Unpacking Embodied Carbon

An increasing number of building professionals are taking this challenge seriously and prioritizing the decarbonization of the buildings they design, build and operate. According to Architecture 2030, the operation of buildings is responsible for 28% of global emissions, while building materials and construction account for 11%. These two emissions sources are often referred to respectively as operational carbon (emissions from energy use over the life of the building), and embodied carbon (emissions from extraction, manufacturing, transport, and installation of building materials). Until recently, emission reduction strategies have focused on operational carbon, but embodied carbon is now receiving increased attention. There are several reasons why:

• The International Energy Agency estimates that the existing global building stock will almost double by 2050, representing significant embodied carbon emissions.
• The emissions associated with materials and construction are front-end loaded, with cumulative operational emissions not catching up until 20 to 30 years after construction. This burst of emissions with each new building accelerates climate change.
• Electricity production is rapidly decarbonizing, with less coal generation and more renewable energy feeding the grid each year. This, combined with increasingly efficient buildings, means that embodied carbon could soon surpass operational emissions.

Before reviewing reduction strategies, let’s get clear on terminology. Embodied carbon could more accurately be considered “disembodied” carbon, as it is not resident in the subject building material but rather represents the greenhouse gas emissions released into the atmosphere from energy use and other processes required to create the material. This contrasts with “stored” carbon, which refers to the atmospheric carbon taken in by plants and stored in the biospheric carbon pool (e.g., forests). When trees are harvested, about 50% of stored carbon is transferred to the long-lived “anthropospheric” carbon pool in lumber products, where it is stored, likely for decades, until the material burns or decomposes. Much of the remaining stored carbon from a harvested forest returns to the atmosphere in short order through decomposition and burning of waste and disposal of short-lived biomass like pulp and paper. It is best to avoid using the word “sequestered” as there is disagreement on the definition, and it primarily refers to engineered processes to capture CO₂ emissions and permanently sequester them in the geosphere.

Embodied carbon in buildings can be reduced with both building- and material-focused strategies. Building level approaches include:

• Reusing existing buildings, as even major renovations have less than half of the carbon footprint of new construction.
• Maximizing the durability and maintainability of the building enclosure and finishes, by selecting durable materials that are easily refurbished, and designing and constructing high-performance enclosures that resists moisture intrusion and air infiltration and conducting building enclosure commissioning (BECx).
• Designing for adaptability and flexibility, so that the building will continue to be valuable and useful as programmatic needs change over time.
• Using less material, with choices like avoiding basements (the most carbon intensive part of a structure) or using off-site construction (up to 98% material waste reduction).

Material approaches to embodied carbon reduction include selecting specific products with lower carbon intensity or substituting conventional products with low-carbon alternatives. For example, concrete is the most carbon intensive material in traditional construction, primarily due to the emissions from cement production. Iron slag, coal fly ash and limestone calcined clay can all be substituted for part of the cement in some concrete mix designs, significantly reducing embodied emissions. Another example can be found with insulation. In place of extruded polystyrene (XPS) insulation which has very high embodied carbon, a project team can specify mineral wool board with a fraction of the emissions. The increased focus on emissions reductions is also leading to manufacturing innovation, including capturing CO₂ and injecting it into concrete as it is mixed, or using renewably-produced hydrogen in electric arc furnace (EAF) steel production or biomass reductants (e.g., sugar cane, eucalyptus) in basic oxygen furnace (BOF) production.

Embodied carbon is measured in carbon dioxide equivalent (CO₂e) emissions per unit of material, accounting for the global warming potential (GWP) of the various greenhouse gas emissions (methane, hydrofluorocarbons, etc.) relative to the GWP of CO_2. This is determined by conducting a life cycle assessment (LCA) which inventories the environmental impacts, including global warming potential, of a product’s life cycle from cradle to gate (raw material extraction through manufacturing stages) or cradle to grave (extraction through end-of-life). The results of an LCA can be summarized in an environmental product declaration (EPD), a standardized, third-party verified accounting of a product’s environmental impacts. There are several databases of EPDs including the Transparency Catalog by Sustainable Minds. EPDs are recognized by the United States Green Building Council (USGBC) in the LEED rating system, by the International Living Future Institute (ILFI) in their Declare label, and Living Building Challenge and Zero Carbon certification, and by other international sustainable building frameworks like BREEAM and Green Star.

The embodied carbon of an entire building can be estimated during design using tools like Tally (a BIM plug-in developed by Kieran Timberlake) or One Click LCA, and during construction procurement using the Embodied Carbon in Construction Calculator (EC3), developed by the University of Washington Carbon Leadership Forum with support from an industry consortium including Skanska and Microsoft.

There is no doubt about the need for all of us to act on the global climate crisis. Material manufacturers, architects, structural engineers, and contractors all have a role to play in developing, specifying, and procuring low-embodied carbon materials. Educate yourself on the best ways you can engage, ask manufacturers about the embodied carbon of their products, specify lower-carbon materials on your next project, and help your peers to do the same.

Alan Scott, FAIA, LEED Fellow, LEED AP BD+C, O+M, WELL AP, CEM, is an architect with over 30 years of experience in sustainable building design. He is a senior consultant with Intertek Building Science Solutions in Portland, Ore. To learn more, follow Scott on Twitter @alanscott_faia.