Life Cycle Assessment of Embodied Carbon in Buildings: Background, Approaches and Advancements

Abstract

The environment demands a reduction in greenhouse gas (GHG) emissions, as building and construction are responsible for more than 40% of the energy consumed worldwide and 30% of the world’s GHG emissions. Many countries have aligned themselves with the Paris agreement, following its target of achieving net zero carbon emissions, although some governments are focused on the operational energy efficiency part of the equation instead of the whole equation. This study emphasizes the significance of incorporating the minimization of embodied emissions into all parts of the building, with a focus on the measurement of embodied carbon, concepts of its management and strategies proposed and enacted for mitigation. As estimate is an important part of any debate, the measurement approach covers the uncertainty analysis from diverse points of view through a novel approach; management covers the early design tools, and the significance of the lifecycle stages; mitigation covers the reduction strategies of embodied carbon, although reduction in embodied carbon is a subjective topic and depends on region. The analysis covers the ideal approaches for mitigation irrespective of the region.

1. Introduction

The major source of carbon emission is continued to be buildings and construction, accounting for 40% of all emissions connected to energy. Of the 40%, 12% comes from embodied carbon (EC), which is associated with different stages over the life cycle of the building. The remaining 28% represents the carbon offset, which represents different energy usages in building operations such as heating, cooling, and electrical appliances [1,2]. According to research, with the surge in urban sprawl, GHG emissions may be doubled in relation to the construction and building industry in the following 20 years if significant improvements in building efficiency are not made [3]. The different stages of a building’s life over which the emission of carbon and energy use occur are: (I) material extraction, (II) material processing and component manufacturing, (III) construction and assembly, (IV) operation and service, and (V) end of life (EOL); these stages cover the assessment of the building from cradle to grave [4]. In addition, the transition between these phases accounts for considerable emissions related to transport, which is a consequential aspect, that must be considered in carbon emission estimation.

Embodied carbon generally refers to the carbon dioxide (CO₂) emissions associated with the construction and material life throughout a building or infrastructure’s whole life span. It includes any CO₂ produced during the extraction, transportation, and fabrication of building materials, as well as the transportation of those items to the project site and the construction methods utilized. Simply put, embedded carbon refers to a building’s or infrastructure project’s carbon footprint before its completion. It also refers to the CO₂ emitted when maintaining and eventually deconstructing the structure, as well as transporting and recycling the garbage. Carbon that is produced through electricity, heat, lighting, and other sources is not the same as carbon that is embodied.

Building embodied carbon assessments can be compared to the more widely used and standardized life cycle assessment approach in terms of methodology (LCA), which focuses on quantifying carbon emissions throughout a building’s life cycle. Single-point estimations of an average numerical output based on deterministic data typically lack the relevance or variability of that number. In comparative studies, the LCA point value findings are superposed and directly compared, for example, when the performance of two buildings is evaluated. The ostensibly less harmful option is chosen without regard for the possibility of making a mistake.

Many fields evolve slowly across the generations; some are guided by a small neighborhood of methodical experts, and some disappear as the world continues to evolve. However, there exist certain topics in which the whole world has come to understand the significance and has started to escalate the research, such as the topic of this research. This study aims to understand the process of the construction of the built environment and possible pathways necessary to understand and assess the changes from the inside to evoke changes through policymaking. The sudden interest in the topic may lead to an avalanche of disconnecting ideas.

This inquest aims to set out a comprehensive and holistic approach to data analysis, with the probable risks and assumptions involved in it and paths for a low carbon future via the approach of measuring, managing and reducing embodied carbon. In this study, a detailed assessment of the current methods for dealing with embodied carbon is covered, and based on the study, research and review articles are selected to compare the studies that have been undertaken over time and their impact in enhancing the assessment methods. These assessment methods include current measurement, life cycle measurement and Monte Carlo simulations, and the influence of the methods in minimizing the uncertainties that are identified in the study over the life cycle of the building. Further, an analysis of the introduction of the assessment process from the early stages of building design is undertaken in the study. Here, sustainable technologies that are currently easy to incorporate in different stages, such as the construction, operation and demolition of a building, are studied for their effects in minimizing the embodied carbon of a building. After the detailed assessment of the embodied carbon at the building level through a bottom-up approach, a perspective of the embodied carbon through a top-down approach is also given. In the top-down approach, various mitigation and management strategies that are currently being followed over the globe at the neighborhood, urban and national levels, such as single-region input–output (SRIO) and multiregional input–output (MRIO), are studied.

2. Estimation of the Embodied Carbon of Buildings

Currently, life cycle assessment (LCA) is considered the most practiced methodology used to evaluate environmental issues in the context of buildings [5]. It offers a framework for measuring and evaluating environmental effects over the whole life cycle of a system of goods or services, from conception to disposal [6]. It streamlines the estimating approach, and is thus commonly used in evaluating a building’s energy and carbon footprint. The International Organization for Standardization has formalized four major steps of the LCA: goal and scope definition, inventory analysis, impact assessment, and interpretation (ISO-14040, 2006) [6]. The objective of the system, as well as the system’s boundaries and functional units, must be defined in the initial step of any LCA application. This is because it is believed that the model and the simulation assumptions will have an impact on the LCA output. By doing this, the likelihood of incorrectly interpreting LCA results is reduced [5]. Buildings on the other hand are unique and differ from controlled industrial processes comprehensively. Figure 1 shows the steps that are crucial for the whole life cycle assessment of buildings. Due to factors such as buildings’ long lifespans, different material uses globally, variable site-constrained construction techniques, the distinctive nature of each building, the evolution of function, maintenance, and retrofitting, etc., variability in the study of LCA is increased, and it is not easy to generalize even for a region. Because of that, many LCA studies have been constrained to specific objectives and limitations [7,8,9]. The second stage of any life cycle assessment is the development of a life cycle inventory (LCI), which is also a crucial step for the generalization of the process, and it includes resource flows as well as externalities that come with the product under assessment [5]. The third step is the evaluation of the potential environmental impact and risk associated with estimation using data from the LCI, which is also known as the Life Cycle Impact Assessment (LCIA).

3. Research Methodology

The research aims to review approaches and advancements in the life cycle assessment of buildings over time, and to review the processes that have been used, as shown in

Table 1. Assessment of the approaches and advancement in LCA of buildings.

Title	Goal	Methodology	Relevant Conclusion	Reference
Life-cycle assessment: Inventory guidelines and principles	Identification of a generalized method for inventory development through a template and to provide set of rules for necessary assumptions.	Developing a general framework and addressing general issues through literature study.	Establishment of the life cycle assessment process, and for the implementation of product life cycle assessment, development of guidelines and principles.	[10]
A comparative life cycle assessment of steel and concrete framed office buildings	Assessment of the environmental impact of a structurally framed office building based on material and operation.	Case study of two office buildings and comparison with possible renovations.	CO2 emissions from building materials can be used as a relevant environmental parameter for LCA.	[11]
Status of Life Cycle Assessment (LCA) activities in the Nordic Region	Comparison between the framework provided by ISO 14,040 and the process followed on the ground.	Case study of 350 studies from industrial companies and research institutes from the Nordic Region.	LCA has not been used in the strict sense presented in the standard and the system approach has also been modified according to need.	[12]
Life Cycle Assessment on Environment	Covers background and possible future advancement of the LCA.	Literature study.	The possible intervention of software systems and databases in the LCA process.	[13]
How to improve the adoption of LCA	Assessment of the progress in LCA	Literature review and survey.	For better LCA adoption, the focus should be on the research of simple methodologies and the need for LCA experts.	[14]
Life cycle assessment Part 2: Current impact assessment practice	Assessment of the development of LCA in different fields and possible collaborations to provide future indicators	Literature review.	LCA of each stage is interlinked with each other through various parameters that are neglected in the assessment.	[15]
Environmental life cycle assessment of a commercial office building in Thailand	Environmental LCA of A commercial office building	Case study: using input–output process.	Assessment of building from construction to demolition based on ISO 14,040 methodology.	[16]
Life-Cycle Assessment and the Environmental Impact of Buildings: A Review	LCA review considers current advancements from a building perspective.	Literature review and case study.	Need for LCA in the building sector and its importance as a decision-making support tool.	[17]
Life cycle assessment of buildings: A review	To identify the environmental impact of the entire service life	Literature review of existing assessments of service life of buildings	The life cycle of a building operation phase constitutes the most emissions.	[9]
Life cycle assessment (LCA) and life cycle energy analysis (LCEA) of buildings and the building sector: A review	Review the literature on LCA, life cycle energy analysis and life cycle cost analysis	Literature review	Generally, studies are carried out on low-constructed buildings and not on traditional buildings.	[18]
Life cycle assessment in the construction sector: A review	Overview of the current situation of LCA in the construction industry	Literature review	Phases like choice of material, construction, demolition etc., also play a major role in the minimization of LCA.	[19]
System boundary for embodied energy in buildings: A conceptual model for the definition	To develop a comprehensive system boundary model for life cycle energy analysis and quantifying embodied energy.	System proposal based on literature review.	Minimization of incomplete, inaccurate, and inconsistent data from numerous parameters through the proposed system.	[20]
Impact of building service life models on life cycle assessment	Development of a process for the whole LCA of the building.	Literature review and case study.	Examination of the effect of materials and systems in building operation, maintenance, repair, and replacement by modelling process.	[21]
Scope-based carbon footprint analysis of U.S. residential and commercial buildings: An input–output hybrid life cycle assessment approach	To assess the carbon footprint of the selected building based on different criteria or scopes.	Case study: based on hybrid analysis.	Transportation is identified as an important factor for the assessment, which links through every life cycle stage.	[22]
Using Life Cycle Assessment to Inform Decision-Making for Sustainable Buildings	Identification of the long-term environmental consequences of various design strategies	Case study: based on hybrid analysis.	Development of a framework that considers future refurbishment.	[23]
A review of life cycle assessment of buildings using a systematic approach	Recent advancements in the LCA of buildings	Bibliometric approach.	Provides pattern of growth of interest in building LCA.	[24]
Life-Cycle Assessment of Buildings	To evaluate the impacts of products from cradle to grave in building life cycle	Review paper.	The life cycle perspective of the whole building’s life cycle serves to evaluate the environmental benefits of reducing building energy.	[25]
Life Cycle Assessment of building stocks from urban to transnational scales: A review	To evaluate the environmental impact of building stock from urban to translational.	Review of existing case studies.	Limitations and opportunities of LCA of different building stocks are discussed and analysed for future work.	[26]
Life Cycle Assessment of Geotechnical Works in Building Construction: A Review and Recommendations	To address the geotechnical works in the LCA of buildings	Literature review.	Development of a unified framework to address the geotechnical works.	[27]
Life cycle assessment of the building industry: An overview of two decades of research (1995–2018)	Background and future scope in the LCA of buildings	Bibliometric approach.	Variation between different studies based on different parameters has been identified.	[28]
Assessment models and dynamic variables for dynamic life cycle assessment of buildings: a review	Comparison between the present methodological progress and synthesized dynamic assessment models	Literature review and proposed dynamic model.	Dynamic LCA offers important inferences for environmental practice.	[29]

. For this purpose, research, as well as review articles based on the life cycle assessment of buildings, have been selected from the period 1990 to 2022. The first criterion for selection is the relevance of the study to the topic and its contribution to future research. The second criterion for the selection is the approach of the study and the impact of the approach in identifying the results or reaching a new conclusion. General keywords such as life cycle assessment, building energy analysis, embodied energy, and embodied carbon have been used for the initial selection. After the initial selection, 97 papers were selected; based on the 94 selected, further selection was based on the three parts of abstract, methodology and conclusion. Initially, 47 review and research papers were selected out of 94, and based on criteria such as the number of citations over the years, indexing, and the relevance of the methodology to the existing discussion, 21 papers were finalized and reviewed based on the methodological aspect covered in order to pursue the LCA. Based on the review, three general approaches (process analysis, input–output analysis, and hybrid analysis) that have been advanced over time are identified; in these approaches, possible uncertainties have been identified and possible approaches to address those uncertainties have also been analyzed.

3.1. Process Analysis in LCA

The compilation of life cycle inventories (LCIs) has been traditionally approached by process analysis. It entails a thorough examination of resource usage and environmental discharges from on-site manufacturing, as well as the suppliers’ contribution of inputs deemed important by the analyst. There are two common approaches to process analysis [30]: the matrix inversion technique pioneered by Heijungs, and the widely utilized process of the flow diagram approach [31]. “In contemporary economies, every industry sector is dependent on every other industry, and this industrial interconnectedness continues inescapably upstream through the whole life cycle of every good, like infinite tree branches” [32]. It has been reported that, depending on the industrial sector, the truncation error induced by sketching the system’s boundaries in a process flow diagram might be up to 50% [33]. To address the scarcity of real data in the building sector, a framework for uncertainty analysis was developed by combining data quality indicators (DQI) with the probabilistic technique and assessing them based on different uncertainty studies conducted for the process-based assessment of the building’s embodied carbon [34]. The truncation issue in the matrix inversion technique used for process analysis is that it does not account for further upstream inputs, although it may take into consideration infinite orders of interactions between the upstream processes already contained inside the system’s boundary [32].

3.2. Input–Output (IO) Analysis

Leontief developed the IO LCA approach, often known as the input–output life cycle assessment method, in the 1970s [35]. Although global models exist today, they generally depend on national account economic transaction matrices. When commerce happens in one sector, the IO tables simply keep track of how the extra value distributes across the economy. The environmental effects of a transaction in one sector can be evaluated across all sectors of an economy by integrating sectoral environmental burden data in the IO tables. Pure IO LCAs, on the other hand, suffer from a downward bias since the “gate-to-grave” (from usage to decommissioning) portion of the emissions is intrinsically missing from assessments. The approach is not the most often used LCA method, owing to its many serious issues. For starters, it has an aggregation error imposed by each IO sector in an IO model that generates a weighted average of emissions from a variety of real-world economic sectors [36]. IO LCA has, however, been used in a few pre-use evaluations for residential buildings. The range of outcomes is similar to that of the LCA process; however, the published estimates are on average greater than the process LCA estimates. Over a ton of CO₂ per square meter has been estimated.

3.3. Hybrid Analysis

The main strength of both processes (completeness and IOA specificity) has been focused on the execution of hybrid approaches [36]. The advantage of the hybrid analysis is that it reduces curtail errors of operational analysis while retaining enough product detail to equate two alike products or systems [37]. In the literature review process, three types of tests of hybrid are used consistently: meta-hybrid analysis, input–output-based hybrid analysis and hybrid analysis at several levels. In a multi-level combined analysis, process-based data are used for the mining and release phases, and several other important upstream processes. In this case of analysis, the two existing datasets are simply joined together, along with the other upstream processes that are modeled using input–output analysis [30]. This strategy uses conventional detailed process analysis, adding IOA to what is “missing” in the process [37].

4. Uncertainty in LCA

Certainty refers to our ability to know the truth with assurance and precision. The reality of certainty and truth avoids self-defeating philosophical skepticism arguments, and not accepting their existence is also part of admitting a fact with certainty [38]. The foundation of the concept of uncertainty is the reality of truth, admitting that something exists but cannot be fully comprehended. Apart from the sensation of existence, there is no uncertainty in objects; they only exist in our minds or intellect [38]. As a result, doubt is highlighted by our inability to know the truth. In truth, there are other approaches to dealing with uncertainty, but the probabilistic approach is one of the most popular. Probability can be considered as the language of uncertainty that describes our awareness of the truth’s limitations. Therefore, as will be explained later, many domains of knowledge have turned to probability to assist them to overcome this barrier. LCA is no exception [39].

During the 1990s, many studies were devoted to handling the uncertainty in LCA. In LCA, uncertainty analysis is described as “the study of the spread of unintended deviations” to determine “those areas where product and process improvement leads to the greatest environmental gain” [40]. Similarly, uncertainty analysis in LCA was shown to be effective in guiding decision-makers toward the relevance of any discrepancies in product comparisons, alternatives for product enhancement and eco-label assignment [40]. The expected quality of data is outlined by data quality goals (DQG), and the semi-quantitative values that offer information about the data quality are data quality indicators (DQI); these were initially discovered by Weidema and Wesnaes and used in an LCA context. Funtowicz and Ravetz’s entirely qualitative suggestion served as the basis for the methodological development, which is referred to as the “pedigree matrix” in LCA parlance. It is one of the most popular approaches to semi-quantitatively resolving data uncertainty in LCA [41]. Generally, life cycle assessment models progress from deterministic models to stochastic models, which are introduced by a probability distribution, and along with this DQI offers early probabilistic ways to deal with the existing data uncertainties [42]. However, at the beginning of the twenty-first century, a comprehensive framework was developed and investigated for separating different forms of uncertainty and variability in the life cycle assessment. These frameworks are critical because they distinguish between different forms of variability and uncertainty in LCA and acknowledge that uncertainty and variability of different types may require different management. Possible ideas of different types can be seen in Figure 2 [43]. The data quality and evaluation matrix, and the transformation matrix, are considered the two most effective techniques used in DQI assessment. DQI is still vulnerable to limitations in the context of assessment accuracy, due to the subjective assessment of data quality. The results nevertheless frequently tend to be underestimated as described, even though the present qualitative uncertainty assessment methods complement the quantitative analysis approaches to decrease variability [44].

While uncertainty denotes a lack of comprehension of the truth, variability denotes the inherent variances that exist within a group as a consequence of natural value heterogeneity. As a result, although uncertainty can be reduced, variability cannot; it can only be better quantified, by enhanced sampling, for example. For the sake of convenience, in the further research of this paper, both categories of uncertainty and variability are referred to as “uncertainty” [38,45]. Based on the variability and variance, uncertainty is of three different types. First, uncertainty in which parameters are known to us tends to follow a statistical randomness pattern (things we already know). Second, uncertainty in which factors are known to us presents problems that are not known to us (things we know but do not know). Third is the uncertainty in which the factors impacting the system or problem are not known (things we do not know that we do not know).

Based on the types and pathways of uncertainty, there are some uncertainties that are frequently encountered by professionals in fields who are decision-makers working on construction projects, and these have an impact on all facets of design, construction, and management:

• Incomplete and insufficient data (for example, lack of information on construction materials, and building geometry);
• Inaccuracies in modeling models (for example, energy analysis software, building utilization projections);
• The undefined character of the future (for example, changes in usage, service life, technology advancements, socioeconomic shifts, and so on).
• When it comes to the problem of measuring or estimating embodied carbon, there are four types of uncertainty to consider [28]:
• Uncertainty concerning the embodied carbon emissions in different stages of building components, materials, and the entire structure;
• Uncertainty regarding the amount of carbon that will be embodied in the future and the service life of construction materials and components, with technological advancement;
• Uncertainty about future occurrences that will take place during the service life of built assets, such as component replacement, component life span, use modifications, and end of life;
• Uncertainty over the capabilities and methods of measurement.

4.1. Probabilistic Approaches

4.1.1. Current Measurement

The ideal approach for assessment (as shown in Figure 3) is to assume that a building element or building that has already been in existence has an exact value in terms of the amount of carbon that it contains, but it is impossible to virtually determine what this is; the identified value lies somewhere between expected apprehension and absolute ignorance. There exist many ways of representing uncertain knowledge of continuous variables. The following list includes approaches with the weakest [46].

• A single figure that indicates the best estimate is referred to as a single-point estimate or “best guesstimate”;
• A range of possible values is listed in ascending order with no indication of where the actual value falls. This is defined when the lowest and highest potential values are uniformly divided;
• An estimate of three points, also known as 3PE, is based on the lowest, most likely, and possible credible values. This can be defined by a triangular probability distribution. The 3PE approach is an excellent approach to capturing the expertise of seasoned practitioners who can advise on the lowest conceivable estimate, and for an unknown variable with the highest possible values;
• An empirical distribution of data from similar situations where the carbon embodied was assessed. The instances can be utilized as a probability distribution if they reflect a representative sample of the range of potential values.

The range of possible values and their probabilities, such as lognormal, normal or a binomial distribution, can be represented with a mathematical probability distribution. Empirical data are frequently used to calibrate such a distribution. The possible approaches discussed are plotted in Figure 4, which shows a shared region implying that among all the distribution methods, there exists an area of common embodied carbon estimate for a particular assessment, which can be considered as the “safe estimate”. This strategy is considered appropriate when the distribution accurately represents reality; for example, the prediction of the component life that follows a mathematical distribution when the yearly likelihood of component failure is known. For the purpose of mathematical simplicity, it might be tempting to choose an unknown variable with an arbitrary distribution, but this accomplishes nothing and may result in illusory precision.

4.1.2. Life Cycle Measurement

The focus of the study up to this point has been on the beginning condition of structures or building components, where an exact measurement for embodied carbon is expected, but it is still not possible to know how accurate it is. When looking at the problem from a lifetime perspective, the situation is different, since embodied carbon from different life cycle stages, as well as the maintenance and replacement of building components during the operational stages, has not yet occurred, and hence cannot be estimated. In addition, it is necessary to analyze this possible future embodied carbon to carry out a life cycle assessment over the buildings’ life. Figure 5 is one of the approaches for the LCA, and it is necessary to consider how to take the necessary steps to include possible changes over lifetime.

Because crucial factors, such as the amount of embodied carbon in a kilogram of building material like steel, are expected to fluctuate over time from present levels, future estimations of embedded carbon generally have more uncertainty than current measurements. This element is frequently overlooked, and current values are used in life cycle calculations; life cycle measures of this sort may only be relied on to a limited extent.

4.1.3. Tree Representations

A tree that is designed to capture possible future values through its branching can be used to represent the fan of uncertainty [46]. Throughout time, the variable will take just one path through the number of possible pathways through the tree, although many potential paths are available to choose from, and the best path to be taken in advance is unknown. Each generation of a lognormal tree consists of two-way branches that are generally isolated by a year, with probabilities (equal or unequal) associated with branches upward and downward. The possible range of values expands with each generation, but the possibility of each one decreases. The tree for the variable develops the probability distributions algorithm at each generation, and they become larger progressively, reflecting increasing uncertainty.

4.1.4. Monte Carlo Simulation (MCS)

In this case of progressive broadening and flattering, a second approach to estimate future values of the different life cycle stages, Monte Carlo simulation (MCS), is more applicable. There can be numerous variables in Monte Carlo simulation that vary from generation to generation with each probability distribution.

The probabilistic measurement of embodied carbon is done by MCS based on countless simulated scenarios. From several predefined variables, values are randomly sampled that predefine probability at each step, and the variable that determines embodied carbon measurement begins with known values in each scenario. Each scenario can be to a branch of a giant tree with variable future possibilities that is too large to identify all of its branches in themselves. The numbers from different perspectives together create an embedded carbon system with a probability distribution. The coefficient of variation (CV), which describes the uncertainty level based on the output from the MCS, is used to determine the limit to which the uncertainty of a scenario may be taken into account. The potential value for each activity related to building life cycle activity is indicated by the distribution, given the limitations and shortcomings of the availability of data in the building sector [42,46]. MCS is carried out using 10,000 repetitions of the Crystal Ball program. The number of iterations mostly influences the stability of the final product. Generally, the excessive repetition of the MCS in a single scenario is viewed as a less effective way to perform MCS, since the outcome will converge with an increase in the number of simulations. To assess the right number of repetitions, it is therefore valid to examine convergence. Then, using the standard deviation and mean of a large number of values that were obtained from simulations with various iterations, the trend of convergence was investigated [47].

4.2. Decision-Making under Uncertainty

The principal function of embodied carbon measurements in buildings is to compare different approaches, such as the use of material, at the design stage and decide which to use. When embodied carbon measurements are ambiguous, the situation shifts due to the decision-maker’s subjective attitude toward uncertainty. Subjectivity has the effect of causing various decision-makers to interpret the same ambiguous measurements differently and pick different options. Two crucial components of the subjective evaluation of ambiguous facts are time preference and risk aversion. A probabilistic methodology is required because risk aversion affects both life cycle analysis and existing embodied carbon measurements. Only LCA is relevant, but it applies to probabilistic and deterministic approaches [48].

4.3. Flexible Strategies

The goal of life cycle carbon assessment is to assist present decision-makers in maximizing the predicted performance of embodied carbon across different life cycle stages. However, additional decisions will be made during different stages, such as the replacement of a component with a shorter service life than the study duration.

For the benefit of future knowledge, different opportunities for future decision-making must be incorporated into the current design strategies. Although this concept is not new, with recent advancements, it has evolved into a precise and quantitative feature [46].

5. Introducing Early Environmental Assessment in the Design Process

Modern architectural practice is competent in examining the implications of new structures operationally, notably in the context of operational energy usage and carbon emissions. Embedded emissions design experience is still limited, despite social trends urging the incorporation of environmental considerations into the design process at the very beginning phases.

The design process is defined in both theoretical and experimental literature as a series of repetitive decision methods that gradually elevate the design to a superior complexity level while decreasing uncertainty. Beginning with the early conceptual phases, when many factors are ambiguous and the design team analyzes a wide variety of strategic and parametric possibilities, and concluding with the project’s completion when the final building eliminates all uncertainty, the design changes with time. If environmental effects are to be successfully included in the early design phase, simpler methods that can deliver higher accuracy while using a few generic parameters are required.

• Generally, construction professionals utilize a simplified geometric model, which predicts the areas and quantities of building components based on restricted geometric input data. A model, which can calculate both the areas and consumption of building elements, might be used early in the design phase to estimate the influence of building form on embodied carbon. Through this method, a link between the geometric model’s accuracy and its use time can be established;
• Rather than limiting the embodied carbon assessment of building elements to after the completion of a chosen building design, a more effective method would be to select suitable building elements from an inventory of a large number of predefined building elements with embodied carbon results at the early design stage. This method would have a greater impact.

A simple solution can be presented by integrating these two techniques, making embodied carbon data more available and usable for non-technical customers and construction professionals for the early design process with the development of low-embodied carbon buildings.

• Building Geometry Calculation: Based on minimal geometric input data, a simpler geometric building model is built, which calculates the area and quantity of construction elements.
• Building and Material Lifespan: As the building comprises different elements, and each element has a different lifespan within the life span of the whole building, it is necessary to set out a process that identifies both.
• Parametric Variation: The building elements that are predefined cover the diversity of design solutions for all internal and external elements, primarily related to the typological variations’ specification; therefore, the first step is the selection of predefined building elements according to the requirements.
• Embodied Carbon Calculation: Every typological variation is subjected to the evaluation of embodied carbon throughout its life cycle. The data must be compiled into an inventory of pre-defined building elements and carbon data.
• Tool for Embodied Carbon Design: The design tool pairs geometric data with specified construction parts to quickly assess the embodied carbon of structures.
• Detailed Building LCA vs. Simplified: The results obtained from the simple tool are compared to those of a thorough building LCA, and the differences are addressed.

It is feasible to construct simpler embodied carbon tools, to save time, and this can also deliver enhanced assessment results such as the LCAP tool, which gives more precision in the early design process by utilizing fewer generic parameters [49]. For the secondary building parts and services, a similar approach must be followed, which is now not included in the model, and would assist the ongoing development of the tool. This would include calculating material consumption per square meter of floor space for doors, staircases, heating/ventilation systems, and other construction components. This would enable the tool to learn and expand with these elements, increasing the instrument’s accuracy even further, enabling it to be utilized for later stages of the building’s life cycle.

Generally, for the environmental performance of the product, two methodologies are established, life cycle assessment and carbon footprint; such is the case of building assessment. With the established methodologies, common quantitative claims of life cycle assessment exist in two forms, environmental product declaration (EPD) and product carbon footprint (CFP). For the lifecycle-based quantitative claims, it is necessary for the product to be open to the sources of data, the boundaries of the system, the recycled product’s impacts and the choice of measurement [50]. In the building’s life cycle assessment, two established methodologies play a vital role to achieve the lifecycle-based quantitative claims. As the building constitutes different materials and shows different maintenance requirements in different stages of its life cycle, the EPD and the advancement in the modeling options of the material sources and system boundaries according to region and requirement lead to a detailed life cycle assessment of the building in any stage. The EPD application in the early stage leads to the detailed analysis of CFP in any stage of the building’s life cycle. However, for the EPD to support comparable EPDs, product category rules (PCRs) are considered mandatory to define specific rules for products serving the same function [51]. For the PCRs to develop, ISO 14,025 [52] presents the procedures and content required for a PCR, as well as requirements for comparability. However, standards present a defined set of rules and will not be sufficient for the development of PCRs for every requirement, which can also be called supplementary requirements. In the building sector, several documents and standards have been published other than ISO 14025, such as EN 15,804 [53], EN TR 15,941 [54], EN 15643-1 [55], and many more, which promise to serve the purpose of comparison and comparative assertion, but they also require a certain form of PCRs to carry out the promise. For this purpose, EN 15084 + A1 [53] is presented for the further development of EPDs [56]. With the contribution and collaboration of developers, PCRs are developed according to regions and requirements. Although such developments are the case in Europe or other developed nations, developing nations still fail to understand the concept of EPDs and PCRs.

6. Embodied Carbon in Different Stages of the Building Life Cycle

Construction, maintenance, and demolition trash have become a major source of solid waste for the environment and society in developing and developed countries, due to the tremendous rise of the construction sector globally in recent decades due to population increase; 20–60% of the world’s solid waste stream is produced by the construction industry [57]. Construction-related garbage makes up roughly 20–30% of all solid waste produced in the European Union (EU), 30% in Canada, 29% in the United States, 26% in Hong Kong, and 30–40% in Australia. [58,59,60,61,62]. As a result, while construction contributes greatly to global progress and money, it also has a severe influence on the environment. Waste is sometimes disregarded because it is seen as negligible in comparison to waste generated during operations.

Landfilling and incineration are two typical waste disposal methods that have wreaked havoc on society, both economically and environmentally, by accumulating garbage and exacerbating the problem of global warming by emitting carbon dioxide (CO₂) during procedures [63,64].

Rapid population expansion has accelerated construction operations, resulting in increased trash output and embodied carbon across the phases of an existing building’s lifetime. Demolition operations contribute the majority of the waste, whereas construction trash makes up the smallest portion of the entire waste portion. Despite the fact that the construction stage generates the least waste, it is possible to reduce the waste rapidly and effectively, with the option of better site management and improved information flow and coordination among members of the design team during the design and construction phases. Maintenance waste, on the other hand, has generally gone ignored among the three, despite having a higher potential of embodied carbon content than building waste—almost six times more over 50 years. The durability and quality of materials will dictate how often they must be maintained, replaced, or repaired. Design rethinking and embracing lifecycle thinking are thus critical steps toward reducing waste and maximizing possible recovery.

EOL waste output and related embodied carbon are by far the highest. Because of different sorts of obsolescence or other factors, the destruction of the EOL of the building is sometimes unavoidable. It is possible that the materials or components can be reused or refurbished during lifecycle management, and design-out-waste concepts are employed from the early design stage, preventing landfilling. Furthermore, well-designed and -maintained buildings can survive longer, preserving most of the embodied carbon contained in the construction.

If appropriately collected, handled, and recycled, the wastes created at different phases of a building’s lifetime may be reused and become important resources for the construction industry. The waste can be considered a valuable resource for substituting raw materials and reducing embodied carbon and landfill waste. The role of transportation in embodied carbon’s impacts on waste has been demonstrated. As a result, minor changes in travel lengths may cause the outcomes to worsen. As landfill sites are rapidly filling up and possible new sites for dumping waste are located further away from built-up regions, requiring a longer journey, this is becoming the case. The most essential technique to minimize the embodied carbon in the three stages of buildings is to avoid waste formation so that resources are not wasted, and the environment is not harmed. Addressing waste minimization through design optimization and early stakeholder involvement is key to the strategy’s success.

7. Sustainable Technologies and Embodied Carbon

Buildings and their activities are recognized to account for a sizable share of total global energy end-use. In total, 40% of the world’s energy and one-third of global greenhouse gas emissions are because of the building sector and its activities [65]. To offset this impact, new buildings built with the concept of sustainability that consume significantly less, if not zero, energy are required. Using sustainable technology is one of the most efficient methods among several available methods to achieve this goal. “Sustainable technology” refers to instruments that use materials that do not have a long-term negative carbon dioxide equivalent (CO₂e) impact on the environment to power, ventilate, heat, and/or cool a structure. This environmental influence could be determined using a widely accepted method known as life cycle assessment (LCA).

Many new sustainable technologies are available on the market, most of which are intended to cut energy use in buildings, and there is a vast list to choose from. The consideration of the use of sustainable technology in a building should be based on the needs and adaptability of the building to certain technologies. Generally, these technologies frequently outlive the building itself, but because of the developments in building-integrated photovoltaics, for instance, each one is becoming more resilient and connected to the structure of the building. Because each system requires power to work, these technologies produce CO₂e indirectly once installed and in operation on the building. The electricity generated by a solar PV system is generally utilized to power the different systems of the building, including the heating and cooling system, which is considered to be the most demanding system. The aim of using any renewable source of power is to minimize the reliability on the grid system, although renewable sources are still under research, including improving the decarbonization and adaptability of the grid at the user end instead of in the building, and adjusting demand response with the integration of renewables, and possible developments in the future will help in the transition of the building’s user from a consumer to a prosumer.

On the effects of sustainable technology throughout life, there is still a shortage of reliable data and knowledge. Given the growing popularity of these technologies, this is a sizable gap. When evaluated on a whole-life basis, the cost of new sustainable technologies is much lower than that of conventional systems. However, because each construction is unique and uses a unique combination of technologies, generalizations of the process need to be avoided. Using sustainable technology has environmental advantages. However, there is one major issue that must be addressed: funding, and the additional capital expenditures necessary to install and maintain the technology being explored.

Building Materials Design Strategies for Low Embodied Carbon

There are five key design solutions for reducing the embodied carbon effects of building materials. Reusing and recycling materials, using building materials with low carbon intensity, sourcing materials locally, and using durable and long-lasting materials are some of them. Reducing the amount of space and resources utilized is another.

To avoid shifting the burden of manifesting carbon emissions to later stages of the building’s life cycle, it is important to consider methods of incorporating carbon reduction early in the design stage, and transparency in the design and construction process. The early assessment of low-embodied carbon building materials can save time, energy, and money. One of the most successful low-embodied carbon design solutions is to use less space and materials. Other options include the use of recycled and repurposed materials, low-carbon substances, local resources, and materials with a high strength and long service life.

8. Mitigation of the Embodied Carbon

Macro- and micro-level methodologies have been traditionally utilized for the embodied carbon evaluation of buildings, based on a single building project. I-O life cycle assessments have been used for the assessment of carbon emissions in the entire construction sector in previous research, and the entire construction sector is considered inside the system boundaries of the entire economy at the macro level [66]. Hybrid and process-based LCA methodologies are commonly utilized at the micro level to assess carbon emissions from a specific building or construction project. Despite these theoretical advancements, high-efficiency technology, and a range of environmentally beneficial rules implemented in the construction industry, lowering embodied carbon emissions is proving difficult.

8.1. Macro Perspective

8.1.1. National Level

In 1970, Leontief proposed single-region input–output (SRIO) analysis, and it has been used as a validation approach to examine the “externalities” of goods or services by calculating the inter-industry dependency connection throughout the whole economic system using data that are readily accessible to the general public [35].

However, SRIO was carried out under the presumption that the technology of manufacturing in domestic production processes is similar to the technology used in foreign regions, because of which it failed to take into account characteristics that are region-specific, such as the climate, geographical location, natural resources, and supporting economic activities that have a direct impact on regional environmental changes. This oversight highlights the issue of seeing projects from a regional and industrial sector viewpoint.

8.1.2. Regional Level

To address the challenges identified in the embodied carbon assessment of the building sector by the SRIO, the multiregional input–output (MRIO) model was adopted. The MRIO model has been regarded as a systematic approach to quantifying environmental consequences that take geographical inequalities and technical differences into account [67,68]. Regions may differ in terms of building materials, construction methods, and transportation options, as is the case in many nations. As a result, the regional productivity of a particular building under study may range greatly from the level of the national average. The errors between the simulated value and the real emissions may be further illustrated by such a disparity. To acquire a reliable estimate, it is therefore important to measure the variations in manufacturing techniques [34,69,70].

Given the intensity of embodied carbon emissions, in average circumstances, it is easy to determine the carbon emission contained in the final demand. However, since this study only focuses on the building sector, further explanation is essential to derive a better understanding of the regional construction sector’s embedded carbon emission intensity and interregional carbon emissions transfer. Embodied carbon emission intensity, which illustrates the total amount of carbon emissions per unit of money, can be used to determine direct and indirect carbon emissions generated throughout the construction sector’s supply chain. This section will also provide a supply chain-based method for structurally analyzing embodied carbon emissions in the building sector. This technique aids in identifying the crucial production chain segments and routes wherein economic connections with other industries have a substantial impact on the final product [71,72].

8.2. Micro Perspective

Process-based models, input–output (I-O) models, and hybrid models are the major quantification approaches for embedded carbon evaluation at the building level. There is a tendency for relevant research to increasingly turn away from process-based individual examples and toward a more hybrid and global perspective. There are two popular processes: hybrid-based LCA and process-based LCA.

• The I-O-based hybrid model was created to give process-based data for the I-O analysis’s most energy-consuming pathways.
• I-O analysis is used to determine the product under study’s initial overall environmental load.
• Using I-O analysis to break down a complicated upstream process and identify the main pathways that have a big environmental effect.
• Key route modification using process-based inventory data on both supplied amount and energy intensity.
• Taking the original overall environmental effect determined by the I-O model and subtracting the corresponding I-O value of the main routes reflected in the process inventory.
• Including updated energy routes from process-based analysis in the remaining unmodified I-O framework.

9. Analysis and Discussion

Assessments of the embodied carbon studies are carried out at many levels (building level, neighborhood level, urban level and national level), and many frameworks have been developed over the years, but still, there are very few studies available at the national level that discuss embodied carbon. Even in these few studies, there are few in which future uncertainties are mentioned [73]; in most cases, uncertainties are excluded, keeping the changes constant or changing some factors. For the analysis of a large-scale building-by-building method and the archetype, this method is the most commonly used; among the two archetypes, it is the method that can be combined easily with LCA, tree representation and MCS, but the major problem with the archetype is that it is suitable mostly for developed countries or countries that are planned—for unplanned cities, the archetype method is not suitable. Generally, for unplanned cities, the two modeling approaches are the stochastic modeling approach and the point cloud model approach. The stochastic modeling approach is considered efficient for cities that are planned, or which are planned at first but over a period show new developments, and are then considered unplanned. In the stochastic modeling approach, the probabilistic calibration of building stock and parametric screening through Bayesian calibration is undertaken, which comes under the life cycle measurement and MCS approach of the measurement depending upon the selection made for Bayesian calibration. The point cloud approach is considered for unplanned cities that are considered “organic” in nature. In the point cloud approach, with the generation of a 3D model of the city based on geo-information, the 3D model developed must have a minimum level of detail (LOD) of three for the assessment, which means the model of the building accurately resembles what is installed; the higher the LOD level, the higher the accuracy of the result.

With the measurement and analysis parts covered, we then address the management and mitigation section, with the analysis of an existing city, neighborhood, or nation. Here, certain effective strategies as discussed regarding sustainable technology and embodied carbon may be identified, which are easy to implement, cheap in terms of investment, and show efficiency in minimizing the embodied carbon at any level. As in the case of India, solar panels are promoted throughout the nation to overcome the problem of electricity with government incentives; in the case of Ireland, heat pumps are promoted to minimize heating through fossil fuels, and also through government incentives. Although the approaches in different countries are structured by different factors, they eventually help in minimizing the embodied carbon of the nation generally in the operational stage of the building. So, the assessment of renovation policies at different levels will not only help in minimizing the embodied carbon, but also satisfy other needs too.

For a particular building’s LCA, many studies have been performed that followed many approaches. In building analysis, there are typologies of buildings—commercial, office, and residential—and there are many studies that have focused on the LCA of commercial and office buildings. Although residential buildings are identified to be most common typology in any city, the analysis of the residential sector is still not as progressive as compared to the other typologies. In the analysis of embodied building stock at the national level, it is identified that the embodied carbon of the residential sector is about 1.5 to 2.2 times the embodied carbon of other typologies [74]. We analyze the existing studies around the globe on the embodied carbon assessment of high-rise residential buildings—we focus on high-rises because any important city of any country always contains more population than it was designed for, and instead of building horizontally, they develop vertically, and this will be same for most of the cities of the future. An analysis of the embodied carbon of high-rise residential buildings based on the structure and life cycle inventory technique is carried out, as shown in

Table 2. Comparison of emissions of high-rise residential buildings.

Location	Structure	No. of Floors	Floor Area (m2)	Life Span (Years)	Life Cycle Inventory Technique	Emission (Kg CO2/m2)	Uncertainty	Reference
India	Reinforced concrete	31	28,445	75	Hybrid	414	Excluded	[80]
China	Steel	6, 11,18	5524, 5544, 9574	30	Process-based	800–1000	Excluded	[81]
USA, Norway	Timber	7, 12, 21	6097, 10,542, 11,823	60	Process-based	−234.8, 441.8	Excluded	[75]
Australia	Prefabricated modular	8	3943	50	Hybrid	864.04, 578.23, 629.46	Excluded	[82]
Turkey	Reinforced Concrete	13	7445	50	Process-based	105	Excluded	[83]
Norway	Concrete	9	3700	30	Process-based	228.41	Excluded	[84]

. The studies selected aim to cover different approaches in the different cities of different countries, and in those different cities, different structures are assessed to analyze the role of structure in emissions. Studies with uncertainty excluded are purposely selected to avoid large variations in the results, because analyses with uncertainty are still not common around the globe. In the analysis, it is identified that structure plays a vital role in the emissions of a residential building, as shown in Figure 6 and one of the research projects, in which when the concrete frame of the structure was completely replaced by timber, the emissions became negative [75]. However, this reduction does not mean that using timber in every nation will provide similar results, because timber is not common in every country, such as in India, and in China the demand for buildings is so high that it cannot be met with timber and without damaging nature. The conclusion of the study is the identification of such materials that can replace concrete and steel in the structure and that are widely available in the respective country. Much research is being performed [76,77,78] to minimize the carbon content of concrete, as concrete is considered a universal material for construction in developing countries, using fly ash and many recyclable materials in the mixture [79].

10. Conclusions

The built environment has a large and complex carbon footprint, and there are few well-established methods for lowering it. To meet strict carbon budgets and climate mitigation objectives, the embodied emissions of different building stages, transportation required in those stages, and energy systems set up in their construction and maintenance must be reduced. In this study:

• Embodied emissions in all aspect of construction are covered, with the main focus of study being on the measurement, management and mitigation strategies of embodied carbon;
• As estimate is an important part of any debate, so our measurement covers the uncertainty analysis from diverse points of view through a novel approach. Management covers the early design tools and the significance of the life cycle stages, and mitigation covers the strategies in place to reduce the embodied carbon, although reductions in embodied carbon are a subjective topic and depend on region. Analysis covers the ideal approaches for mitigation, irrespective of the region;
• In this study, for the measurement of the embodied carbon, different probabilistic approaches have been studied, such as current measurement, life cycle measurement, tree representation, and Monte Carlo simulations;
• It is identified that life cycle measurement is appropriate for existing buildings, but in most of the studies, uncertainty is excluded, and the demolition and service life requirements of the buildings are considered constant. MCS, on the other hand, can estimate the embodied carbon based on future probability, and is more accurate than the other two, but is not used frequently because of the complexity of the process.

Comparing the scenarios of the study with the embodied carbon assessment studies, as mentioned in Table 2, it is identified that many studies do not consider uncertainty during the assessment of embodied carbon. Although this is a simpler approach, it is still considered successful in minimizing the embodied carbon of the building sector, because this sector is the least explored in terms of carbon minimization. As this assessment of embodied carbon is common in the building sector, understanding the uncertainty is crucial for future minimization in the sector.

For the management and mitigation of embodied carbon, considerations of both are important to employ the approaches identified on a large scale, such as the neighborhood, urban or national scale. Some examples of this management include the integration of different renewable sources of electricity for energy consumption, which will help in lowering the emissions in the operational stage of the building. For the construction and demolition stages, different low-embodied carbon building materials and design strategies, such as reduce, reuse and recycle, low carbon, and local material, help in minimizing the embodied carbon. For mitigation, different approaches such as SRIO, MRIO, hybrid-based LCA and process-based LCA, which are currently employed at different micro and macro levels, and their performances have been analyzed.