Residential and commercial buildings contribute to almost 30 percent of the United States’ total greenhouse gas emissions. That breakdown includes both general building operations and the embodied carbon of materials used to build the structure itself — the sum of all greenhouse gas emissions generated as a result of mining, processing, manufacturing, transporting and installing the materials.
As more corporations are held accountable for their carbon footprints, interest in reducing embodied carbon for new construction and building retrofits has gained more traction. At GreenBiz Group’s virtual clean economy conference, VERGE 20, in late October, industry experts discussed ideas not just for cutting back on that embodied CO2 but also for using materials that turn buildings into another carbon capture opportunity.
Buildings as carbon sinks
Kate Simonen, executive director of the Carbon Leadership Forum, an organization focused on radically reducing carbon emissions associated with buildings, said she wants to see a bigger push than just reducing embodied carbon, through the creation of carbon sinks in buildings. The forum is pioneering research into techniques that do just that.
"We [can] actually take carbon out of the atmosphere, put it into building materials and then store it as long-life products in the building," she said. "So if we can successfully do that, we can create buildings that are carbon sinks storing carbon in them as they're being in use."
We [can] actually take carbon out of the atmosphere, put it into building materials and then store it as long-life products in the building.
Chris Magwood, executive director of the Endeavour Center, a building design organization, suggested four key principles that enable buildings to store more carbon than it emits.
The first is to use materials with the lowest possible emissions to begin with by completing life cycle assessments and using modeling software to substitute in different materials in different areas of the structure. Completing this process prior to purchasing material allows for a variety of scenarios, from having "240-plus kilograms [of carbon emissions] per square meter of floor area, all the way down to offering net storage of 130 kilograms per square meter," Magwood said. "Net storage" becomes possible when the carbon contained in the material is greater than the emissions used in making the material.
Magwood is also a proponent of using biogenic carbon-storing materials wherever possible, which he described as "plant-based materials that have drawn carbon out of the atmosphere while they’re growing."
Examples that Magwood has used in building designs include wall panels made from straw or hemp using mycelium-based insulation, and wood chip foundation blocks. Magwood claims that "there is essentially no part of the building where you can’t find a reasonable biogenic material to substitute for one that isn’t."
His last two core principles involve designing for modularity and designing for disassembly and reuse, which often go hand in hand. By building using modular components rather than one cohesive structure, individual components can be taken apart and put together multiple times, Magwood said. That means when a building comes to the end of life in that particular area, it doesn’t need to be completely demolished. Rather, when modular components are included, parts can be removed and reused for other construction projects, and carbon storage continues beyond the lifespan of just one structure.
Making the case for mass timber
Kristin Slavin, associate director of building innovations at infrastructure developer Sidewalk Labs, discussed other alternatives to reducing embodied carbon in buildings. Slavin has been working on materials for prefabrication of buildings — the practice of assembling parts of a structure offsite and then transporting the assembly to the site. She is particularly looking at mass timber, a relatively new technology out of Europe, which uses large wood panels for wall, floor and roof construction as an alternative to stone, masonry and concrete construction.
There are many benefits to this practice, Slavin said, said from reducing the time, cost and carbon emissions of new buildings. "[Using mass timber] will bring construction off site as opposed to on site into a controlled and more automated environment, providing high-quality jobs and more safety," she said.
Slavin said mass timber works for prefabrication because it is lighter than concrete and steel, and it reduces embodied carbon significantly because it can be transported generating fewer emissions. However, the practice is not yet cost-competitive with concrete, and it is unfamiliar to many developers, adding a level of risk and uncertainty to Sidewalk Labs’ goals.
If you're looking at the impact of straw on atmospheric drawdown, that straw is growing in the same field every year. It was going to go back to the atmosphere quickly if we didn't put it into a building.
Although the company has not explicitly stated the sequestration benefits of mass timber, it continues to research timber management, procurement, end of life and recyclability. Slavin said Sidewalk Labs is able to do a detailed analysis of components in its kit of parts, which is crucial to the process. "By doing a bill of materials and incorporating body carbon in our bill of materials approach, we can make sure that every single material that's going into our pieces is accounted for," she said.
Some audience members expressed concern over Sidewalk Labs’ approach to mass timber, most notably, about the potential for deforestation. Slavin countered those comments by explaining that the company practices "preplanting" to account for its impact, which is planting new trees to replicate the estimated number of trees used to build.
It’s all material
Magwood advised organizations concerned about embodied carbon to pay particular attention to the differences between sustainability assessments of wood products (such as mass timber) and alternatives such as straw, used frequently in his projects.
"If you're looking at the impact of straw on atmospheric drawdown, that straw is growing in the same field every year," he said. "It was going to go back to the atmosphere quickly if we didn't put it into a building."
Another contrast is that timber is not a residual material, unlike the materials used in Magwood’s practices. "These are carbon sources that weren't produced to be a building material; they're sort of byproducts of grain farming," he said.
As the world continues to grow and new structures are rapidly developed, it is crucial to be thoughtful about the way in which new construction can mitigate environmental damage and even counteract it through storage, according to the panelists.
As Magwood explained, there has to be a more intentional focus on the impact of embodied carbon. "Think about it not as something that you necessarily have to do tomorrow but you plan the things that need to happen to that building… I can't really think of a place in a building where you couldn't be substituting carbon-storing materials," he said.