The other side of the coin: Understanding embodied carbon

It’s well understood that built environment operations produce carbon emissions related to heating, cooling and other services. But buildings are also responsible for a hidden share of carbon emissions during their lifecycles. These hidden emissions have come to be known as embodied carbon.

Before a single light or air conditioning unit is switched on in a new building, the asset’s carbon footprint will already be sizeable. The extraction of materials used in the construction phase, along with associated manufacturing and transportation processes all generate carbon emissions. The same is true during demolition and disposal. Clearly, embodied emissions are not generated directly by the asset in-use, but they would not occur if the asset were not built. Only by adding these embodied emissions to operational emissions can we accurately estimate the building’s environmental impact.

Across the world, building regulations have been adapted to include climate provisions. But efforts have focused almost exclusively on limiting operational emissions. This is at least partly because operational emissions are believed to account for a larger share of total emissions over the building lifecycle. In reality, however, ratios between embodied and operational emissions can vary significantly. A building with low energy demand can still have a large carbon footprint if it was built using carbon-intensive materials, such as glass and steel. And if we look at infrastructure, we can see clearly that that embodied carbon counts often heavily outweigh the operational equivalents.

An inefficient building can be renovated and improved to reduce future in-use emissions, but the clock cannot be turned back on embodied carbon. And while operational emissions are spread over the whole building lifecycle, most embodied emissions occur in high volumes over short spaces of time. Improved practices around embodied carbon management can spare a lot of damage upfront. This, of course, does not mean that operational emissions should be neglected. What is required is a whole life approach to decarbonisation.

Unsurprisingly, this has proved to be challenging. Firstly, embodied carbon is difficult to measure. Operational carbon is calculated either through a mathematical modelling (which can be a basic spreadsheet or a sophisticated simulation), or by closely monitoring energy consumption. Embodied carbon meanwhile is estimated as part of a holistic lifecycle assessment (LCA). It’s a methodology that aims to account for every possible source of carbon associated with the lifecycle of an asset, product or service. This is a complex enough undertaking even when looking at small products. To perform a thorough LCA on a plastic bottle, for instance, the assessor must have access to significant amounts of data. And even with that information, it will be necessary to make a number of “best-guess” assumptions, such as future disposal scenarios.

At the building scale, an LCA is usually conducted by summing the results of individual LCAs for every product and service feeding the building. Underlying assumptions and methodologies across these individual assessments need to be compatible. In fact, LCAs can be performed using widely different methodologies, although there is general convergence towards the process-based method set out in ISO 14040. This method forms the basis of the European set of standards developed by the CEN Technical Committee 350 for the Sustainability of Construction Works. It’s also used to produce Environmental Product Declarations which quantify environmental impacts, including average carbon emissions, associated with “functional units” of a product. A functional unit could be a kilogram of steel or a ton of sand. Although use and accessibility of LCA data has greatly improved in the last decade, significant issues remain around reliability and comparability of results.

The second challenge is how to actually reduce embodied emissions in buildings. This is best approached at design stage, when there is maximum scope for intervention. Following the principles of lean construction, designers can minimise the need for carbon-intensive materials in the first place. As a rule of thumb, heavy building materials (such as glass, steel, concrete) have high embodied carbon. Clever design can reduce the overall need for such materials, but often significant quantities will still be required. Thus designers, and successively procurement officers, can choose to use low-carbon versions of those materials. Carbon embodied in concrete can be reduced by using recycled aggregates and industry by-products. Designers can also strive to use carbon-neutral and even carbon-positive materials – i.e. materials that actually absorb carbon from the atmosphere. These are, by and large, bio-based materials such as timber or hemp fibre. Carbon-positive materials can greatly offset carbon emissions embodied in other components of the building, though they are not entirely unproblematic: issues around performance, availability and cost persist.

In recent years designers have started to pay more attention to embodied carbon in buildings. Nonetheless, many still believe that regulatory intervention is needed to shift from a voluntary choice to an established practice. The UK Architects Climate Action Network, for example, is calling on regulators to add a dedicated embodied carbon section to national building regulations. But how can embodied carbon be regulated? The first step is to require the assessment of embodied emissions. Then, caps on emissions can be applied to individual building components, or to the building as a whole.

Embodied carbon assessments are slowly becoming part of planning, building and procurement requirements – though often at local or regional, rather than national level. The London Plan requires development proposals to include whole lifecycle emissions calculations and list actions that will be taken to reduce life-cycle carbon emissions. Several Austrian regions have established building subsidy policies which reward efforts to reduce embodied carbon.

Globally, public sector organisations are, perhaps unsurprisingly, leading the way. In Norway, the state building commissioner, property manager and developer Statsbygg has mandated 40% carbon reductions across new project lifecycles. The Buy Clean California Act, meanwhile, has been hailed as a ground-breaking piece of legislation. It introduces limits to whole life carbon emissions associated with products procured for public works. It is, though, currently applied to only four construction materials.

The first mandatory requirements on embodied carbon have been implemented in the Netherlands. Since 2018, Dutch building regulations have capped whole life emissions in new construction projects. Designers must assess and balance both operational and embodied emissions if they want a the greenlight for their proposals. Notably, the cap is measured by Euros-per-square-meter (designers are provided with a method to convert carbon emissions into currency value). On the one hand, this approach could be said to conceal carbon emissions. But it also translates them into a language that both designers, clients and the public at large can more easily understand.