The consequences of global warming have created an imperative to significantly reduce greenhouse gas (GHG) emissions. For buildings, this means considering not only the operational emissions but also the embodied emissions. This entails the development of legal requirements as well as changes in building design and construction practices. Embodied carbon is becoming a topic of growing importance, especially because its absolute and relative share in the life cycle of buildings is currently increasing. Questions arise for all those involved in policymaking, the real estate and construction industries, as well as design who may now be dealing with this topic for the first time.
The specific questions addressed in this briefing note are:
There is a long track record to reduce the demand for construction materials, as well as for the raw materials and energy required to produce them. A century ago an interest in the minimum amount of coal required to manufacture products arose, while 50 years ago the ‘energy crisis’ shifted the emphasis on crude oil. Over the last decade, the recognition of a growing ‘climate crisis’ broadened the embodied energy concept to embodied carbon. Authoritative scientific reports (such as the United Nations’ Intergovernmental Panel on Climate Change—IPCC) have highlighted why embodied carbon requires urgent action.
Building regulations have been focused on building operational energy demand (and the associated emissions). However, GHGs are caused and emitted at every stage of a building’s life cycle. From a macro-economic perspective, today approximately 10% of total global anthropogenic GHG emissions result from the manufacture of building construction materials such as steel, cement and glass (UNEP 2021). To meet the stated societal goals of radically reducing GHG emissions, it is necessary to assess and reduce the building-related embodied part (the embodied GHG emissions are called ‘embodied carbon’ here in line with common terminology). This task can be assisted by using harmonised assessment methods, the improvement of data quality for construction products, and the deployment of user-friendly design and assessment tools.
Today, several developments are driving the consideration of embodied carbon (Figure 1). Several formal standards provide the basic principles for assessing life cycle GHG emissions of buildings and construction products at both the European (European Committee for Standardization (CEN), e.g. EN 15978 and EN 15804; CEN 2011, 2019) and international (International Organization for Standardization (ISO), e.g. ISO 21929 and ISO 21930; ISO 2011, 2017) levels. Based on these standards, several organisations such as The Royal Institute of British Architects (RIBA), The Royal Institution of Chartered Surveyors (RICS) and the World Green Building Council (WGBC) have implemented methods, guidelines and tools to assist their members. International research projects such as the International Energy Agency’s Energy in Buildings and Communities Programme (IEA-EBC) Annex 72 are underpinning the development of methods, guidelines and tools to ensure they are robust and fit for purpose, and to create the basis for a next generation of guidelines, standards and legal requirements.
In the policy and market realms, there is a trend to establish whole-life carbon (WLC) as a criterion in building sustainability assessment systems. The reduction of embodied carbon is becoming a requirement in green public procurement, as well as a prerequisite for the allocation of subsidies. Building developers and buyers are increasingly being informed about the carbon footprint of buildings (e.g. EU Taxonomy and the current draft of the Energy Performance of Buildings Directive (EPBD) in Europe). Several countries (Figure 1) have now set mandatory requirements to report life cycle-based environmental performance assessment results of buildings, including embodied carbon, and some have even introduced or will soon introduce binding embodied carbon or WLC limits (e.g. France, Denmark and Sweden). In some countries, local authorities take the lead and demand such assessments (e.g. Greater London Authority (GLA), City of Vancouver).
What is meant by the term ‘embodied carbon’? This is not the carbon physically contained in the building. Rather, it is the allocation of GHG emissions that arise in the production of construction products of all kinds, their transport to and from the building site, the processes of building construction, maintenance, replacement and deconstruction, as well as the end of life of building components and the building. There are initial/upfront, recurrent and end-of-life-related emissions.
GHG emissions include not only CO2 but also other gases with a global warming effect. CO2 emissions dominate GHG emissions with 75% of the global average and in some countries up to 90%. These are typically reported by converting them into kg CO2 equivalents to express their global warming potential.
Concerning the physical carbon content in a building, the indicator ‘biogenic carbon content’ was recently introduced in European standards to describe it (kg C).
The quantification of embodied carbon of building products is often achieved using the life cycle assessment (LCA) methodology. LCA is the systematic analysis of the energy and material flows and the resulting effects on the environment, including the climate. The case of determining the embodied carbon of a building as part of a WLC assessment is an applied LCA. It is usually sufficient to link product quantities with life cycle-based environmental data for products and services determined using LCA—although variation can occur between similar products due to different primary energy sources or manufacturing processes. The result is also referred to as a building-related carbon footprint, where embodied carbon is seen as a partial carbon footprint.
The development of more stringent operational performance requirements has increased the importance of embodied carbon from buildings. Often, high-efficiency buildings require more materials and technical equipment which increase the embodied part of the carbon footprint. However, high operational energy performance does not necessarily have to result from high embodied carbon. An analysis of hundreds of global LCA case studies of both residential and office buildings (Figure 2) demonstrates the possibility of designing buildings with low embodied carbon that comply with ambitious building energy-efficiency regulations (Röck et al. 2020). To put it differently, the optimisation of both embodied and operational emissions is technically feasible and necessary to achieve net zero emissions over the life cycle.
The life cycle of a building has several stages and a framework exists to define each stage (Figure 3). Each stage requires a calculation of embodied carbon. The range of values for embodied carbon found in the literature is often due to the methodological choices behind their calculation. The interpretation of these values depends heavily on the defined object of assessment, i.e. what building components are included in the building model and what life cycle stages are included or omitted in the life cycle model. In combination with communication of an assessment result, it must be made explicit what scope of assessment and which system boundaries were applied.
Carbon footprint assessments differ significantly in these two scopes. Regarding the building model, the variation with the most critical effect is the inclusion/exclusion of building services such as heating, ventilation and air-conditioning (HVAC) systems. These could account for nearly 40% of embodied carbon of technology-intensive buildings (Hoxha et al. 2021). In the past, the focus was on the structure so the technical systems were neglected due to limited data availability. However, this has changed and today the aim is to have an as complete building model as possible.
In future, a more intense focus on emissions released before the building operation begins (often called upfront carbon emissions) is expected as they are responsible for more than 60% of the total embodied carbon and are immediately consuming the remaining global carbon budget (Röck et al. 2022). However, the inclusion/exclusion of the impacts associated with the replacements of building components during use stage can have a considerable effect (Birgisdottir et al. 2017). It is important to stress that post-handover results strongly depend on assumptions about the service life of building components (Goulouti et al. 2020) and end-of-life processes, among others.
The design team faces many design and material choices. Quality assured data for construction products and processes are vital for evaluating the carbon implications of these choices. Reliable data exist for construction products and construction processes based on published LCAs, as part of quality-proofed environmental product declaration (EPD), in quality-verified databases, integrated in quality-verified assessment tools or from other sources. These require regular updating to provide temporal and geographical validity. For example, the energy mix used in the manufacturing of a large proportion of products as well as the efficiency of the manufacturing processes themselves change over time (Alig et al. 2020). In the long run, data taking into account the results of the decarbonisation processes in the sense of forecasts for 2030, 2040 and 2050 are needed to model future replacement.
Building assessment tools are currently being developed worldwide that are easy for designers to use even without knowledge of the details of an LCA (Melton 2019; Marsh et al. 2018). A procedure for identifying the right tool according to specific designers’ or users’ needs is proposed by Di Bari et al. (2022).
Embodied carbon is an important part of the carbon footprint of buildings. Their relative, and in most cases also the absolute, contribution is increasing. For buildings with no or net zero GHG emissions in operation, the share of the embodied part is 100%.
A prerequisite for the assessment of embodied carbon for buildings is the availability of corresponding data for construction products. Assessment method, data and benchmarks and/or target values must form a package.
The reduction of embodied carbon makes a significant contribution to reducing the building-related share of global GHG emissions. For this, operational energy efficiency standards need to be augmented with specific target values for embodied carbon and/or WLC. Related policy and regulation are already being developed in some countries. The introduction of regulation will reduce uncertainties, define system boundaries, and make both the assessment process and reporting more transparent.
The assessment and reduction of embodied carbon is a design task. Targets should already be formulated in the client’s brief or integrated into voluntary/legal requirements. The system boundaries need to be communicated transparently and should be based on international or European formal standards.
Clients have a responsibility for decisions throughout the initiation, design and implementation of a building project. Clients may be individuals, institutional and public organisations or commercial project developers. Standard EN 15643 (CEN 2022) recommends formulating requirements not only for the functional and technical performance in the client’s brief but also for environmental performance. In this context, the aim to reduce embodied carbon can also be addressed. However, it is first necessary for the client to make full use of the possibilities to act, given as examples in Table 1.
|AVOID CONSTRUCTION||REDUCE CONSTRUCTION|
A sufficiency strategy and space optimisation can avoid building anew. The willingness to forgo a new building can be allayed if the functional and technical requirements can be met by an existing one. Clients can already receive support from design professionals and consultants with these tasks. Another client task is to provide designers with adequate time and money to compare and assess design solutions and to actively commission them to do so. For many clients, the benefit of lower embodied carbon is already obvious. This can lead to better results in sustainability assessments, marketing, valuation and financing as well as insurance conditions. The associated cost needs to be examined more closely. Despite the wide range, there are examples of only minor additional costs for low embodied carbon buildings (Jungclaus et al. 2021).
Clients/investors and their associations (amongst others) should:
Options for designers to influence embodied environmental impacts, including embodied carbon, are listed in Table 2.
|DESIGN DECISIONS||MATERIAL CHOICES|
These possibilities fall within the efficiency and consistency (regenerative) strategy. It is important to determine, assess and influence embodied carbon during design. Designers and their associations (amongst others) should:
Since upfront emissions are an important part of embodied carbon and immediately consume parts of the remaining global budget for greenhouse gas (GHG) emissions, it is particularly important not to delay efforts to reduce them in both new building and refurbishment projects in a coordinated action of clients and designers but also policymakers and the construction product industry.
The authors thank all colleagues involved in International Energy Agency’s Energy in Buildings and Communities Programme (IEA-EBC) Annexes 57 and 72 for their collaboration. The respective results provided an essential basis for this briefing note. They are also thankful to the four anonymous reviewers for providing constructive comments which improved the text. The authors’ contribution to these projects was supported by the Federal Ministry for Economic Affairs and Climate Action (BMWK) and the Project Management Jülich (PTJ).
The authors have no competing interests to declare.
Alig, M., Frischknecht, R., Krebs, L., Ramseier, L. & Stolz, P. (2020). LCA of climate friendly construction materials. treeze. https://treeze.ch/projects/case-studies/building-and-construction/climat-1
Birgisdottir, H., Moncaster, A., Wiberg, A. H., Chae, C., Yokoyama, K., Balouktsi, M., Seo, S., Oka, T., Lützkendorf, T., & Malmqvist, T. (2017). IEA EBC Annex 57 ‘Evaluation of embodied energy and CO2eq for building construction’. Energy and Buildings, 154, 72–80. DOI: https://doi.org/10.1016/j.enbuild.2017.08.030
CEN. (2019). EN 15804:2012+A2: Sustainability of construction works—Environmental product declaration—Core rules for the product category of construction product. European Committee for Standardization (CEN)
Di Bari, R., Horn, R., Bruhn, S., Alaux, N., Saade, M. R. M., Soust-Verdaguer, B., … & Frischknecht, R. (2022). Buildings LCA and digitalization: Designers’ toolbox based on a survey. In IOP Conference Series: Earth and Environmental Science, 1078(1), 012092). DOI: https://doi.org/10.1088/1755-1315/1078/1/012092
Goulouti, K., Padey, P., Galimshina, A., Habert, G., & Lasvaux, S. (2020). Uncertainty of building elements’ service lives in building LCA & LCC: What matters? Building and Environment, 183, 106904. DOI: https://doi.org/10.1016/j.buildenv.2020.106904
Hoxha, E., Maierhofer, D., Saade, M. R. M., & Passer, A. (2021). Influence of technical and electrical equipment in life cycle assessments of buildings: case of a laboratory and research building. International Journal of Life Cycle Assessment, 26(5), 852–863. DOI: https://doi.org/10.1007/s11367-021-01919-9
ISO. (2011). ISO 21929-1:2011: Sustainability in Building Construction—Sustainability indicators—Part 1: Framework for the development of indicators and a core set of indicators for buildings. International Organization for Standardization (ISO).
ISO. (2017). ISO 21930:2017: Sustainability in buildings and civil engineering works—Core rules for environmental product declarations of construction products and services. International Organization for Standardization (ISO).
Jungclaus, M., Esau, R., Olgyay, V., & Rempher, A. (2021). Reducing embodied carbon in buildings: low-cost, high-value opportunities. RMI. http://www.rmi.org/insight/reducing-embodied-carbon-in-buildings
Melton, P. (2019). Embodied carbon tools: Assessing the options. BuildingGreen. https://www.buildinggreen.com/news-analysis/embodied-carbon-tools-assessing-options
Röck, M., Saade, M. R. M., Balouktsi, M., Rasmussen, F. N., Birgisdottir, H., Frischknecht, R., Habert, G., Lützkendorf, T., & Passer, A. (2020). Embodied GHG emissions of buildings—The hidden challenge for effective climate change mitigation. Applied Energy, 258, 114107. DOI: https://doi.org/10.1016/j.apenergy.2019.114107
Röck, M., Sørensen, A., Tozan, B., Steinmann, J., Le Den, X., Horup, L. H., & Birgisdottir, H. (2022). Towards embodied carbon benchmarks for buildings in Europe—#2: Setting the baseline: A bottom-up approach. Zenodo. DOI: https://doi.org/10.5281/zenodo.5895051
UNEP. (2021). 2021 Global status report for buildings and construction: Towards a zero-emission, efficient and resilient buildings and construction sector. United Nations Environment Programme (UNEP). www.globalabc.org