Metal Building Products and their Embodied Carbon

Because of this, embodied carbon in building material and its reduction is gaining industry awareness. This upfront CO2 (the global greenhouse gas (GHG) emissions) goes into making a building. It’s the carbon that’s generated from building construction, including extracting, transporting and manufacturing materials—carbon emitted before a building is even occupied. While architects are familiar with reducing GHG emissions from energy used in building operations, many architects are unfamiliar with embodied carbon in building materials. This is a problem that they are now being forced to confront.

“To limit global temperature rise and mitigate the worst effects of climate change, we need immediate and extensive reductions in the carbon footprint of building materials over the next few years,” says Michael Cropper, vice president, structural engineering, Thornton Tomasetti, Washington, D.C. “Since the embodied carbon footprint of buildings is locked in once the materials are in place, it has a large upfront impact on climate change. Energy use from building operations, on the other hand, may contribute to more emissions over time; however, it’s those short-term solutions that are critical to tackling climate change.”

Fabian Kremkus, AIA, LEED GA, principal, CO Architects, Los Angeles, agrees that we can’t reverse embodied carbon once a building is built. “We can retrofit energy-efficient systems such as photovoltaics and heat pumps, but the building’s carbon imprint from its foundation, floors and walls will never go away. We do design for maximum operational efficiency by considering heat gain/insulation and other factors to reduce energy consumption. One example is using metal to help with shading. Further, future legislation will likely restrict the amount of carbon used to manufacture building materials. [And,] aside from killing the planet, [architects are] particularly conscientious of embodied carbon on LEED projects to help achieve the highest rating possible.”

BUILDING MATERIALS

Bob Redwine, team carbon advisor, KL&A Engineers & Builders, Golden, Colo., explains there are several primary building products used in most larger buildings that consume large amounts of fossil fuels in their production and emit additional greenhouse gas emissions in their process-related chemical reactions: steel, aluminum, cement and glass. “As the commercial building market is a large consumer of these materials, it is also the source of about 11% of all annual global carbon emissions, with metals representing 20% to 30% of that total.”

How does metal measure up against other building materials in terms of carbon emissions? Kremkus explains that the biggest advantage of metal skins is their weight savings. “They don’t place as much burden on the structure and require less material than wood or cementitious alternatives. Cement products are particularly fossil fuel intensive to manufacture. Steel can have a significantly lower carbon footprint, particularly if the material is manufactured using electricity generated by renewable power. Recycled steel further reduces embodied carbon. It eliminates ore mining, requires less refining and minimizes waste. Virgin-steel manufacturing produces slag that is either discarded or repurposed as lower-grade material.

Fortunately, almost every piece of steel we use includes some degree of recycled material. Even better—but rarer—is up-cycling, when waste from one manufacturing process is used for a different purpose. An example might be excess sheeting from drink-can tops made into fencing material. Up-cycling limits the amount of additional energy required to make the different products.”

Amy Hattan, corporate responsibility officer, Thornton Tomasetti, Portland, Maine, agrees the carbon footprint of metal building materials is greatly influenced by the amount of recycled content and the energy consumed during production. “These two factors can have a significant impact on the amount of embodied carbon in a particular product.”

Hattan cautions that comparing the global warming potential of metal to another material is not accurate without knowing the qualities of a specific product and the whole building system. “Although it is tempting to compare the global warming potential of various materials on a weight or volume basis, you need to look at the whole building system to get the full picture. Each material has its strengths and weaknesses, and there are a range of factors to consider when choosing products for a particular project. These include building type, use, size and height, cost, aesthetics, schedule and market availability, among others. When comparing similar materials, environmental product declarations (EPDs), which provide information on the global warming potential of a particular product, can be a helpful tool.”

Brent Trenga, LEED AP BD+C, WELL AP, director of sustainability, Kingspan Insulated Panels North America, Deland, Fla., stresses looking at the EPD for the product you are specifying. “Look at type 3 specific EPDs. There are free, industry-wide tools available—like an embodied carbon calculator—to help everybody.”

Redwine explains comparing various building materials’ embodied carbon rankings is complicated. “Taking into consideration the structural capacity per pound of embodied carbon and using results from a life cycle assessment (LCA) is the best way to respond to this. When considering exterior enclosure materials (roof deck and exterior cladding) using the same approach, LCAs for functionally comparable enclosures result in the lowest embodied carbon intensity for wood materials, a higher embodied content for steel and the highest for concrete.”

Is metal at a disadvantage? Redwine believes the answer depends on what the metal is being used for. “When considering life cycle costs of exterior enclosures, metal may be competitive with wood and has an advantage over concrete products. For interior acoustical metal ceilings, steel and aluminum are at a disadvantage to the more typical mineral fiber ceiling tiles, with a much higher embodied carbon intensity.”

Trenga contends metal is not a disadvantage. He cites clean-energy electric arc furnace (EAF) technology. “There are more EAF mills coming. We have a major competitive advantage with how clean steel is here in the U.S. It’s some of the best in the world because Asia, China and even Europe are using blast oxygen furnaces. [Also,] metal has [a high carbon embodiment rank] because of how much steel we are using on a building. If you are going to wrap an entire façade with steel, that’s a lot of steel on a building.”

Scott admits that based on their volume used in construction, concrete and metal have the highest impact, but pound-for-pound, materials like rigid insulation might have a higher embodied carbon content. “The important thing is to consider, ‘What is the most appropriate material for a specific use?’, and to seek the products with the lowest embodied carbon within their product category. For example, for a particular mid-rise building, structural steel may be at a disadvantage compared to mass timber regarding embodied carbon in the structure, but even this wood building will have a concrete foundation, metal connectors, and quite possibly metal roofing, flashing, cladding, sunshades and other components. The lightweight, durability, longevity, flexibility, recyclability or other attributes may give metal building products a superior life cycle advantage for these components.”

ACCENTUATING METAL’S BENEFITS

What can be done to accentuate metal building products’ embodied carbon features versus other building materials? Kremkus explains the industry can highlight that metal doesn’t emit greenhouse gasses once manufactured; wood out-gasses methane as it decays. “[Also,] where advantageous, publicizing the products’ point of origin and percentage of recycled content when known might help metal’s reputation. Metal that is manufactured with electricity produced by renewable resources such as solar or hydro can be an environmental selling point.”

Cropper says, “To demonstrate its commitment to reducing embodied carbon, the metal construction industry can help promote high recycled content materials and low energy intensive production methods. It can also strive for greater transparency in terms of embodied carbon data and EPDs.”

Trenga says it’s about producing the documents; it’s not about marketing. “Show me the EPD for your product and compare it to a similar material alternative to what you are using. Ask manufacturers to do an LCA and to produce an EPD that is product specific. Compare like for like, compare the system for system. Look at the data that proves what the carbon of those materials is.”

Scott suggests moving beyond simple judgements of one material over another. “The metal construction industry can support informed decisions on material choices with LCAs of building components and whole-building systems to highlight and quantify circumstances where metal building has advantages. The industry can also encourage metal material supplies to adopt raw material procurement and manufacturing innovations to reduce greenhouse gas emissions, embodied carbon and other environmental impacts, and they can favor suppliers who innovate.”