Material Benefits: lowering embodied carbon in the built environment

It is now commonly known that the building sector is a significant contributor to global carbon emissions. These emissions come in two categories: operational carbon – generated from the operations, heating and cooling of a building throughout its lifespan – and embodied carbon, which is produced during the manufacture and transportation of building materials, as well as the construction and demolition of buildings.

Until now, much of the construction sector’s efforts towards reducing carbon emissions have related to the operational aspects. Operational carbon constitutes an enormous portion of a building’s footprint, and much of this can now be easily mitigated, so that focus is paying off. However, as Head of Technical at the Green Building Council South Africa (GBCSA) Georgina Smit explains, now that buildings are performing at a much higher level operationally, we are seeing that, proportionally, embodied carbon is becoming a much larger issue. “The conversation about carbon has evolved,” she adds.

What’s the hype?

Embodied carbon refers to the carbon that is produced during the manufacture of building materials, their transportation to a building site, the construction of the building, and the eventual demolition of the building. This is often referred to as a ‘cradle to grave’ process. Tessa Brunette, Associate and Head of Sustainability in Africa at Arup, highlights the World Green Building Council’s target of all new buildings being net zero in operational carbon, and reducing embodied carbon by 40%, by 2030. “We, as designers, have agency and can have a significant impact here,” she says. “We are already working on buildings that will be completed in 2030.” So, the design decisions we make now will have an impact for decades to come. “What an exciting opportunity,” Brunette adds, “this is disruption!”

Embodied-carbon measurement

Several tools for measuring embodied carbon are being developed globally. Some, including Arup’s own, have started being used locally. But, as Brunette explains, the available data is still limited, and carbon factors are not available for the African context. However, momentum is building to develop the data and tools. Brunette emphasises that the focus must be on starting to assess carbon, not on which tools are available or how much context-specific information there is. Calculations can be as simple as sums on paper. She encourages designers to just begin calculating, then share knowledge and collaborate to develop a contextual data network, and further develop helpful tools.

Smit highlights that the GBCSA is supporting the market transformation in this area through the updated Green Star rating tool, which has more emphasis on embodied carbon in its ‘Positive’ category.

Low-carbon materials in sa

‘Mass timber’ is increasing in popularity as a low-carbon building material. Managing director of XLAM, South Africa’s first manufacturer of cross-laminated timber, Jamie Smily, explains that the term refers to a number of engineered timber materials that employ methods such as lamination to increase their strength. Being a natural material, timber requires very little processing, therefore producing a much lower volume of carbon than other traditional building materials.

“Timber has the added advantage of being able to capture carbon during the trees’ growth, and then locking it in when used as a building material,” Smily explains. “Locally produced materials are even better as the transportation carbon [factor] is lower than that of imported products.” The most commonly available mass timber products in South Africa are glue-laminated timber (GLT or Glulam), where the timber is laminated in the same direction and is used in structural elements such as beams and columns, and cross-laminated timber (CLT), where timber is laminated in layers at 90 degrees to one another, creating panels that can be used for walls, floors and roofs. Laminated veneer lumber (LVL), made up of shaved layers of timber, is also becoming popular globally.

It’s easy to adjust to designing with mass timber products, and products like CLT and GLT are SABS/SANS-certified, so there are no special requirements for using them, explains Smily. They are very good in fire situations, as they have predictable char rates, they offer great options for off-site prefabrication, and bank finance is available for building with them.

Boshoff Muller is the managing director of Cape Town-based Afrimat Hemp, which is pioneering and beneficiating the industrial hemp value chain in Africa. Muller explains that hemp is arguably one of the fastest carbon dioxide (CO2)-to-biomass tools available today. Hemp absorbs significant amounts of CO2 from the atmosphere and does it in only a three- to four-month growth cycle.

Lime, a more sustainable binder than cement, is added to form ‘hempcrete’. Although lime goes through a similar manufacturing process to cement, there are two fundamental differences from a sustainability point of view: lime is calcined at a lower temperature – at 950°C, versus cement at 1 400°C – and cement needs water to set and form crystals, while lime relies on carbon absorption for its setting method.

According to Muller, a reputable hempcrete company claims that for every cube of hempcrete that is used, 75kg of CO2 is removed from the atmosphere. Afrimat Hemp is completing its own lifecycle assessment of the entire value chain and hopes to achieve similar verified results. He cautions teams to verify that bio-based materials are tracked to ensure that there is no green-washing.

Using reclaimed or upcycled building materials, or materials with a high recycled content can also contribute to lower embodied carbon, although the processing and transport would still need to be factored in.

Touching the earth lightly

Zutari’s technical director of structural engineering Chris Greensmith explains that ultimately the goal of embodied carbon reduction is to build less, and the best way to do that would be to not build anything new at all. “We need to start by challenging the client’s brief and ask, ‘Is a new building really needed?’ Philosophically, this is a conundrum in our industry because if the answer were always ‘no’, then we are all out of employment. However, by asking this, if the answer is ‘yes’, we can move more easily towards assessing the client’s actual needs. We can look to renovating existing buildings or challenge the architectural and structural designs to produce less material-heavy configurations, before we address efficiency by tackling material choices,” he says. Changes are easier and less costly at design development stage than during construction.

Once it’s established that a new building (or a renovation) is required, and the intent and scope has been defined, the design team on any project needs to start engaging with specialists and suppliers early in order to start detailing correctly from the start. Christo van der Hoven, chief technical officer at the Sustainabuild Group – which encompasses a number of companies specialising in sustainable construction technologies – elaborates on the factors that influence embodied energy:
Material selection looks at prioritising materials that don’t require a lot of processing, such as sustainably grown timber or natural fibres over concrete, steel or aluminium. Resource depletion should also play a role here.

The distance the materials travel affects the embodied energy. The closer to site the source is, the better.
The manufacturing process should be considered. Materials like aluminium require high temperatures to manufacture, concrete uses a lot of water, and polymers require lots of chemicals. All of these factors increase the embodied carbon.

The design and construction of a building plays a role here too. Complex, excessively large, or unconventional designs may require more energy to construct. Considering modular grids can reduce this.

A building or material’s lifespan factors into its embodied carbon too. The longer it exists, the less its overall contribution to emissions. Ideally, a material should be able to be reused and reconfigured multiple times before being sustainably disposed of.

Greensmith explains that the largest proportion of embodied carbon is usually in the main structure of a building. Studies on structural elements have shown that small changes to the structure and the main envelope of a building can have an enormous impact and not necessarily add cost. Identifying the main carbon culprits and making suitable changes may mean replacing concrete slabs with CLT on steel beams, or perhaps replacing concrete or steel columns and beams with GLT. He believes that, eventually, “mass timber could essentially replace steel and concrete in building structures, but we need to start by integrating it into the mix of materials in order to build with confidence”.

Embodied carbon for the future?

Rebecca Dilnot, Advancing Net Zero programme officer at the World Green Building Council, reiterates that the main consideration is that setting net-zero embodied carbon targets is important as early in a project as possible. Echoing Greensmith’s views, she adds, “WorldGBC advocates a prevent-first approach to embodied carbon, in that opportunities to renovate and repurpose buildings are maximised before choosing to build new.”

Making even small changes now will have a significant impact over the lifespan of a building. As Brunette and others constantly maintain, we need to just start! Begin by calculating and collaborating to increase the available data, and start changing the way we design buildings, even if it’s a bit experimental at first.