per unit of mass [13], there is much evidence to suggest that the use of timber results in buildings with lower EC [22].…”