Thinking Differently about Embodied Carbon

Richard O'Hegarty

The greenest building is … one that is already built’ wrote Carl Elefante, ex-president of the American Institute of Architects, in 2007. A phrase which is today so frequently cited with equally dogmatic language that one might assume there exists a compelling evidence-base behind the phrase. The phrase was not, however, a scientific conclusion, it was a provocation. A welcome one.
Carl’s nine-worder provokes us to consider something greener than the shiny low-energy certification hanging on our newly constructed walls, or the solar array covering our recently laid roof, something ‘that is already built’. He was referring to the significant, and at the time of writing, largely unquantified embodied carbon emissions in the materials of our buildings. Suggesting that if we save a building which already exists, we won’t need to build a new one, and therefore we will be emitting less carbon.
At a very high level, saving buildings by repurposing old ones seems like a no-brainer, but in order to ensure this message reaches those who make decisions, we need a greater understanding of what embodied carbon is and greater evidence of its significance.

Understanding embodied carbon
Understanding starts with language, and the abstract nature of the term ‘embodied carbon’ is worth thinking about for a moment. When we detach ourselves from the built environment lexicon and interrogate the term in its literal sense, it makes us think of a building as something which stores carbon, almost as if the carbon is there, physically, in the materials.
That, however, is not what embodied carbon is referring to. It is referring to the carbon emitted during the process of making a building, all the way from raw material extraction, right through to demolition. The materials do not physically embody carbon dioxide, they embody the responsibility of the carbon emitted further up the supply chain.
The physical materials are therefore accountable for those emissions. Steel, concrete, glass, aluminium, timber and brick account for almost all emissions embodied in an average Irish building and all require some form of energy-intensive processing to convert their raw ingredients into useful building materials. Concrete and timber are slightly different to the other materials in that they do physically ‘store’ carbon, albeit in fundamentally different ways.
Concrete, for example, sequesters carbon dioxide from the atmosphere over time through a chemical process known as carbonation. Timber, on the other hand, does not sequester carbon dioxide during its product lifetime, it instead ‘stores’ carbon dioxide which was previously sequestered when growing in the forest in an entirely separate chemical process called photosynthesis.
So while the term embodied carbon does not refer to physically stored carbon dioxide, some of our buildings possess the ability to both sequester and store carbon.
That stored carbon dioxide is, however, only a fraction of the total carbon emitted during the processing of those materials.


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