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How buildings become carbon sinks

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Without carbon sinks, we will not achieve the climate goals. This was the clear message made in the World Climate Report written by international scientists. By using the right construction methods, buildings have the ability to store carbon in the long term and thus offer major potential. Experts discussed how this can be achieved at the DGNB Annual Congress.

The second digital annual congress of the DGNB took place on 14 and 15 February. The event featured a variety of open discussions, providing plenty of ideas and inspiration on different aspects of sustainable building. This series of blog posts offers a look back at the event with a summary of key insights.


How buildings can become carbon sinks: the topic of the digital panel discussion really went to the heart of the matter – at least that’s how moderator Martin Prösler sees it, not only due to the large number of people who tuned in to the session, but also because the same issue was recently discussed with colleagues on the DGNB Innovation Committee. There has been a great deal of talk among politicians and the general public about decarbonising our planet, i.e. cutting everyday carbon emissions by changing how we run the economy and act as a society.

This compares to a topic that gets less coverage in general discussion: the idea of actually removing carbon dioxide from the atmosphere. Industry still seems to be trying to get out of the starting blocks and this must change, says Dr Anna Braune, head of research and development at the DGNB and one of the three participants in the panel discussion. To achieve global climate goals, it will be necessary to start removing carbon from the atmosphere in parallel to decarbonisation. According to Braune, this means placing more value on soils and plants, seeing natural carbon sinks as our allies, and building capacities in technology sectors.

Two ways to remove carbon from the atmosphere

This is about two approaches to carbon removal. The term carbon capture and storage (CCS) is already known to some. CCS refers to the technological process of capturing carbon dioxide, reprocessing and condensing it, and locking it away – like placing it in long-term storage. The other approach is to exploit the potential of natural ecosystems to store carbon. For example, this can involve using natural reservoirs such as forests, soil, peatland and oceans. Preserving such resources is thus essential for achieving climate goals. Consequently, it’s a bad idea to cover up or seal land that is urgently needed as carbon sinks.

Seeing buildings as carbon sinks

Despite these challenges, Braune is convinced that the building industry has an active contribution to make to climate change mitigation. For instance, buildings can be transformed from major carbon emitters into active promoters of the energy transition. How? By ensuring buildings are used or operated in a carbon-neutral manner and that they actually become carbon sinks by cleansing the atmosphere. This is possible by ensuring building structures permanently store carbon. Also, spaces can be left uncovered to allow plants to grow. First and foremost, Braune points to building renovations, because in any case, new buildings should only be erected when absolutely necessary.

Whereas Braune approached the topic by introducing the technical aspects of carbon removal, the discussion that followed focused exclusively on natural sinks. The panel members clearly see major potential in this area from an immediate perspective. Also involved in the discussion were Dr Philipp Bouteiller, Managing Director of Artprojekt and Professor Harald Garrecht, Director of the Institute of Construction Materials at the University of Stuttgart.

Exploiting the diversity of bio-based materials

One of the first building materials people point to when it comes to carbon reduction is wood. Unsurprisingly, it has become hugely popular in recent years. It is also highly suitable for building purposes. According to Dr Philipp Bouteiller, for many years managing director of Tegel Projekt and responsible for a model district of urban timber construction in Germany, one tonne of wood sequesters between 600 and 800 kilograms of carbon dioxide. However, using wood makes it crucial to approach building from a very different angle, warns Braune. For example, it makes no sense to use more wood than necessary, just in order to leverage carbon reduction effects in an effort to lower the carbon footprint of a building.

Braune also wonders why wood is being hyped so much if it grows so slowly, pointing instead to mushrooms or cultivated mushroom mycelium, which is already on the market in the form of acoustic panels, or natural fibres such as seaweed, hemp or flax, which can be used for insulation purposes. “These are superb developments. Tender little plants that entered the scene years ago at universities, and now there are companies looking quite intensively at using these kinds of bio-based materials.” Materials produced with renewable raw materials are gradually becoming the norm, says Prösler. He also described a bridge that is being built using flax fibres and biopolymers. Materials now span the entire range of applications, from insulation to structural elements.

The panel members saw particular potential in the use of biomass originally destined to become waste products. “In Italy, for every ten tonnes of rice produced to make risotto, there are eleven tonnes of husks and stalks, which are often burnt in the field,” said Prösler. One start-up he knows converts these by-products into building materials. Expanding on this, Garrecht mentioned a former PhD student whose doctorate looked at rice straw concrete used in agricultural buildings.

Combining bio-based materials and minerals

“And what about fire protection?” – a question often asked by the audience. Garrecht sees major potential in combining organic and mineral material systems. There are a number of examples from the past that show how materials can be combined to use more biomass in the building industry in keeping with fire protection requirements. For example, research was already started in Baden-Württemberg twenty years ago to look at hemp and limestone composites. When the products were finally ready for market, hemp farming subsidies were discontinued and the supply of hemp eroded. The material is still highly successful in construction in France and the UK.

Another example provided by Garrecht was wood concrete. Instead of using gravel and sand in concrete, wood chips made from waste produced by the timber industry is used, before adding cement and water. One problem with this process, however, is that materials sometimes react with one another and compromise stability. It is also still difficult to produce large building elements, says Garrecht, although this is currently the subject of further research. One alternative he sees to carbon-intensive cement is to lower the clinker factor or use biopolymers. “I strongly believe that this offers plenty of potential to build over groundwater storeys.”

We all need to pull in the same direction

Ensuring materials made with hemp, straw or wood actually make their way into buildings is a key objective of project Baucampus, which is currently underway at Artprojekt. According to Bouteiller, sometimes the stumbling block is actual implementation on the building site, because builders and tradesmen lack training. “Knowing how things work is not enough; we also have to provide more training and ensure it’s applied on the building site,” he highlights, also appealing to the companies that provide training. Braune petitions companies to build broad-scale capacities: “We need to understand that without these sinks, and without products that stabilise carbon levels, there will be no stable climate for this planet.” The recipe for success – especially when it comes to the widespread use of carbon-reducing building materials – is nothing new, but this makes it all the more imperative that we all pull in the same direction.

Curious? You can watch the whole panel discussion “New role for the building sector: How buildings can become carbon sinks” here (in German).

Filed under: Impulse


Pia Hettinger is working at the DGNB as an editor in the PR, communications and marketing team. She focusses on discovering relevant topics and tailoring them for her audiences. After having worked at different agencies and corporations, as a qualified expert in media science, she joined an architectural office, where she managed the corporate communications division. Her increasing interest in sustainability led her to become part of the DGNB team here in Stuttgart.

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