Buildings are massive sources of what is termed “embodied carbon” — the carbon emissions that come from making the cement, concrete, steel girders, rebar, insulation, glazing and finishing materials used in buildings ranging from single-family homes to skyscrapers. Taken as a whole, this embodied carbon accounts for between one-tenth to nearly one-quarter of the world’s carbon emissions, depending on how it’s measured. That makes it a key target for reduction to combat climate change.
But low-cost ways to cut buildings’ carbon footprint are already available to U.S. architects, contractors and construction materials suppliers, according to a new report from think tank RMI and global construction firm Skanska. (Canary Media is an independent subsidiary of RMI.)
In fact, according to the report's case studies, deploying methods to make use of materials and standards that are already in place today could shave between a quarter to nearly half the typical building’s embodied carbon at little more than 1 percent extra cost, compared to business-as-usual approaches.
These methods require some significant changes to how architects and engineers plan for the type and strength of materials that go into their buildings, as well as careful coordination across the supply chain that delivers them to construction sites. They could also benefit from refinements to still-emerging embodied-carbon measurement standards across multiple sectors, not to mention government regulations and private-sector guidelines to encourage their use.
But as regulations for lower-carbon buildings start to emerge at the county, state and federal levels, and as demand for lower-carbon buildings grows among public- and private-sector customers, those that haven’t built the capacity to work these lower-carbon materials into their businesses may find themselves being left behind.
“That’s the current market advantage that we see right now — people are going to demand low-embodied-carbon materials, and the people who can supply these products will own that market,” said Victor Olgyay, a principal in RMI’s Carbon-Free Buildings Program and co-author of RMI and Skanska’s Reducing Embodied Carbon in Buildings report.
The report surveys a wide array of technology and business-process options to drive lower-carbon outputs from the cement and steel industries, which account for roughly 5 percent and 7 percent of global carbon emissions, respectively. A companion Concrete Solutions Guide presents opportunities to cut nearly one-third of the carbon-intensity of concrete, the second-most-used material on earth besides water.
The report also takes on insulation, an under-acknowledged source of carbon emissions that can be reduced through the use of alternative materials, some of which use biological components that can serve as a sink of atmospheric carbon, Olgyay noted. Glass, paint, carpet and other finishing materials that go into buildings also have lower-carbon options available.
Some of the biggest gains to come in decarbonizing cement and steel will rely on technologies that are still in early development, such as the use of zero-carbon energy sources for the heat required for making both.
But according to Olgyay, there are “things that can be done immediately” to shave big chunks of embodied carbon from materials while the world awaits these technology breakthroughs.
“We can get to lower-embodied carbon at cost parity — less, or slightly more, but without any significant cost implications,” he said in a Monday interview. “That’s been a rumor for a while,” and RMI’s new reports “try to put that on a real factual basis.”
How to cut embodied carbon from buildings today
That’s where the report’s case studies with Skanska, a major builder in Oregon and Washington, come in. The partners modeled three types of buildings: a five-story concrete-and-steel office building, a six-story lumber-framed multifamily building built on a steel-reinforced concrete slab, and a tilt-up concrete building typical of warehouses and big-box retailers.
They then applied some key carbon-reducing interventions: Adding specifications for lower-carbon concrete, steel and other building materials, and carrying out one-for-one replacements of higher-carbon materials with lower-carbon alternatives.
The results show that the first concrete-and-steel building could cut 46 percent of its embodied carbon footprint. Lower-carbon concrete cement drove the biggest reduction, as well as using steel rebar with a higher proportion of recycled metal in more energy-efficient electric arc furnaces using lower-carbon electricity.
The “stick-built” midrise building could achieve a 41% embodied carbon reduction, with the biggest single reduction coming from replacing traditional extruded polystyrene insulation, which uses chemicals with high global warming potential.
The tilt-up concrete structure showed a lower, but still significant, embodied carbon footprint reduction potential of 24%, almost all of it from alternative concrete sourcing. It was also the most expensive reduction to obtain, although still just 1 percent more expensive than the alternative.
Charles Cannon, a senior associate in RMI's Climate Intelligence Program and author of the concrete report, highlighted the various routes that builders can access today to source lower-carbon concrete. One major approach is using “supplementary cementitious materials” to replace some of the cement that makes up the most carbon-intensive portion of concrete.
While cement manufacturing tends to be highly centralized, most concrete is made much closer to where it’s used, to avoid the cost of transporting the sand and rock aggregate that makes up the rest of the concrete. That means “the carbon content of concrete is not set in stone until it reaches the local ready mix supplier, who is ultimately responsible for delivering the final mix of concrete to the site,” he said.
That opens the door for these ready-mix producers to substitute materials like fly ash from coal plants, slag from steel mills or other materials that can reduce the amount of cement needed in a mix to achieve the same performance — if two things can be lined up. First, there has to be a source of the substitute material within about 100 miles of the construction area to cost-effectively transport it to the site. Second, there has to be certainty that the new mix will be accepted by engineers, contractors and building inspectors.
“It can take a year or so, and tens of thousands of dollars, for a ready-mix company to fully test and vet a new blend [to] offer on the market,” he said. Once the first company has done it, it's easier and less costly for others to follow suit — but not if “the demand hasn’t been signaled strongly by developers or by policies.”
With those prerequisites in place, however, new mixes can actually save money for ready-mix companies and their customers by reducing their cement costs, he said. “The cost of that leap is the upfront cost. [On an] ongoing [basis], lower-cement-content concrete will tend to be cheaper.”
Getting the policies and markets aligned
That’s why RMI's “current focus is trying to get more policies passed to increase the demand,” Olgyay said. “To get people excited about it, there has to be a more compelling business case.”
These policies are starting to emerge at many different levels of government. States including Colorado and New York have passed laws mandating that state agencies must set maximum acceptable global warming potential rates for different building materials used in public construction projects like roads and public buildings. California lawmakers are debating bills that would mandate an overall reduction in cement production carbon-intensity over the next 15 years and offer preferences for bids for public projects with the lowest-carbon concrete.
The General Services Administration, the agency that serves as the property manager for all federal facilities, is in the process of implementing a low embodied carbon procurement policy that “will potentially have a huge impact to drive demand for these materials,” he said.
There are complications involved in setting such standards. Measuring the carbon-intensity of different materials is a work in progress, Cannon noted. Today’s state-of-the-art standard is the Embodied Carbon in Construction Calculator (EC3) tool, which allows one-for-one comparisons of different building materials like concrete, steel and insulation.
But it doesn’t yet include metrics to compare choices of different materials, such as replacing concrete with mass timber. That makes it hard to verify the value of “whole-building life cycle assessments” that offer the deepest potential carbon-reduction impacts, compared to simply replacing one material with another. Getting accurate and verifiable carbon data from upstream suppliers is also a significant challenge, he said.
There’s a huge spread in the embodied carbon of different building materials, from highly carbon-intensive cement, steel, gypsum board and extruded polystyrene, to plant-based insulation materials that actually sequester carbon and thus come with negative embodied carbon ratings.
“It’s clear that the way we make materials has a huge impact on our climate,” Olgyay said. “To get it into the economy, we need people to recognize that’s it’s a better way to build for both people and the climate.”
(Lead image: Scott Blake)