Cost Model: Low Carbon Frames

The built environment is a major contributor to greenhouse gas emissions, and the construction industry is tasked with delivering new residential buildings while reducing carbon output to neutral levels, in order to fulfil the requirement of producing net zero carbon emissions by 2050.

Eliminating or negating embodied carbon will be the biggest obstacle, from both a cost and a viability perspective, to achieving net zero. Cutting operational carbon in residential buildings is generally well understood, but embodied carbon encompasses everything from the carbon content of the materials specified to the carbon output of the transport used to deliver materials to site, and the carbon intensity of eventually decommissioning the building.

Design considerations

For a net-zero residential project to be achieved, the concept of net-zero must be addressed from the very first meetings with developers and housebuilders, and be a fundamental principle of the project.

While reducing operational carbon can be done retrospectively to a degree – for example, by switching to renewable energy supplies – the embodied carbon of a project must be mitigated prior to construction and on into the planning stage. At this stage, trade-offs often have to be made between design intent, materials selection, and net-zero ambitions.

Looking ahead, residential design will also probably evolve to better incorporate passive environmental design principles.

Construction issues

A growing emphasis on passive carbon reduction principles could bring a return to more traditional construction techniques. Such an approach might include hiring local workers on projects, thereby reducing the travelling distances and carbon output of on‑site staff.

It could also mean sourcing local masonry and timber, and building homes whose construction inherently allows good ventilation through the use of naturally breathable and fire-retardant materials like wool and minerals as insulation – passive carbon-cutting concepts that would have been familiar to housebuilders centuries ago.

This thinking is in contrast to the use of more modern materials and techniques which have sealed up homes in a bid for energy efficiency and, post-Grenfell, raised safety issues. This means there could also be tension between delivering energy-efficient homes and delivering net-zero ones.

If Passivhaus-style construction principles are applied, contractors must pay special attention to detailing, ensuring good seals around service penetrations and junctions. This requires specific training and extra time on-site to ensure quality.

A question of materials

The structure holds most embodied carbon. For a medium-sized residential building, LETI says 46% of embodied carbon is in the superstructure, 21% in the substructure, 16% in internal finishes, 13% in the facade, and just 4% in mechanical, electrical, and plumbing engineering.

The primary focus must therefore be on reducing emissions from structural components. This is typically concrete, and more specifically cement, and steel, in the form of reinforcement or structural framing.

Cement replacements in concrete, such as Regen ground granulated blast furnace slag (GGBS) and pulverised fuel ash (PFA) – by-products of steel manufacturing and coal-fired power plants, respectively – are one way to cut embodied carbon from concrete.

GGBS reduces the carbon impact by about 20% for every 25% of cement that it replaces. However, as the steel production industry is on a carbon-reducing journey of its own, the by‑products needed to make GGBS may become less available as that industry becomes cleaner. There are also new cement-free concrete products being developed, such as Cemfree and Earth Friendly Concrete, but as these are not yet in the design codes they are not widely used at present.

For reinforcing bars, a similar saving can be made by choosing lower carbon reinforcement steel, produced with a high recycled content via electric arc furnace.

Future approaches – and what to do today

The industry can no longer ignore the issue of embodied carbon, nor can we work in silos when it comes to reducing it. Carbon must be considered right from the outset as a core part of a building’s design, purpose and value. Decisions must be made on a project-by-project basis, incorporating as many variables and considerations as possible and making the most of the technology and data available to us at present.

The availability, quality and interoperability of embodied carbon data will make a huge difference to the pace at which we progress in reducing embodied carbon. We are only just starting to have widespread access to reliable, comprehensive carbon data across all elements of a building, which will allow developers to make smart, informed choices.

Right now, the most effective solution to reduce embodied carbon is to deploy the best of emerging data and materials technology – while also staying mindful of time-honoured techniques for being frugal and thoughtful with materials use, designing with simplicity and functionality at the fore.

Beyond building life

Carbon studies compliant with BS EN 15978 can include an assessment of module D impacts. Module D sits outside a standard lifecycle assessment (modules A to C) and enables supplementary information beyond the building lifecycle, such as the potential for reuse, sequestration and recycling, to be incorporated in studies. This divides opinion.

For some, the most important step in transitioning our building designs to limiting embodied carbon is to take action to mitigate the immediate extraction of raw materials and address our carbon emissions now, before it is too late. Technology will advance – demolition and reprocessing of materials could very well be net zero in 28 years’ time – and it is impossible to guess how materials will be handled at end of life.

For others, standard lifecycle assessments oversimplify these complex factors, which can materially affect study outcomes and choices made. Structural steelwork with a high recycled content and manufactured in electric arc furnaces can still yield higher carbon footprints than reinforced concrete when protection and fireproofing is taken into account and comparison is made on an assessment of modules A to C. That steelwork can be relatively easily repurposed without reprocessing at end of building life is not often taken into account.

Timber sequestration is another hot topic. Trees absorb carbon from the atmosphere as they grow in a process called carbon sequestration. Once absorbed, carbon is locked away in the timber until the end of its lifespan. Allowing for sequestration can significantly reduce the embodied impact of timber.

Greater guidance needs to be developed with respect to reasonable assumptions if we include module D and end-of-life scenarios, and we should probably be reporting an upper and lower embodied carbon range on projects. There needs to be more emphasis on the way a building has been detailed and the design intent for the frame, as well as on the ability for the specified material and engineering to be disassembled and repurposed on another project at end of life.


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iCRBN is a new and unique city-scale carbon accounting analysis tool that supports decision-makers for cities, communities, and large real estate portfolios.

It accelerates decarbonization strategies through multidisciplinary design thinking that evaluates carbon emissions through a defined life cycle.

iCRBN is one of KEO’s key solutions for reducing the impact of climate change.

Source: Building UK