The building construction industry (including commercial, residential, and industrial buildings) accounts for 5% of global energy use and 10% of global GHG emissions 
. A primary source of these emissions is the manufacture of building construction materials such as steel, cement, and glass. To significantly reduce the GHG emissions associated with manufacture of these materials, step-change reductions are needed in the embodied energy and carbon. As aggressive building energy codes push new construction towards net-zero-energy and net-zero-carbon operations, corresponding efforts to reduce embodied energy and carbon from building construction materials must be pursued to achieve building sector decarbonization goals. For example, Chastes et al. reviewed 90 case studies and found that embodied carbon accounted for 26–57% and 74–100% of the total life cycle carbon emissions of low-energy and near zero-energy buildings, respectively 
In the past couple of decades, progressive building energy codes as well as the underlying research on reducing the operational energy and its related greenhouse gas emissions have stimulated changes of practice in building design and operation. Significant progress has been made in reducing the operational energy of a building during its use phase, largely the result of energy codes.
In contrast, strategies to reduce embodied carbon in the remaining life-cycle stages of a building are less defined and studied. The World Green Building Council defines the embodied carbon of a building as the carbon emissions associated with the materials and construction processes throughout the whole life cycle of a building 
. The selection of building materials and systems is largely unregulated, as long as minimum health, safety, and performance standards are met. One challenge is that upstream energy use and carbon emissions resulting from the production of building materials and equipment are more difficult to measure and track than operational energy use and emissions. Relying on the self-assessment and reporting by the manufacturers of building products alone cannot provide an accurate assessment of the embodied carbon of a building. Assessing the embodied carbon of a building has remained challenging for building design teams because a building consists of hundreds or thousands of materials and components, which require specific expertise, training, and ready-to-use tools. Building design teams often do not process such broad expertise or tools. For instance, a 100% recycled-steel beam, produced using renewable energy, may have significantly lower levels of embodied carbon compared to a virgin-steel beam produced using a coal-fired furnace even though their structural performance is the same 
. The location of steel manufacture not only determines the type of energy source from the local grid, but also plays an important role in the amount of carbon derived from transporting products to the construction site. Furthermore, the complexity of global manufacturing and supply chains makes it even more difficult to measure carbon emissions from material extraction to product assembly.
A major barrier to incorporating embodied-carbon related policies, standards, and regulations into practice lies in the complexity of embodied carbon counting, which requires standardized methodologies at the national level, robust analyses based on adequate data, and thorough evaluations of the readiness and maturity of the required techniques, technologies, and materials. To the knowledge of the authors, there is no comprehensive review on embodied carbon research and practice in the United States.
2. Gaps in Database Development across All Levels
It identified four primary issues pertaining to database quality and the data collection process. The first issue relates to data availability for the use stages (B) and the end-of-life stages (C), since they are both highly dependent on the assumptions of the assessor about how a building may be used and maintained 
. Predictions of repair and replacement are particularly challenging and can be very subjective. Within a building, each building component has its individual lifespan. For example, roofing materials and window units have different lifespans and repair cycles, where each timespan depends not only on the product quality but also on the use conditions. Such information can only be obtained at the local level, which becomes a bottleneck for data collection partially due to the lack of awareness, knowledge, skills, and tools to include the total cost of ownership and the associated embodied carbons in building design and development. Building energy standards provide operational assumptions when calculating the whole building operational energy use, but similar assumptions do not exist for product life cycles. This may introduce potential bias by favoring products that have lower upfront embodied carbon (in stages A) but a short life span (in stages B). The lack of information on product durability may also introduce conflicts between resilience and embodied carbon. Both are critical components that must be addressed to combat climate change.
The second issue lies within the pre-use stages (A). There are limited published data for the A4 (transport to site) and A5 (construction and installation) stages in comparison to that for the A1–A3 stages. More standards have been developed and knowledge been gathered for products from sub-stages A1 to A3, which are associated with raw material extraction, transport, and manufacturing. Research into reducing the A1–A3 sub-stage-related carbon emissions has stimulated practice and policy changes in the building industry. One example is the EPDs and product category rules (PCRs). A growing number of local, state, and federal procurement policies require EPDs for reporting the embodied carbon of eligible products 
. However, the A4 and A5 stages are critical components of building construction and should not be neglected. Otherwise, another potential bias would favor low-carbon materials or products that are heavy, shipped over a long distance, or require energy-intensive equipment or processes to assemble onsite. For example, wood is a low-carbon construction material, however, its embodied carbon (in A4) may significantly increases as tree logs are shipped from their origin to a second location for primary processing (e.g., producing timbers of specific size and dimensions), to a third location for secondary processing (e.g., making more specific building products), and to a warehouse of a distributor, before reaching the construction site.
One potential cause of the lack of data for the A4 and A5 sub-stages may be that those data are highly context- and project-dependent 
. After products are shipped away from their original manufacture, it is difficult or sometimes impractical for the manufacturer to track how and where the products are shipped to and where they are used in the final destination. For instance, a stone product wholesale distributor may import products from different countries and store them in warehouses across the United States. Those products would then be shipped to different project sites within or outside of the country. Therefore, the embodied carbon in the A4 and A5 sub-stages is out of reach of the manufacturer and is often neglected in tracking. Such tracking and reporting responsibilities should be shared by manufacturers, distributors, and project developers. Guidelines or standards should be developed to guide data collection and tracking in these stages.
The third issue concerns missing guidelines for supply chain-specific and facility-specific data collection and reporting. In the existing and proposed legislations on embodied carbon, all EPDs are required to be third-party verified. Half of these legislations require product-specific EPDs (a specific product and manufacturer across multiple facilities), the other half require either supply chain-specific EPD (using supply chain-specific data for key processes) or facility-specific EPDs (attributing to a single manufacture and manufacturing facility) 
. Supply-chain and facility-specific EPDs aim to incentivize individual manufacturers to better distinguish their low-carbon products. However, PCRs only provide guidelines for calculating industry-average and product-specific EPDs. To improve data accuracy for comparison, the Carbon Leadership Forum (CLF) suggested that EPDs and PCRs should be improved by including supply chain-specific upstream data for processes with large impacts (e.g., production of cement for concrete (A1), manufacturing of mineral wool board (A3)) and developing corresponding guidelines 
The fourth issue revolves around the lack of consensus on how to integrate the benefits and loads of reuse, recycling, and recovery potentials (Stage D) into the whole life-cycle assessments. So far, the majority of the embodied carbon assessment tools and databases focus on the life-cycle stages A through C. Stage D is a particularly important stage, which could provide incentives to promote recycled and reused products and materials. There are very few studies that include stage D, due to the lack of data. As such, it is a huge missed opportunity. A new building that is 30 percent more efficient than average may take at least 10 years, and up to 80 years, to offset the emissions generated from the construction process 
. Although building codes and standards traditionally do not address the end-of-life building activities, guidelines are needed to calculate the avoided carbon emissions from reused building components, structures, or even whole buildings (e.g., adaptive reuse).
3. The Lack of Quantitative Research on Embodied Carbon at the Building Level
Currently, very few activities are focused on collecting data on embodied carbon or embodied energy at the whole building level. Unlike operational energy, which has abundant data collected from a variety of building types, embodied energy or carbon has few benchmarking references for comparison and target setting. For example, the US EPA ENERGY STAR program publishes the technical reference for US Energy Use Intensity (measured by Btu/sf2
) by Property Type annually. This helps a design team compare its building energy use with similar properties across the nation 
. However, a consensus does not exist on how to baseline or benchmark the embodied carbon of a building today.
The most comprehensive assessment of embodied energy in buildings was published in 1979, in a publication entitled “Energy Use for Building Construction” 
, commissioned by the Advisory Council on Historic Preservation. It aimed to assess the benefits of restoring and rehabilitating existing buildings, hence, it proposed methods to measure the energy needed to produce, replace, repair, and demolish the building materials. The assessments of building materials were then aggregated to calculate the embodied energy of the whole building 
. Embodied energy can be used as a proxy for embodied carbon, although the latter includes carbon emissions from non-energy sources. The Advisory Council assessment included three separate embodied energy categories corresponding to different life cycle stages: embodied energy (A1-B5), demolition energy (C1-C4), and operational energy (B6).
The Advisory Council-sponsored research project was a collaboration between the University of Illinois at Urbana-Champaign and Richard Stein Associates, Architects, of New York City. To date, this pioneering effort remains the most thorough evaluation of the embodied energy assessment at the whole building level by different building type, which has ever been produced in the US. The building materials in the report were based on construction industry data from 1967. Obviously, since then, there have been significant changes in building products, technologies, and manufacturing processes. Steel beams, for example, are now made with continuous casting, avoiding the billet reheating of earlier times. Thus, the base numbers for embodied energy from the 1976 report are likely to be outdated, so revision and updates are required.