The construction industry is increasingly focused on embodied carbon as a critical part of curbing the climate impact of buildings. With the anticipated reentry of the U.S. in the Paris Agreement, we can expect that post-pandemic economic recovery will include investments in buildings and infrastructure, with a focus on cutting carbon dioxide (CO2) emissions. In addition to reducing operational carbon emissions (emissions from energy use over the life of the building), we also need to reduce embodied carbon (emissions from extraction, manufacturing, transport and installation of building materials).
Buildings are responsible for approximately 40% of annual global carbon emissions, with embodied carbon in materials accounting for almost one-third of these. Concrete and steel generate the bulk of embodied carbon emissions, followed by aluminum, glass and insulation.
As buildings become more efficient and renewable energy increases its share of energy sources, limiting embodied carbon emerges as critical to meeting climate targets. A consortium of architects, contractors and material manufacturers (including Gensler, Skanska and Interface Flooring) called materialsCAN (Carbon Action Network) has committed to reducing embodied carbon in buildings. Additionally, investors, including the Institutional Investors Group on Climate Change and Black Rock, are setting expectations for carbon emission reduction that will touch all major industries.
Mass timber construction (e.g., cross-laminated timber or CLT) is gaining traction as a low-carbon construction technology. Depending on the timber source (sustainable or conventional forestry) and the carbon intensity of the extraction, manufacturing and transportation, mass timber can have a carbon footprint significantly lower than the steel and concrete it replaces, and in some instances, may even be carbon negative (sequestering more carbon than generated). While mass timber is growing in popularity, there are limitations to its production and applications, so it will not displace all concrete and metals in buildings.
As the building material with the highest total carbon emissions (due to volume used and carbon intensity of cement), concrete is the primary focus of emission reduction efforts, but metals are getting increased attention. Metals remain important building materials for structures, exterior cladding and interior finishes due to their strength, durability and versatility, but the metal building industry must also address its significant carbon footprint and offer owners, architects and contractors the information and options they need to design and build low-carbon buildings.
Manufacturers of metal building materials and systems can proactively decarbonize their products by sourcing raw materials from lower-carbon suppliers, using renewable energy in manufacturing, increasing recycled content, and implementing innovations in fabrication and distribution. The steel industry is currently among the three largest global producers of carbon dioxide (~8%).
Each ton of steel produced emits approximately 1.85 tons of carbon dioxide. The production of aluminum has a smaller carbon footprint (~0.8% of global emissions) but it is more carbon intensive by weight (12 to 17 tons of emissions per ton of aluminum) and demand is increasing. Renewable energy and biomass in metal production can significantly reduce carbon emissions, and even achieve carbon neutrality. For example:
- The traditional production of steel from iron ore in a blast furnace/basic oxygen furnace (BOF) is carbon intensive because it uses coal (coke) as a fuel and reductant (de-oxidizing agent). Renewably produced hydrogen or biomass reductants (e.g., sugar cane, pyrolyzed eucalyptus) can be used in place of coal in a BOF to produce direct reduced iron (DIR). Hydrogen has been challenging as a renewable energy storage medium for transportation, but it lends itself well to steel production.
- Hydrogen can also be used for steel production in electric arc furnaces (EAF).
- Fossil fuel-based electricity (coal and natural gas) used for electrolysis accounts for approximately two-thirds of the greenhouse gases associated with aluminum production. With costs coming down and availability increasing, energy-efficient aluminum smelters can be powered with renewable energy, significantly reducing emissions.
Beyond the sourcing of lower carbon raw materials, increasing recycled content can significantly reduce carbon emissions in metals. Virgin steel has five times the embodied carbon of high recycled content steel, and virgin aluminum has a carbon footprint more than six times that of recycled aluminum. High-quality steel can be produced with efficient EAFs using high-quality scrap or lower quality scrap combined with renewably produced DIR, and aluminum is readily recyclable while maintaining quality. Manufacturers should work toward a circular economy (closed-loop recycling) by designing composite products to be easily recycled, and by investing in efficient material recovery infrastructure.
Another avenue for embodied carbon reduction in metals is careful consideration of manufacturing and distribution processes. Transportation can be 5% to 15% of the embodied carbon of a material. In some cases, factory assembly of modular or panelized building components could reduce the waste associated with on-site assembly of system components, while also optimizing distribution logistics. In other cases, site forming of metal building components (e.g., roof and wall panels) from coil reduces the transportation-related emissions (requiring fewer truck loads), while also reducing waste from cut-offs and shipping dunnage.
Manufacturers can support the informed specification and procurement of low-carbon materials while differentiating their products’ advantages by taking the steps noted above and quantifying the reductions with life cycle assessments (LCA) and environmental product declarations (EPDs). These will feed into databases like Circular Economy (ICE Database), ecoinvent and GaBi and analysis tools such as Tally and the Embodied Carbon Calculator for Construction (EC3). This material transparency also supports building certifications like LEED and the International Living Future Institute’s (ILFI) Living Building Challenge and Net Zero Carbon. Further, the European Union and several U.S. states are beginning to regulate embodied carbon in construction materials, so savvy manufacturers will want to stay ahead of the curve.
The building industry is taking embodied carbon emission reductions very seriously in the selection of materials and systems. The metal building industry should be proactive in reducing its carbon footprint and reporting material transparency to stay competitive and demonstrate leadership. Low embodied carbon is now a primary attribute of preferred building materials.
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.