A return to traditional building materials
Construction still relies on concrete and steel – at a high cost to the climate. But interest is shifting back to natural and reusable materials.?
As it seeks out materials to build a low-emissions future, the construction industry is rediscovering tried-and-tested options such as wood, earth and straw. “Building materials made from renewable resources produce far fewer greenhouse gas emissions,” says Guillaume Habert, Professor of Sustainable Construction at ETH Zurich. “As these resources grow, they absorb CO2 from the atmosphere – and when they are used in construction materials, that carbon remains locked away for the long term.”
Switzerland has plenty of such natural materials at its disposal. Take earth, for example: each year, around 50 million tonnes of soil are excavated during the construction of homes and other buildings, and most of that ends up in landfills. If the clay-rich soil were to be set aside and used as a building material, the loop would be closed at its source, saving resources, money and countless lorry journeys. Yet most builders simply have the excavated material carted away – and pay for the privilege. Choosing the right building materials benefits not only the environment, says Habert, but also the local economy. And it can even protect people’s health: studies show that natural building materials can reduce problems such as allergies and asthma.
Sustainable by design
New processing methods
The key is to find new ways of working with traditional materials, says Habert: “If we want to use earth, wood or straw in today’s economic environment while meeting expectations for quality, durability, cost-effectiveness, aesthetics and comfort, we can’t keep processing them the way we did 200 years ago.”
Part of the answer lies in production: prefabricated components and the use of robots can simplify manufacturing and speed up construction. Researchers are also looking at how to fine-tune the materials themselves – for example, by using additives to improve workability or increase water retention. Habert’s research includes investigations into different formulations of liquid earth. This is produced by blending earth, a natural building material, with chemical additives – all more or less natural. These make the earth fluid, so it can be poured like concrete into formwork, where it hardens. “The way we process the material may be new,” says Habert, “but the material itself is still the earth beneath our feet – with all of its advantages.” Earthen materials are abundant, low-?carbon, cause very low CO2 emissions and produce a healthy indoor climate; they also provide sound insulation and help control humidity. Other bio-based materials, such as hemp and straw, share similar qualities. Straw, for example, regulates humidity and provides effective thermal insulation. In principle, straw bales can even be used like bricks to build entire houses. In practice, though, buildings in which straw serves as the primary load-bearing material are still rare: to ensure stability, walls must be very thick, either increasing the building’s footprint or reducing usable floor area. The same drawback applies to earth, which is why Habert currently sees its main role in inter?ior use. Given its limited strength, using earth for entire buildings remains a major challenge.
Forgotten traditions
This is a challenge his colleague Roger Boltshauser – architect and ETH Professor of Architecture and Regenerative Mater?ials – is eager to embrace. For over 20 years, he has explored the properties of earth and has also used it in load-bearing structures. One of the world’s oldest building materials, earth served as a model for concrete when the latter emerged in the 19th century and, at least in Europe, went on to supplant it as the dominant material. “The first concrete formwork was copied from formwork for earthen constructions,” says Boltshauser.
One of his goals, Boltshauser explains, is to revive the forgotten tradition of earth as a building material and reintroduce it into architecture in new ways: “We’re seeking a new architectural language for how we use earth and ways of integrating it efficiently into construction processes.” Helping him in this task is Felix Hilgert, an entrepreneur in earthen construction and head of research at Boltshauser’s chair. With a market share of less than one percent, earth remains very much a niche product, says Hilgert: “Earthen materials don’t have the benefit of 150 years of research and development, so it’s still inefficient and expensive to build with them.”
The two researchers work with rammed earth, a form of earthen construction that has been used for centuries. A mixture of clay, sand, gravel and water is placed into formwork in successive layers, each compacted with a rammer, either by machine or by hand. This produces a dense, stable mass. After ramming, the formwork is removed and the earth is left to harden in the open air, developing load-bearing strength in the process. “We hope to use earth for larger structural elements such as walls and slabs,” says Boltshauser. “A pure earth mix is ten times more sustainable than concrete, but it has only a tenth of the load-bearing strength,” he notes. To give earth greater compressive strength – and therefore make it an attractive option for multi-storey residential buildings – the team is investigating the use of reinforcement techniques, as in concrete construction. At the same time, they are experimenting with additives, including trass lime, a mixture of lime and ground volcanic rock (trass). “This renders the earth more durable, but it also makes it harder to reuse the material,” says Boltshauser.
About
Roger Boltshauser is Professor of Architecture and Regenerative Materials in the Department of Architecture at ETH Zurich.
Ingo Burgert is Professor of Wood Materials Science in the Department of Civil, Environmental and Geomatic Engineering at ETH Zurich and co-head of the WoodTec research group at Empa.
Guillaume Habert is Professor of Sustainable Construction in the Department of Civil, Environmental and Geomatic Engineering at ETH Zurich.
Modern rammed-earth buildings are typically two or three storeys high. In theory, however, they could reach 40 metres, says Boltshauser. Taller buildings are possible even with unstabilised earthen elements, he adds, citing the kiln tower that he and his students designed and built for the Brickworks Museum in Cham. Constructed from prefabricated rammed-earth elements, the kiln tower – the world’s first prestressed earthen structure – is reinforced with steel cables. This made it possible to dispense with an intermediate floor inside and to make the load-bearing exterior walls thinner.
A further strategy is to pair earth with timber for load-bearing work. On its own, earth has very limited tensile capacity, but wood excels in this respect – hence the rise of prefabricated rammed-earth elements framed in timber. Such hybrid elements can be used to build more sustainable walls and facades. Like earth, wood is a sustainable material that is plentiful in Switzerland – and it is becoming increasingly popular as a building material. In newly built single-family and two-family houses, timber already accounted for 17 percent of structural elements in 2023, and that share is still rising. In cities pursuing densification, this lightweight material is a popular choice for adding extra storeys, where it accounts for an even larger share of the materials used. There are even a handful of timber high-rise projects. Nonetheless, wood remains underused as a building material, says Ingo Burgert, Professor of Wood Materials Science at ETH Zurich and co-head of the WoodTec research group at Empa. “Wood is a great material to build with: it offers high strength and stiffness and has a lower density than some other materials. And, of course, it’s renewable,” he says. Another key benefit of wood, he adds, is that it locks in carbon: “If we can manage to use more timber in construction while maintaining a sustainable forestry industry, we will store additional carbon in the building stock for decades to come. That will buy us valuable time in the fight against climate change until new technologies for capturing and storing carbon more effectively and efficiently become available.”
Burgert’s team is exploring ways to enhance wood’s properties and improve its durability. Wood is susceptible to water, sunlight and fire and is also impacted by fungi and insects – damage which also causes the locked-in carbon to be released. “If we want to significantly expand the use of wood in construction, we need to extend its lifetime or even break the process of degradation in the most environmentally friendly way possible, while also better protecting timber against fire and colour change caused by sunlight,” says Burgert. To that end, the team is testing natural, largely bio-based substances, ensuring that wood’s sustain?ability is not compromised by problematic chem?icals. Among the candidates are shellac – a resin secreted by lac insects – and tannins, which are plant-based molecules.
Forests of the future
In Switzerland, timber construction relies mainly on conifers, especially spruce. Poorly adapted to drought, spruce has been hit hard by climate change, prompting the sector to look for alternative species to feed into existing processing chains. This is no easy task: trees grow slowly, while the climate is changing fast. “What?ever replacements we choose, we have to remember that we won’t be harvesting them for decades,” says Burgert. In future, Switzerland’s forests are likely to contain far more broadleaf trees. Less suited to conventional sawing, these species will require additional processing technologies. Burgert’s team is exploring the production of wood elements from split wood, a method traditionally used to make shingles, which can increase the yield of material per tree.
In recent decades, timber construction has shifted toward prefabrication, with modules manu?factured in factories and then assembled on site. This streamlines timber construction and alleviates the problem of wood changing its dimensions due to fluctuations in ambient humidity. Prefabrication and automated production are essential for deploying sustainable building materials at scale. All three ETH professors see major opportunities for Switzerland in this field. “With advanced industrial automation, a broad-based research community, innovative start-ups and a construction sector eager to find alternatives, we’re seeing the development of new materials and processing techniques,” says Habert.
There are already examples of buildings made from sustainable materials that are cost-?efficient, Habert explains, yet these remain the exception rather than the rule. “We need more manufacturers supplying sustainable building materials. And we need to generate confidence in these buildings,” he adds. One initiative designed to help is the Atlas of Regenerative Materials, a recently launched web platform showcasing architects, construction companies and initiatives across Switzerland that work with bio-based materials. By highlighting projects built in this way, it aims to show that these construction methods can become standard practice – and ultimately that the buildings will continue to perform as intended years after construction. Mention a straw house, and some people still picture the three little pigs and the wolf, says Habert. “We need to change that image and show that a straw house could be right next door – and that it looks just like any other house. That’s the only way to shift perceptions.”
Materials of the future
ETH Zurich has created two new professorships to rethink how materials are produced, used and reused and to embed sustainability in materials, processes and products from the outset. Initial funding from donors will accelerate technological advances to enable the development of efficient and sustainable solutions for tackling climate change.
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