by Emily Bergeron
Every ton of carbon emitted stays in the atmosphere for hundreds of years; these emissions are cumulative. Buildings generate around 39 percent of annual carbon emissions worldwide. Twenty-eight percent comes from operating existing buildings, and 11 percent from new construction.[i] A significant cost to the built environment is embodied carbon. As the world’s population approaches 10 billion, the global building stock is expected to double in size, making understanding and addressing this issue of ’upfront carbon’ critical. In this lesson, we will look more closely at the emissions associated with the materials and construction processes throughout the lifecycle of buildings and infrastructure.
A building’s carbon footprint is composed of embodied and operational carbon. Embodied carbon is the CO2 created in producing building materials (e.g., materials extraction, transport to manufacturing, and manufacturing), transportation to the site, and during construction. It is also the carbon produced while maintaining and ultimately demolishing, transporting, and recycling those materials. Operational carbon comes from energy consumption – heat, lighting, etc. Embodied carbon is largely produced at the beginning of the building life cycle. These emissions are fixed, unlike operational emissions, which can be periodically altered by upgrading lighting, mechanical systems, or other equipment. Once a concrete foundation is poured, the carbon emitted during manufacture and transport can never be recovered.[ii]
Efforts to curb carbon emissions from building operations have shown some success – partly attributable to converting the grid to renewable power sources. Between 2005 and 2019, CO2 emissions from construction operations declined by 21 percent in the United States, even with an increase in total floor area.[iii] As we continue to reduce operational carbon, embodied carbon will account for a more significant part of buildings’ overall carbon footprint. Between now and 2050, it is anticipated that embodied carbon will account for nearly 50% of the overall carbon footprint of new construction.
Embodied carbon can be challenging to measure – having been called the building industry’s “blind spot.”[iv] This is partly because of a need for more agreement on measuring embodied carbon in buildings.[v] And a need for comprehensive data on the issue. Such emissions data on building operations is available, but currently, a government agency needs to curate this information. One generally accepted strategy for measuring is called life cycle assessment (LCA), which looks at the environmental impacts of a building from raw material extraction through end-of-life and disposal. Site activity emissions are also sometimes added to the calculation. The embodied carbon calculation typically multiplies the quantity of each material or product by a carbon factor (customarily measured in kgCO2e per kg of material) for each lifecycle stage being considered:
Embodied carbon = quantity × carbon factor
Project lifecycle showing both the scope of the definition and need for whole life consideration. From the report Bringing Embodied Carbon Upfront by the World Green Building Council.
Materials Acquisition. Raw materials extraction, transport to process, process/manufacturing.
Construction. Transport to site, construction, and installation.
Use. Use, maintenance and repair, replacement, refurbishment, and operational use.
End of Life. Demolition, transport, waste processing, disposal, and recycling.
There are a variety of tools available to aid in collecting the data for these assessments: calculators, online or spreadsheet-based tools, which help determine the order of magnitude of embodied carbon early in the design stages before modeling begins, design-integrated LCA tools that allow you to calculate the environmental impacts of building material selections directly in a design model (e.g., Tally, a plug-in for Revit), product selection/procurement tools that collect product data and aid in comparing options, and professional LCA software.
For guidance on how to calculate embodied carbon, see:
There are many ways to deal with the impact of embodied carbon. The AIA lists ten ways to reduce embodied carbon:[vi]
Suppose we are to reduce the emissions associated with the construction industry. In that case, it is critical that we improve building energy performance, decrease building materials’ carbon footprint, increase investment in energy efficiency, and be mindful of embodied carbon. For example, construction energy consumption could be addressed by changing how we produce materials. For example, with concrete, innovation could focus on lowering cement’s carbon footprint or finding an alternative. Cement kilns could utilize alternative methods for heating cement kilns rather than fossil fuels. Another possibility for reducing impact is recycling construction waste. Rather than the increased emissions that result from creating all new materials, projects can use recycled materials instead.
Another way that this aspect of construction’s carbon footprint is to utilize the existing building stock better. When you consider the amount of new construction projected and assume the role of embodied carbon, it is also vital that we look at existing building stock for adaptive reuse. Historic buildings have embodied energy that is lost when a building is demolished. According to a study commissioned by the federal Advisory Council on Historic Preservation (ACHP), about 80 billion BTUs of energy are embodied in a typical 50,000-square-foot commercial building, the equivalent of about 640,000 gallons of gasoline (ACHP, 1979). If a building is demolished rather than reused, that expended energy and carbon is essentially wasted, and even more is spent on the demolition process and new construction. In upcoming lessons, we will consider if old buildings can be as green as (or greener than) old ones.
For more on embodied carbon:
[i] International Energy Agency, 2018 Global Status Report: Towards a Zero Emission, Efficient and Resilient Buildings and Construction Sector(Nairobi: United Nations Environment Programme, 2018).
[ii] Amy Cortese, “The Embodied Carbon Conundrum: Solving for all Emission Sources from the Built Environment,” New Buildings Institute, February 26, 2020.
[iv] Anthony Pak, “Embodied Carbon: The Blind Spot of the Building Industry,” Canadian Architect, July 3, 2019, 26–29.
[v] Manish K. Dixit, “Life Cycle Recurrent Embodied Energy Calculation in Buildings: A Review,” Journal of Cleaner Production (March 2018): 732.