Embodied carbon

The need to urgently address the impact of building construction in response to the climate emergency we are facing is becoming increasingly evident across the sector.

According to data from the International Energy Agency, the construction sector accounts for 38% of global greenhouse gas emissions. Of this total, 27% corresponds to emissions from energy production for building operations, and the remaining 11% is attributed to the extraction and processing of materials—so-called embodied carbon—which is largely emitted before a building enters its use phase.

Having addressed the issue of emissions derived from a building’s energy consumption—whose regulation has been in place for years—the focus is now shifting to emissions during the construction phase, long before the building is even in operation.

It is during the production phase of the materials that make up a building that, according to prevailing standards since industrialisation, high levels of energy are consumed and large amounts of carbon are emitted.

Unlike widely implemented regulations related to energy consumption over the life of buildings—such as nearly zero-energy buildings (NZEB)—embodied carbon, despite accounting for 11% of global greenhouse gas emissions, is still not regulated, with a few exceptions in some European countries such as Denmark, Finland, France, or the Netherlands.

To move towards a decarbonised construction sector, it is necessary to address embodied carbon in our buildings during the construction, refurbishment, and dismantling phases, in order to carry out a decarbonisation process aligned with the goals of the Paris Agreement in time and thus avoid a climate catastrophe.

The impact of the construction sector

According to 2022 data from the International Energy Agency, buildings generate 38% of annual global greenhouse gas emissions, of which 27% are derived from building operations, while the remaining 11% corresponds to the materials industry and transport used in this construction—what is known as embodied carbon. [1]

In Spain, the building sector plays a crucial role, as it accounts for 30.1% of final energy consumption and 25.1% of emissions, of which 8.2% are direct emissions associated with fuel consumption in the residential, commercial, and institutional sectors.

To a large extent, this is because Spain has an ageing and inefficient building stock that does not meet the habitability needs of the 21st century. [2]

Carbon emissions in architecture

When we talk about carbon emissions in the architecture sector, it is necessary to take into account all emissions generated throughout the building life cycle. That is, we must include not only emissions during the use phase, but also all processes that take place before the building is brought into operation—material production, transport, and construction works—as well as refurbishment processes or the dismantling of systems at the end of their service life.

To do so, we distinguish four main phases in a building’s life: the product phase, in which materials are manufactured; the construction phase; the use phase; and the building’s end of life, which may involve demolition or transformation through refurbishment.

Each of these phases, shown in the following chart, corresponds to different associated carbon types.

Chart showing the building life-cycle information modules for assessing sustainability in accordance with UNE EN 15978. [3]

Types of carbon emissions throughout a building’s life cycle

In addition, it is necessary to distinguish between the different types of carbon emissions released throughout the phases of a building’s life cycle:

– Upfront embodied carbon (Upfront carbon): emissions caused during the production and construction phases, before the building’s service life begins.

Also referred to as energy content, grey energy, or hidden energy, it refers to the sum of emissions from construction materials throughout their life cycle, from extraction, manufacturing, and construction through to maintenance and disposal.

Unlike other emissions categories, these have already been released into the atmosphere before the building is occupied or the infrastructure begins operating.

Upfront embodied carbon corresponds to modules A1-3 of the production phase and A4-5 of the construction phase.

– Use-stage embodied carbon (Use stage embodied carbon): emissions related to the materials and processes required to maintain the building or infrastructure during use, as well as for refurbishments.

This occurs in parallel throughout the use phase alongside operational carbon, emitted due to heating and cooling needs, as well as energy consumption for all kinds of appliances.

It corresponds to the use, maintenance, repair, and replacement modules B1-4, and module B5 for refurbishment.

– End-of-life embodied carbon (End of life carbon): carbon emissions associated with the building’s end-of-life phase, i.e., occurring after its use.

In this phase, embedded or embodied carbon corresponds to the modules for deconstruction/demolition (C1), transport from site (C2), waste from processing stages (C3), and disposal (C4).

– Operational carbon (Operational carbon): emissions associated with the energy required to make the building operational as infrastructure, i.e., the energy consumed during the building’s use phase to maintain habitable indoor conditions, referring both to HVAC use (heating and cooling) and other uses (domestic hot water, appliances, cooking, and lighting).

Operational carbon corresponds to module B6. [4]

Embedded or embodied carbon

Embedded carbon (from the English: embodied carbon) or embodied carbon includes the upfront carbon emitted during the building’s construction, the use-stage carbon corresponding to its refurbishment, and the end-of-life carbon that includes dismantling.

Embodied carbon corresponds to greenhouse gas (GHG) emissions associated with material production, their transport, as well as construction processes throughout the entire building life cycle: production, construction or refurbishment, use, and end of life.

Embodied carbon is therefore generated across all phases of a building’s life cycle.

In addition, we can add another subtype of embodied carbon: beyond-the-life-cycle embodied carbon (Beyond the lifecycle). These are carbon emissions or savings incurred due to the reuse or recycling of materials, or due to processes of biogenic carbon reabsorption by the construction materials themselves, which are accounted for as avoided or negative emissions.

Life Cycle Assessment

Given this reality, the importance of assessing environmental impact and having reliable data to support decisions on material selection that limit carbon impact becomes clear.

To calculate impacts in the construction sector, the most effective method is life cycle assessment (LCA), a methodology that considers multiple criteria and the full environmental impact throughout the entire life of a product or service. [5]

Life cycle assessment (LCA) is a method for calculating the environmental impact of a product or project across all life stages.

An LCA of a building involves assessing all phases throughout its conception, construction, and dismantling—from raw material supply, manufacturing of construction products, and the construction process, to use, demolition, and material recycling.

Accounting for embodied carbon goes hand in hand with the development of a common taxonomy that describes and establishes the foundations of concepts or, where applicable, calculation methods such as LCA.

Once all phases prior to building use are taken into account—and the share they represent, which can exceed three quarters of the building’s total impact—understanding a building’s LCA helps ensure embodied carbon is no longer a hidden part of a building’s climate impact, but is considered, quantified, and begins to be regulated.

Below is the full breakdown of the phases of a building’s life cycle and an approximate percentage of carbon emissions associated with the different phases:

Environmental Product Declarations

To carry out a building life cycle assessment process, we need to be able to assign impact values to each of the materials it comprises. These data may be incorporated from a generic value (for example, generic emissions for a ceramic brick) or from a value that analyses the emissions of a single product developed by a manufacturer.

This information must be contained in so-called EPDs—Environmental Product Declarations—which provide the life cycle assessment and impacts of a specific material, carried out by each manufacturer through standardised normalisation.

This tool is used to assess environmental impact and resource use throughout the product life cycle in accordance with EN 15804 and its Product Category Rule (PCR). [9]

We should not forget that both LCA and EPDs are 2 of the 6 macro-objectives established by the Level(s) certification, a common European framework that is currently voluntary and aims to become mandatory to improve building sustainability.

With regard to EPDs for materials, Level(s) establishes the need to list their service lives, quantify their environmental impact, and the Construction and Demolition Waste (CDW) they generate, with the aim of ensuring material life cycles are circular and resource-efficient.

If it is regulated and made mandatory for all marketed materials to have an EPD, a debate arises regarding the implementation of this standard. If EPDs must be paid for by the manufacturing company—which currently seems the most likely scenario—it should be noted that producing and certifying an EPD is very expensive, as it can currently be around €3,000. [10]

For this reason, most countries that make it mandatory to carry out a life cycle assessment declaring emissions prior to building construction always offer the possibility of starting from standardised product data via EPDs or from generic data by material type.

It is necessary to bear in mind that an EPD must be prepared by an external agent to the marketing company to avoid fraud or misrepresentation of data. This entails a high investment for small companies and could even push them out of the equation, preventing smaller structures from competing with large corporations.

Given this issue, implementing a subsidy system should be considered so that small local manufacturers can lead this change, in a context where low-impact material producers are often start-ups or family-run businesses with limited resources.

In Spain, the standard currently regulating Environmental Product Declarations—EPDs (in Spanish, DAP)—is EN 15804 “Sustainability of construction works. Environmental product declarations. Core rules for the product category of construction products”. [11]

How to reduce embodied carbon?

Beyond technicist predictions, the keys to reducing embodied carbon stem from common sense and consistency with sustainable principles:

Regarding the relevance of material selection based on embodied carbon, it is necessary to add a note on materials to avoid. There are some very high embodied carbon materials that are widely used in the industry, including, among others, aluminium, steel, cement, and plastic-based materials.

There are associations for lower-carbon steel and concrete, such as Responsible Steel Standard Development or the Concrete Sustainability Council, which support the implementation of more sustainable infrastructure for producing these types of materials—whose nature inevitably requires extensive processing—but which will continue to be used, albeit less and less, during the transition towards materials with a lower carbon footprint.

By contrast, some materials have very low or negative embodied carbon values at the point of use: bio-based materials such as softwood, plywood, cross-laminated timber, and plant-fibre insulation. These remove carbon from the atmosphere as they grow and could therefore be used to sequester carbon in the building during its service life and beyond.

In addition, some of the benefits of regulating embodied carbon include preventing building demolitions by incentivising refurbishments, creating a recycled materials industry, raising awareness of healthy buildings by regulating the use of certain materials, driving research and innovation into new materials, and reducing construction costs.