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3 Advances in Sequestering Carbon in the Built Environment

The ability to not only eliminate greenhouse gas emissions, but actually sequester carbon, would transform buildings from net negative to net positive entities. Here, we examine several approaches making this possible.

By now, we know that the global design and construction industry is responsible for approximately 39 percent of total greenhouse gas emissions (GHGs). That is subdivided into 28 percent for all the energy used to heat, cool, ventilate, light, and power our buildings; plus, another 11 percent to mine, extract, harvest, process, manufacture, fabricate and transport all the materials used to construct them. Put more simply, that’s 28 percent for building operations and 11 percent for the embodied carbon of construction materials.

As the design and construction industries work to reduce both of these significant contributors to climate change, the question arises as to whether buildings can actually sequester carbon. Sequestering carbon would transform buildings from net negative to net positive environmental impacts. Below we examine several approaches making this possible.

Wood and timber

As recent advances in wood technology have enabled architects and engineers to make wood buildings taller than ever, many are investigating the use of much more wood in the buildings they design. While glue-laminated (glulam) timber and parallel strand lumber (paralam) have been in use for a long time as columns and beams, an entirely new generation of products referred to as cross-laminated timber (CLT) is now available in many markets. Unlike earlier versions of engineered lumber, CLT is available in large sizes, and can be used as entire walls and floors.

Building Codes have followed this trend; and the 2021 version of the International Building Code (used throughout the United States) allows timber buildings up to 18 stories. Elsewhere in the world, even more ambitious wood construction is allowed. Some recent examples of tall timber construction include:

Substituting wood for steel and concrete has the potential to greatly reduce the GHG impact of buildings, especially if the wood structure can be deconstructed at the end of its life and reused.

Concrete options

The global concrete industry, if it were aggregated as a country, would be responsible for more GHG emissions than all but two other countries — the US and China. The manufacturing and processing of concrete and its essential ingredient — cement — are extremely energy intensive and emissions-heavy. As a result, concrete manufacturers are actively pursuing methods to reduce the GHG impact of concrete. A few promising directions include:

Carbon injection

Waste CO2 from industrial operations can be injected directly into concrete. Once injected, the CO2 is transformed into a mineral, so it is never released back into the atmosphere as CO2, even if the concrete is pulverized for re-use. The resulting concrete is also stronger than traditional concrete. One company, called CarbonCure, is active in the field and claims that its technology could reduce up to 700 megatonnes of global CO2 emissions annually.

Hempcrete

Hempcrete is a composite material that combines the inner woody core of hemp plants with lime binders. It is approximately one-eighth the weight of concrete and is used where concrete block might be used as an exterior or interior wall. Unlike traditional poured-in-place concrete or concrete block, however; it is non-structural and must be combined with other frame elements such as wood or steel.

Recycled concrete

Much concrete is now recycled after the useful life of the building or structure is compete. This has the advantage of eliminating the environmental devastation due to mining raw materials, but it still takes a tremendous amount of energy to demolish concrete and crush it into useful aggregate. Only about 2 percent of the embodied energy of the original concrete remains with this approach.

The role of landscaping

Grounds are often overlooked opportunities to sequester carbon in the built environment. The ability of landscaping to sequester carbon varies widely depending on the approach to plants and hardscaping (sidewalks, drainage structures, furnishings, etc). Minimizing concrete and other high-GHG materials such as steel in favor of lower-intensity materials — such as gravel for paths and wood for structures — can make a significant difference in the initial embodied carbon of a landscape area.

Plants absorb CO2 and release oxygen, which benefits the overall carbon balance of a property. More woody plants on a property help with absorption of CO2 over time. Surprisingly, expanses of bluegrass lawn are not beneficial for carbon sequestration — and, in fact, are usually net-carbon emitters due to the energy intensive fertilization, mowing, and other maintenance required for lawns.

With careful attention to material selection, maintenance procedures and plant specification, landscaped areas can become net absorbers of CO2 within a few years after initial construction.

Conclusion

As designers push to decarbonize the built environment and actively remove carbon from the atmosphere, they will increasingly embrace these strategies and more. Reaching net-zero emissions by the year 2040 or 2050 will require substantial changes in how we design and build, with sequestration increasingly necessary to achieve that elusive goal.

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