Embodied carbon is changing the buildings conversation
December 09, 2020
December 09, 2020
It’s time for us to rethink the process of designing buildings. We must commit to using embodied carbon analysis to drive design decisions
This article first appeared as “It’s all about embodied carbon” in the Stantec Design Quarterly, Issue 10.
Until recently, much of the conversation about sustainability in the buildings industry was centered around operational carbon. That’s the carbon emitted by buildings or the energy buildings consume in their day-to-day operations.
We approached sustainability as a matter of energy performance—of reducing that appetite. The goal was to design an energy-efficient building so that on day one that appetite was lower and so were the utility bills. That approach shaped state energy efficiency codes such as Title-24 in California, Seattle Energy Code, the ASHRAE Standards, and green-building certifications such as LEED, Living Building Challenge, and Green Globes.
The mantra was design or engineer a building so that from day one it uses less energy to operate.
Reducing the operational energy needs of buildings makes sense and will continue to be an essential part of solving the climate change crisis. What we missed by only considering operational energy was the energy investment in the building that it inherits right from day one—its embodied carbon or embodied energy.
Simply put, what volume of carbon emissions did that building contribute to the atmosphere—the air we breathe—before it even opened?
We define embodied carbon as the greenhouse gas emissions (in carbon dioxide equivalent) attributed to manufacturing and transportation of construction materials and the process of construction. Unlike the building systems, which can be replaced with more efficient ones and improved over time, this embodied carbon amount is fixed once it’s been spent. In essence, we (and the planet) live forever with the decisions that the whole project team—from designer to contractor to client—make when designing the original building. A groundswell of interest in embodied carbon from the design industry and the public at large has resulted in the availability of more information about materials and their origin as well as an increasing range of alternative, naturally derived materials.
The climate crisis has forced our hands to act. Science tells us that if we keep on emitting carbon dioxide as business as usual then within the next 10 years, the global temp will rise 1.5° Celsius—and that rise will be irreversible. At the next milestone, a rise of 2°C, we’ll see catastrophic changes in the climate.
Recent reports from the Intergovernmental Panel on Climate Change sternly recommend zero carbon emissions globally by 2050 to stall temperature increase. To target zero emissions, we need a “carbon budget” for the planet. This budget is the max carbon that people can consume annually to keep us on track to phase out carbon emissions by 2050. The lower the carbon budget, the better our chances are of stalling temperature increases. To stall the 1.5°C rise, our carbon budget for the planet is 340 Gt CO2. This carbon budget has a 67% probability to stall the 1.5°C rise and keep us on track to phase out carbon emissions.
Buildings operations accounts for 28% of the global carbon bill. Buildings infrastructure/ materials account for 11% total annual global emissions by sector. Making the steel, concrete, and glass for buildings and transporting them to the site consumes a lot of energy. Our longtime focus on operational carbon has resulted in the possibility of a lopsided carbon footprint for an energy-efficient building. Embodied carbon might account for 66% of the total carbon footprint of a new energy-efficient building with a building life cycle of 30 years.
We’ve come a long way in terms of what’s possible in operational efficiency. But looking at the lifecycle of a building, we now see that there’s a lot more we can do. Something has got to change. When we begin to analyze buildings for their embodied carbon, it’s clear there are opportunities to design differently.
A whopping 8% of global emissions come from the fiery kilns that manufacture cement and concrete, of which Portland Cement Concrete is the most common. This is global phenomena. It’s an extremely energy intensive process to make concrete and the world makes a lot of it.
Fly ash is a byproduct of the cement-manufacturing process and used to be thrown away. But in fact, it has similar binding properties to cement with the right aggregates. And it turns out if we add fly ash back into the cement mix—using 50-60% fly ash instead of 100% Portland cement with aggregates—we can significantly reduce the carbon emissions associated with manufacturing. Studies say adding fly ash reduces the water required to make cement and results in a more workable, pumpable, and stronger concrete product.
Common-sense approaches like this, driven by an awareness of embodied carbon, are going to help us meet our carbon goals as an industry. Similarly, steel manufactured from electric arc furnaces has much lower embodied carbon than that made from a basic oxygen furnace.
What volume of carbon emissions did that building contribute to the atmosphere—the air we breathe—before it even opened?
Broadly speaking, we must look at various ways to sequester carbon—lock up CO2 out of the atmosphere. Innovative technologies such as low carbon concrete, which injects carbon into the concrete mix, promise some sequestration. Another approach is using cross-laminated timber (CLT), where pieces of wood are pressed together to create a super strong timber building material. It’s one of the most promising developments for those interested in designing with embodied carbon in mind. CLT is strong enough that we can use it in buildings over 10 stories. It is procured from managed, sustainable forests and leftover wood scraps. Even accounting for the glue, manufacturing, and transport, CLT is a clear winner.
Just like CLT, natural and biobased materials can lock up CO2. For interiors, this means choosing natural, renewable, and biobased materials like bamboo, cellulose, cork, wood-fiber board, insulation with waste denim, and linoleum. Even straw bales for insulation has benefits regarding embodied carbon. Hempcrete, an innovative product combining the hemp plant core with concrete is strong, elastic, and sequesters carbon.
Building materials can be carbon negative. Plants that have sequestered a certain amount of carbon during their lifecycle are carbon negative. If the volume of this carbon outweighs that used in the processing, manufacture, and transportation of the material, the building material can be considered carbon negative.
Today, if we do an embodied carbon centric design, the first thing architects must do is a whole building life cycle assessment (LCA) analysis. The LCA has been part of LEED rating system since LEED version 4 debuted in 2014. Software tools can help a designer accomplish the LCA. In the LCA, we analyze and report the environmental impacts of a building or product using the metrics of global-warming potential, ozone-depletion potential, acidification, eutrophication, smog formation, and depletion of nonrenewable energy sources. The LCA quantifies the building’s carbon footprint (and that of the site materials), which gives us both a benchmark and a pathway toward specifying materials that can reduce the embodied carbon of the project.
If we can make a case for substituting highly efficient concrete, CLT, or carbon-negative materials, the result will be less carbon-intensive buildings. But to do this we need to target a figure for embodied carbon. We need something like the energy use index for operational carbon, which gives designers and engineers something to aim for relative to other projects. We need a carbon budget for each project. To transform our industry we need more standards, more sophisticated tools for tracking and modeling embodied carbon, and eventually we need baseline codes that capture and reflect the carbon intensity of materials choices.
Of course, the most sustainable approach is to reuse and reposition our existing buildings. However, the desire for new construction is not going to end. With population growth and urbanization, we can predict that humans will require 2 trillion square feet of new building floor area over the next three decades. That is the equivalent of building a new New York City once a month for 32 years. We need to make those buildings low or even negative carbon if we can.
Time to rethink
It’s time for us to rethink the process of designing buildings. We need to rethink about materials and commit to using the results of embodied carbon analysis to drive design decisions.
This means carrying out LCA early on the design process, so we make better long-term decisions about building materials. As informed designers, we must educate clients about the repercussions of the choice of materials in their buildings. And we can’t just talk about this in closed circles on the conference and lecture circuits. We need to bring the embodied carbon conversation to the public even if it means acknowledging the grim reality and how buildings contribute to it.