BIM and Sustainable Construction. Real or Greenwashing?
The built environment, AKA our cities, towns, suburbs, and the infrastructure that ties it all together, has a huge impact on the natural environment. All buildings and infrastructure around the world are responsible for nearly 40 percent of global carbon emissions. Of that 40 percent, 28 percent is operational carbon, i.e. the energy that typically comes from the grid to keep the lights on and the equipment running in a building. The remaining 11 percent is embodied carbon, i.e. the energy it takes to manufacture building materials and the energy used during the construction stage of a project.
The embodied carbon metric is taking a forefront in the architecture, engineering, and construction (AEC) industry as the way to understand and benchmark the environmental impact of a project. So, how does one calculate embodied carbon emissions?
Embodied and operational carbon emissions stages are defined in the Life Cycle Assessment of a building or structure (fig 1). Embodied carbon emissions are from stages A1- A5, the product and construction process stages. It is crucial to calculate the embodied carbon early in the design stages so the project team has the time and scope to explore changes to the project design and materials to reduce the embodied carbon emissions.
Figure 1. Life Cycle Assessment Stages source: The Structural Engineer.org, July 2020.
Where does Building Information Model/Management (BIM) come into the picture to calculate the embodied carbon of building materials?
BIM is an increasingly popular tool used in AEC and the building industry at large. Essentially, BIM is a 3D digital model of a building or structure (like a bridge) created in the design stage of a project. It contains detailed information that goes far beyond typical 2D drawings. BIM is most commonly used for visualization and to improve efficiency in the construction process by using the model to detect design clashes (ex. HVAC, plumbing, electric within the walls) before the build stage to reduce the mistakes, and thus rework, on the jobsite.
The most exciting claim is that BIM can serve as the central model to support the entire development process for all parties involved, from the design stage to the completion of the project and be passed off to the owner as an “as build model” or “digital thread.” In this sense, many declare that BIM is a tool for sustainable construction due to the efficiency it brings to project management. However, where the rubber meets the road is when the embodied carbon estimates of materials are layered into BIM to calculate the emissions of a project.
Although, if embodied carbon tracking isn’t part of the BIM data layers, do the sustainability claims of BIM hold up? This question brings up the validity that process/efficiency improvements can be counted under sustainability action, ESG (environment social governance), or CSR (corporate social responsibility). While process improvements do help, for example, in construction, process improvements can reduce mistakes on the jobsite and thus rework, which requires the use of extra materials. But is that enough given the urgency of taking action to fight climate change and decarbonize our industries or does it fall under the category of greenwashing? Given the critical need to take dramatic action to decarbonize and limit global warming to under 1.5 degrees celsius, reported by the Intergovernmental Panel on Climate Change (IPCC), then process improvements barely make a dent toward progress.
Moving back to BIM and how accurate it is at estimating embodied carbon emissions today, research professor Dr. Shoshanna Saxe, at the University of Toronto in the Department of Civil and Mineral Engineering, and her team set out to answer the question of how accurate BIM is at estimating embodied carbon. And the answer is… not very accurate.
In a recent study, “Embodied greenhouse gas assessment of a bridge: A comparison of preconstruction Building Information Model and construction records,” published 2021 in the Journal of Cleaner Production, Dr. Saxe and her team performed a comparative analysis to determine the estimated vs actual embodied carbon emissions of a highway bridge in Ontario, Canada. The result was that the BIM embodied carbon emissions assessment was 212 percent short of the actual embodied carbon emissions.
Why was the estimate from BIM so inaccurate and where does BIM fall short?
The reality is that architecting and constructing are complicated processes. Part of the reason BIM falls short is because the design stage of a project is often too abstract and that can be reflected in how the model’s systems do not match the real world systems. Also, if BIM is mainly used for visualization and clash detection then the embodied carbon assessment takes a back seat and attention isn’t given to make it as accurate as possible.
However, the most significant reason why the BIM assessment was off is because things change in the process of construction and those changes aren’t accounted for in BIM.
It is rare (if ever) that a building or structure is constructed just as it is designed. The BIM embodied carbon estimates only represent the materials that are designed to be placed in the structure, and thus do not capture all the materials that will be used in the process of construction. It’s standard practice to slightly over-order some materials and stockpile materials on-site to avoid construction delays. Also many of the embodied carbon estimates of certain materials are dependent on the process of how they are delivered and handled on construction sites. So for an embodied carbon assessment to be accurate it requires detailed instructions about construction processes and on-site activities for the workers. None of these things are typically included in BIM.
In this study, Dr. Saxe and her team were among the first to clearly identify the need for construction processes and on-site activities to be incorporated in the embodied carbon assessment. While this isn’t a practice today, one of the major strengths of BIM is that it is technically possible to include construction processes and activities related to calculating embodied carbon in the model. Similar to how construction processes like schedule management and cost management can be included in BIMs today.
The building industry has a long way to go in terms of capturing and tracking data that is relevant to making climate conscious decisions. Technology solutions, like BIM, can serve as powerful tools to help the industry advance their on-site practices and give visibility into their activities. Greater emphasis also needs to be put on education of embodied carbon and the impact the built environment has on the climate crisis.
It is important that industry players, like technology companies and policy makers, collaborate to improve embodied carbon calculation methods and make it common practice on all construction projects. With significant growing global demands to improve legacy infrastructure and build new climate-resilient infrastructure, it is critical that the building industry leverages all the tools and processes available to measure, track, and reduce the embodied carbon footprint of each project.