In the next 30 years, if humans keep building as they have been, the embodied carbon of building materials will have an impact on carbon dioxide (CO2) emissions similar to the operational carbon of buildings. Embodied carbon represents the emissions associated with construction materials—their extraction, manufacture, transportation to the site, construction, and end of life. Operational carbon, by contrast, represents the emissions associated with operating a building. Until recently, most of the green building movement’s focus has been on operational carbon, e.g., energy efficiency. Because embodied carbon represents a significant impact in the short term, it is critical to understand the effects of building materials and what is being done to mitigate their impact.
The following image from the New Buildings Institute makes clear the part embodied carbon plays in the life cycle of a building, illustrating the range of embodied carbon impact.
As with energy efficiency, using an integrative design approach to reduce embodied carbon is most important. Ideally, this includes reusing existing structures and building materials whenever possible and minimizing the use of new materials. When using new materials, green developers should use technologies that reduce CO2 emissions, some of which even use CO2 as a component of their manufacturing process. These technologies are still in the early stages of development; however, there is a growing emphasis on such materials, and technologies are increasing rapidly.
BuildingGreen, an excellent resource for green building materials, has published an article, The Urgency of Embodied Carbon and What You Can Do About It, BuildingGreen, which describes the issues of embodied carbon in building materials. One of its illustrations shows how a designer could choose to reduce the impact of embodied carbon by as much as 58 percent.
In this example, architect Brad Benke studied the impacts of brick façade systems and discovered that five functionally equivalent wall types had very different embodied carbon impacts. Thin brick on metal studs, shown at the far right, reduced embodied carbon 58 percent compared with a baseline wall system (thin brick with precast concrete).
Determining the impact of various materials on embodied carbon requires using tools for performing life cycle analysis (LCA), some paid and some free. An excellent resource is Carbon Leadership Forum (CLF) which has created various toolkits, including one for policymakers, one for owners, and one for architects. The first step is to collect the inputs used in the Life Cycle Analysis calculations, such as Environmental Product Declarations (EPDs) and Health Product Declarations (HPDs). LCA tools use these inputs for the LCA calculations, some of which are based on the whole building and others of which are based on specific components, such as the structural elements.
Recognizing the importance of embodied carbon, recently updated rating systems, such as LEED, now include credit for embodied carbon reductions and even for the mere performance of the LCA calculations. The most credit is given to projects that reuse historic, abandoned, or blighted buildings. The British green building rating system, BREEAM, strongly focuses on LCA.
The following image provides a graphic depiction of the life cycle of materials.
The following sections discuss several building materials with the highest impact on embodied carbon and the work being done to reduce that impact.
The materials with the highest impact on embodied carbon are concrete, steel, and aluminum. The structural systems of almost every commercial building use concrete and steel, but aluminum is used in the construction of windows and curtain walls. All-glass buildings with aluminum frames, therefore, often have a very high impact on embodied carbon and energy consumption.
As discussed in the introduction, the most important first step is reducing the materials used. For new buildings, steel and concrete are generally required. This article will examine ways to reduce the embodied carbon in those materials. In addition, because mass timber construction is becoming more popular, it will be discussed as an alternative.
One of the most essential components in concrete is Portland cement, which requires significant energy for its manufacture. Some have estimated that cement production accounts for five percent to eight percent of global CO2 emissions. For years, other cementitious materials such as fly ash or blast-furnace slags have substituted for some portion of the cement used in concrete creation. Builders should discuss maximizing the use of such materials with structural engineers and the concrete mix companies.
More recently, some concrete companies have started using a product called CarbonCure. See CarbonCure’s Sustainable Concrete Solution. Its manufacturers describe on their website how it works: Concrete is made by combining water, cement, and aggregates like sand or gravel. When carbon dioxide is introduced into this mix, it reacts with the cement and mineralizes, becoming permanently stored in the concrete. The concrete is effectively liquid rock that converts carbon dioxide into stone.
Blue Planet has created a product that uses recycled carbon dioxide to make artificial limestone or calcium carbonate using a chemical process. The aggregate grows to the desired size from a small nucleus in an alkaline carbonate solution and can replace either sand or coarse aggregate. The solution’s raw material (or Geomass) can come from recycled concrete or other industrial sources. See also Blue Planet: Cost-Effective Carbon Sequestration, BuildingGreen.
These products are exciting innovations because they use captured CO2 that would have otherwise been released into the atmosphere, and they reduce the amount of emissions produced by the manufacture of concrete, affecting both the cement and the aggregates. Expect more research and innovations related to concrete soon!
The steel industry is one of the largest emitters of CO2, contributing around seven to ten percent of global greenhouse gas emissions. There are two primary steel manufacturing processes: Electric Arc Furnace and Basic Oxygen Process (Blast Furnace). The electric arc furnace process should be relied upon whenever possible because its raw material is composed of approximately 90 percent recycled steel, whereas the basic oxygen process uses roughly 25 percent recycled steel. Using recycled material as its primary input, the electric arc furnace dramatically reduces its scope by three emissions compared to the basic oxygen process. In addition, with electricity as its primary energy source, the electric arc furnace can also more easily take advantage of renewable energy from solar, wind, and hydro sources. Many steel manufacturers in the United States use electric arc furnaces, which should be required in the construction documents and specifications.
Steel manufacturers continue searching for much lower carbon alternatives. Green Steel World is a global online magazine focused on establishing a global network of professionals involved in producing, distributing, and using low-carbon steel. Some research focuses on using hydrogen, particularly “green” hydrogen, as a reducing agent instead of coal and coke in the steel-making process. See. This process could be promising if the hydrogen is electrolyzed using electricity from renewable energy sources (wind, solar, hydro, etc.). In addition, because steel combustion is concentrated, researchers are studying how to capture the carbon dioxide from that process for potential reuse.
Some steel manufacturers are also exploring carbon offsets for their carbon emissions; however, this is an accounting methodology rather than a way to reduce the embodied carbon of the steel.
With its high strength-to-weight ratio, aluminum is vital to building construction and is most often used for windows and curtain walls. As a high-energy-intensity material, aluminum is often manufactured in places with abundant hydropower, such as Iceland. Aluminum is also infinitely recyclable, which means that aluminum made from recycled materials requires only five percent of the energy needed to produce aluminum from bauxite.
To reduce embodied content of aluminum, it is important to ensure that recycled aluminum is appropriate and available for the intended use and to determine where and how the aluminum is manufactured and transported to the site. Although the aluminum industry is clear about the significant use of recycled aluminum in new aluminum cans, it is the author’s experience that recycled aluminum in construction materials doesn’t meet the same lofty goals.
Wood, particularly cross-laminated wood or mass timber, is gaining traction as a structural building material and a possible replacement for some of the steel and concrete currently used in buildings. Of course, wood has been used in construction for centuries, so its use is not new. However, its growing popularity in larger and taller buildings is recent and partly due to building code changes that make it possible.
Mass timber enthusiasts would likely say it is a low or no embodied carbon alternative to concrete and steel. Though it has many advantages, using mass timber and calculating its embodied carbon are complicated. In a well-researched article by Building Green, Wood, What’s Good, it is suggested that users carefully consider the many reasons why they want to use wood. Replacing steel and concrete as a sole justification may not be practical, and though embodied carbon will likely be reduced compared to those products, the amount of reduction may not be substantial due to many factors. Some of those factors can include where the wood comes from, including forest management and whether the wood is certified (Forest Stewardship Council (FSC) certification is best), the appropriateness of mass timber for the particular project, the manufacturing processes, the life of the project, transportation, and more.
An example of why a project owner might want to use wood is the concept of biophilia. As humans, we are attracted to natural materials such as wood. As a result, we often see mass timber projects as particularly beautiful.
An excellent resource for learning about mass timber is WoodWorks, the Wood Products Council.
This article has addressed structural building materials with the largest carbon footprints: concrete, steel, and aluminum, and a structural alternative to some of those materials: wood. Progress is being made to reduce the embodied carbon intensity of these materials.
Other low-embodied carbon materials are worth considering, although most will not replace the structural materials discussed in this article. Some of those materials include bamboo, hemp, cellulose (e.g., recycled paper), strawbale, and dirt (e.g., adobe and rammed earth). These materials have been used worldwide for centuries and are being revived as we focus on embodied carbon and emissions reductions. In addition to focusing on manufacturing these materials, researchers have developed practical tools to understand embodied carbon and its life cycle analysis better. Third-party reviewed documents, such as Environmental Product Declarations, are available that will help designers make informed decisions.
