Low-Carbon Building Materials
Embodied-carbon emissions originate from activities at the top of the construction supply chain, like the mining and transportation of raw materials and the operation of manufacturing facilities. Nonetheless, opportunities to reduce these emissions are available throughout the design and construction process.
Since the impact of embodied carbon is realized at the beginning of the building life cycle, it is critical to develop low-carbon materials for building construction. The effect of these low-carbon materials can be further amplified when paired with low-carbon building design strategies like materials optimization engineering and building and materials salvage and reuse.
The data currently available for assessments of embodied carbon are of varying quality. These data are typically sourced from national life cycle assessment (LCA) databases, which tend to be generic (with average values for the country) and are rarely updated in the U.S. Environmental data can also be sourced from environmental product declarations (EPDs), which can be reported by manufacturers in either product-specific or industry average reports. In some product categories, suppliers have reported product data, while in other sectors only industry-average data exists. Without standardized metrics to assess embodied carbon, decision-makers have difficulty setting appropriate limits or targets. Alignment on metrics and assessment methods requires often-challenging collaboration across a range of industry organizations including green building programs, government, and industry. In short, embodied carbon data for the building industry must improve in coverage and quality to become more actionable.
Highly codified to protect life and safety, the building industry can be slow to change, making innovation and new approaches difficult. Building codes tend to be prescriptive instead of performance based. This means that the codes often limit the introduction of new technologies that could support embodied carbon reductions. For example, cross-laminated timber (CLT) is a promising low-carbon wood alternative to concrete and steel, but building codes often limit how tall a CLT building can be, which restricts how and where it can be used.
Technology Innovation Examples
Bio-based or biogenic materials are derived from plant or animal sources and have a long history of use in buildings. Among them are engineered wood products, engineered bamboo, hempcrete blocks, and other plant-derived materials. These materials typically require only moderate amounts of processing energy to create effective building materials. As a result, they tend to have very low embodied carbon— often an order of magnitude lower than more highly processed materials such as steel and cement.
Bio-based materials also grow by absorbing CO2 from the atmosphere and using the carbon to build cellulose, with half the weight of most biogenic materials composed of atmospheric carbon. This embodied carbon, when stored within a building for its 50+ year lifetime, remains out of the atmosphere for that duration, enabling these buildings to often have a net carbon benefit.
Iron and steel production are responsible for about 5 percent of global greenhouse gas emissions. Most of these emissions come from the fossil fuels used to convert iron ore into steel through carbothermic reduction, particularly in the blast furnace. Existing cleaner production technologies include direct reduction of iron oxide to steel using natural gas, molten oxide electrolysis, CO2 capture and storage, steel recycling using electric arc furnaces for some steelmaking applications, and the replacement of coal in the steelmaking process with lower-GHG feedstocks.
At present, many of these technologies are not cost competitive with the incumbent processes for primary steel production. The slow stock turnover of industrial facilities also presents a challenge to the rapid diffusion of lower-carbon production approaches. Reducing iron oxide to iron and steel using low-carbon electricity or low-GHG hydrogen (rather than natural gas) is a potentially transformative technology that could substantially reduce steel sector emissions even further.
The production of cement is responsible for about 7 percent of global GHG emissions—roughly 40 percent of which is from the energy used and 60 percent from the CO2 released chemically by the heating of limestone.
Opportunities for significant emissions reductions in cement and concrete include CO2 capture and storage, the development of low-emission material substitutes for cement/concrete, recycling end-of-life concrete for reuse, and the development of processes and materials that consume CO2 (as opposed to generating it) in cement or cement-replacement production—thereby enabling emissions-negative cement production.
Modular or off-site construction is the process of designing, engineering, and producing components for buildings away from the construction site. For example, panelized or modularized components can be used in structural, enclosure, or interior partition applications. This type of construction can have significant benefits over conventional on-site construction, including: 1) significantly more rapid build times on site; 2) higher-performance, tighter tolerance structures; 3) lower overall cost; and 4) reduced job-site and overall waste. While these types of buildings represent only a small share of total new construction today (<5 percent), we expect that fraction will grow as technologies improve and as builders are able to realize the cost benefits associated with modern industrial practices and supply-chain improvements.