Embodied carbon explained
“Embodied carbon” is an imperfect term. The word “embodied” sounds like we’re talking about carbon encapsulated in a material. Instead, it’s shorthand for all the lifetime indirect greenhouse gas (GHG) emissions due to a building—in other words, everything other than emissions from building operations. For example, the GHGs emitted from diesel combustion in transporting a product to the building site are part of the embodied carbon in the product.
Embodied carbon is also known as value chain emissions, upstream/downstream emissions, or Scope 3 emissions.1 The complete carbon footprint of a building includes all of these GHG emissions. A true zero-carbon building would account for and offset its operational carbon as well as its embodied carbon.
Most embodied carbon emissions are upstream or “upfront” of building occupancy—they are primarily related to the manufacturing of materials. This includes the extraction of raw resources, manufacturing of building products, and transportation of those products.
GHG emissions due to material manufacturing, use and disposal are more significant than many people realize. First, these emissions are a big upfront GHG pulse in the life of a building, which makes them a good near-term target for climate change mitigation. Second, as buildings approach net-zero carbon operation, embodied impacts will make up most of the carbon footprint in the built environment.
Embodied carbon has a lot of buzz lately, and that’s inspiring to some design professionals. Kevin Welsh, Senior Sustainability Advisor at Integral Group, is one of them: “It’s great to see the accelerating interest in embodied carbon. It’s the next evolution of our industry’s enthusiasm and dedication towards reducing the impacts of projects.”
How to measure embodied carbon
Embodied carbon reduction begins with data. Without data, we’re just guessing about where to look for improvements, and what decisions are actually beneficial. To bring in data, we need life-cycle assessment (LCA).
LCA is a holistic environmental impact assessment method. A cradle-to-grave LCA for a building accounts for all the lifetime flows between the building and nature, and then estimates the impact of those flows on the planet. An LCA provides multiple results having to do with damage to air, land and water.
Embodied carbon is one of these results—the global warming potential (GWP), expressed in equivalent tonnes of CO2. To calculate embodied carbon requires a full LCA study, although only one result from the study—the GWP—will be used.
Assessing embodied carbon impacts of design and material decisions is always a case-by-case situation involving cradle-to-grave LCA in the context of the whole building. There is no shortcut for this. And whole-building LCA is tricky. But two well-respected North American software tools make it easier for design teams: the Athena Impact Estimator for Buildings (a free standalone tool, produced by the Canada-based non-profit research group that I head) and Tally (a Revit plugin, developed by architecture firm Kieran Timberlake).
One word of caution: embodied carbon calculations are estimates, not absolutes. While LCA is a well-established, rigorous science guided by international standards, it is inexact. There are many variables and assumptions in LCA, and some data gaps and methodology question marks. The uncertainty in results increases with long-lived and complicated products like buildings.
Tactics to reduce embodied carbon
Consumption of anything has environmental impact, and making a building consumes a lot of resources. The easiest way to reduce embodied carbon is to consume fewer resources. That can mean less new construction, smaller new construction, less materials in new construction, and less frequent material replacements.
Another easy win is to be mindful in the use of products with high embodied carbon, such that their use is optimized. For example, cement is commonly over-specified (i.e. wasted) through practices like blanket specifications for 28-day strength concrete. If strength isn’t needed so quickly, the cement content in the concrete can be reduced, thereby reducing embodied emissions.
But otherwise, there are no silver bullets. Looking for prescriptive answers is oversimplifying the problem and risking an unintentionally bad result. It’s very difficult to justify the generic benefit of specific materials, building elements or tactics. Reducing embodied carbon usually requires an iterative process and a balancing of trade-offs. That requires a project-specific, whole-building, cradle-to-grave LCA study.
Getting the most bang for the buck usually means starting with the structural materials, which typically comprise most of a building’s mass. Structural engineers are important partners to engage early for reaching sustainability goals.
What about wood and other bio-based products? These materials store carbon that was removed from the atmosphere by living plants, which makes for an interesting and complicated life-cycle carbon story. On the surface, bio-based products like mass timber from sustainably managed forests might be carbon winners. But there are methodology questions and data gaps in cradle-to-grave carbon accounting for wood products. This creates some uncertainty in embodied carbon calculation results. The bottom line: no single material will solve the embodied carbon problem, and designers need to choose all materials carefully.
Reducing embodied carbon will require a suite of tactics. Mark Lucuik, Director of Sustainability at Morrison Hershfield, has a great example from a recent project: a large bus storage facility in Calgary. With an integrated design process, the team designed a more efficient mechanical system that enabled a lower roof, reducing the height (and therefore material use) of the walls. The team also eliminated some finish materials, customized the concrete mixes, and brought a carbon perspective to the selection of insulation, roofing membrane and other materials. “In the end, we achieved an 18 percent reduction in embodied carbon and, equally important, big reductions in five other LCA measures,” says Lucuik. “We achieved this reduction with decreases in capital cost and improvements in energy efficiency. A triple win.”
This kind of work takes effort and commitment, which is why it doesn’t happen very often. There are a lot of hurdles to be cleared before embodied carbon becomes a common focus in mainstream design and construction.
A case for keeping buildings around a lot longer
Existing buildings represent embodied carbon already in the atmosphere. Choosing to keep buildings in service for as long as possible helps amortize that carbon debt, by avoiding the new emissions that would be caused from demolition and replacement. There are many factors that support the case for building replacement. Can architects create longer-lasting buildings by designing for adaptability? Can architects help make the case for the adaptive reuse of existing assets? Consider the perspective of the University of British Columbia, a large building owner with many aging assets. “UBC Renew” is a program that aims to minimize the financial and environmental impact of construction on campus, by supporting the rehabilitation of existing buildings.
Who owns this problem?
Pledges and calls to action on embodied carbon are easy. But who’s going to actually get this job done? As Richard Hammond, principal at London, Ontario-based Cornerstone Architecture, puts it, “There is no single solution, and progress needs to come from a confluence of economic, social and technical initiatives. Every sector of our society has a role to play.”
Most embodied carbon stems from material manufacturing, so we might look to industry for solutions. And industry is certainly stepping up, with gains especially evident in cement and concrete. “As a significant GHG emitter, our industry has been working hard on improvements for a long time,” says Adam Auer of the Cement Association of Canada. An example is the development of Portland limestone cement (PLC), a replacement for general-use cement that cuts embodied carbon by 10 percent.
Steel is another product associated with high carbon impacts. Mark Thimons, from the American Iron and Steel Institute, says that “since 1990, the North American steel industry has reduced its average energy intensity and greenhouse gas (GHG) emissions intensity by 35 and 37 percent, respectively.” He says the industry is also working to develop an entirely new process for the production of iron. The objective of Flash Ironmaking Technology is to significantly decrease energy use and reduce environmental impacts, especially CO2 emissions.
Industry can come up with innovative, low carbon products, but it can’t force the market to use them. Bob Larocque at the Forest Products Association of Canada notes that building codes are inhibiting the use of new products like cross-laminated timber, which has demonstrated embodied carbon benefits in tall buildings. Adam Auer identifies specification and procurement policies as a big barrier. “For example, government is the biggest customer of concrete,” he says. “If carbon isn’t part of government procurement policy, then PLC doesn’t get specified—and there is insufficient demand for concrete manufacturers to even have PLC on hand.”
Innovation from industry is only part of the solution—this is the supply side of the issue. If embodied carbon is fundamentally about resource consumption, then the demand side of the problem is equally important. Decisions about how to design, what to build, how big to build, and even whether to build at all have a huge impact on embodied carbon.
Material sourcing can be a big deal, particularly given the competitive pressure on domestic industry from imports. For example, Mark Thimons notes that “the embodied carbon of steel produced in North America is considerably lower than the embodied carbon of steel imported from many other countries, especially when transportation is considered.”
Embodied carbon action will be most strongly affected by policy and financial market drivers. Currently, it’s difficult to make a business case for reducing embodied carbon, as noted by architect and Athena Institute chairman Stephen Pope: “Life-cycle accounting for the built environment has been talked about for decades, but there is never money in the budget to do the analysis. With no monetization of carbon emissions, there is little to lure the business community to better behaviour through the use of carbon accounting.”
What if embodied carbon had a price? The Living Building Challenge requires purchase of an offset for embodied carbon. If that approach was widely adopted in policy, then a clear financial driver would be in place to reduce embodied carbon, assuming the market rate for offsets is high enough. Consider the new Joyce Centre for Partnership & Innovation at Mohawk College, which was certified under the CaGBC Zero Carbon Building program and therefore had to declare its embodied carbon. The design focused on achieving a very low operational energy consumption of 73 eKWh/m2/year. However, an offset for the 4,330 tonnes of embodied CO2 in this building would have cost roughly $104,000 at today’s low carbon price of $24 per tonne. As carbon prices rise, the incentive to reduce embodied carbon would get stronger.
Policy requiring embodied carbon disclosure can be a big help in raising awareness and motivating LCA skill development. It’s a great first step in embodied carbon policy and is consistent with the spirit of transparent reporting so evident in other aspects of sustainable design. Disclosure can also support development of embodied carbon baselines and benchmarks. Without those in hand, it’s hard to rationalize performance targets.
We will likely see growing ownership of this problem in the policy sphere, but it will be important that policy does not get ahead of available materials, building systems and the underlying LCA infrastructure. The technical underpinning is crucial to the reliability and comparability of embodied carbon assessments. Materials data, LCA methodology, and benchmarking need some work. A major Canadian initiative2 led by the National Research Council is addressing some of this and will really help move the ball forward.
In the absence of a policy “stick,” maybe a market “carrot” for disclosure will emerge. Kevin Welsh likes the idea of “a new niche certification scheme for buildings, for embodied carbon.”
How low can we go?
There are ambitious calls to action out there for embodied carbon reductions. For example, the 2030 Challenge3 and a new report from the World Green Building Council4 call for zero embodied carbon by 2050. How realistic is that target?
Richard Hammond thinks deep cuts are feasible today, but “getting all new buildings to net zero by 2050 is a very big leap that will depend on a number of factors coming together, including broadly accepted carbon pricing and widely available new technologies.”
With materials and processes available today, embodied carbon reductions in the range of 10 to 25 percent are easily achievable for many projects. But finding significant savings requires an unusually committed client and “a strong integrated design process, with everyone on board from the beginning,” says Kathy Wardle, Director of Sustainability at Perkins and Will Vancouver.
The technical challenge in achieving zero embodied carbon is partly illustrated by looking at one corner of the story: what’s possible with concrete. Adam Auer of the Cement Association thinks a 40 percent GHG emissions reduction for concrete is realistic, but will take collaboration across the construction value chain as well as with policy makers. Switching to Portland limestone cement reduces GHGs by 10 percent. Fuel switching in manufacturing might yield another 20 percent. Optimizing the amount of cement in a mix could yield further reductions of 20 percent or more. Getting to carbon neutrality will require carbon capture at manufacturing facilities, a technology that still faces a number of technological and economic hurdles. But Auer says the cement industry is recognised as an ideal candidate for carbon capture, and is very active in Canada and globally in its pursuit of the technology.
Let’s extend these thoughts to the entire value chain for buildings and consider a few of the things that will need to happen in order to achieve zero embodied carbon without the purchase of offsets. All equipment used in resource extraction and the manufacturing and transportation of construction products would operate without fossil fuel. All structural wood products would be harvested from sustainably managed forests. All manufacturing facilities would have zero carbon emissions by using carbon capture and/or fuel switching to 100 percent non-fossil energy. All equipment on a construction site—as well as equipment used for demolition and the transportation of waste—would operate without fossil fuel.
Or perhaps technologies will emerge that enable carbon-capturing buildings: that is, buildings that actively remove GHGs from the atmosphere during their lifetime, to offset some of their embodied carbon. Exposed concrete already does this somewhat as it ages in place—a process called carbonation, which is not typically accounted for in embodied carbon calculations, and could be optimized by greater exposure to air of concrete in service and at its end of life. Maybe new building technologies will be developed that similarly absorb carbon dioxide.
In the short term, achieving zero embodied carbon will require the purchase of offsets. An innovative carbon trading system would use the revenue from offsets purchased for new construction to create financial incentives for retaining existing buildings and upgrading them for energy efficiency.
But is “how low can we go” the wrong question? In a race to zero, we may lose sight of the bigger picture. For example, building operations is still the biggest piece of the lifetime carbon pie, and needs to be balanced against embodied impacts. Many high-performance buildings have relatively high embodied impacts. “We don’t want to encourage poor operating performance to achieve an embodied reduction—we need to take a life-cycle perspective,” says Mark Lucuik.
In addition, we “don’t have enough benchmark data to know whether the levels of performance proposed by the challenges are appropriate,” says Stephen Pope. In fact, the whole carbon question is even more complex and nuanced than that. Says Pope: “Rather than looking at carbon use as an absolute, we have to look at carbon as a loop cycle. Some of it is always in use, but the use needs to be balanced.”
Current hurdles aside, the enthusiasm for this topic is exciting and encouraging. Kathy Wardle is among those embracing the challenge: “It’s nice to see this momentum building for embodied carbon, because we need to address all of the complex issues around materials—including life-cycle ecosystem and human health impacts.”
Jennifer O’Connor is President of the Athena Sustainable Materials Institute. The Athena Institute is a non-profit research group that advocates for environmental performance measurement and accountability in the built environment and provides free life-cycle assessment resources.
1 Carbon accounting practice widely follows the GHG Protocol and its language (https://ghgprotocol.org). Scope 1 is direct emissions from sources controlled by an entity (site emissions, in the context of a building). Scope 2 is indirect emissions from the generation of purchased energy (source emissions, in the context of a building). Scope 3 is other indirect emissions in the value chain (in the context of a building, this would include embodied carbon, occupant commute travel, and so forth).
2 To learn more, see https://nrc.canada.ca/en/research-development/research-collaboration/programs/low-carbon-assets-through-life-cycle-assessment-initiative.
3 “The embodied carbon emissions from all buildings, infrastructure, and associated materials shall immediately meet a maximum global warming potential (GWP) of 40% below the industry average today. The GWP reduction shall be increased to: 45% or better in 2025, 50% or better in 2030, and Zero GWP by 2050.” https://architecture2030.org/2030_challenges/embodied/ – Accessed Oct 1, 2019.
4 M. Adams et al, Bringing embodied carbon upfront: Coordinated action for the building and construction sector to tackle embodied carbon. 2019. World Green Building Council.