We live in the year in which climate change has finally hit the mind. Heat waves, "Fridays for Future", debates about "flying ash" and the German lignite - the excitement is great. But not only flying and coal energy should cause a headache. There is another problem that has not been talked about so far: the construction boom. It devours gigantic amounts of sand, which is won in the depletion of riverbeds and from the seabed. At least ten billion tons are consumed annually for cement production, most of them in China. Cement production itself is currently responsible for five, maybe even eight percent of global CO₂ emissions. Half of the cement emissions come from the chemical transformation of limestone into burnt lime, an important ingredient for cement clinker and thus for concrete. The construction boom is not limited to China, however. Also in Germany, for example, because of the lack of apartments so much is built as long ago. So much the better that architects around the world have for some time been rediscovering an environmentally friendly building material that has served mankind well for millennia: wood.

Which buildings can you build with wood?

The comeback of timber construction has nothing to do with half-timbered houses or alpine chalets. What the architects think up are smart, sometimes daring constructions that also shoot up. The Mjøstårnet (Mjøsa Tower) rises 18 stories above the Norwegian lake Mjøsa. At 83 meters, it is a real high-rise. The 24-storey HoHo Wien, which will be completed in the seaside town of Aspern near Vienna, lays one more meter higher and will be the tallest wooden tower in the world - with the core of the building made of reinforced concrete. More wooden skyscrapers are already under construction or at least in planning worldwide. Particularly ambitious is the Japanese Sumitomo Forestry Company, which wants to build the W350 in Tokyo - a 350-meter-high tower, which is to consist of 90 percent wood.

While these spectacular structures indicate what might be possible in the future, the new timber construction also offers tangible solutions. For example, for the much-talked-about densification of big cities where new homes are urgently needed. "Because wood as a building material is so light, you can put on one or two floors of existing buildings in timber construction without having to reinforce the foundation," says Andreas Krause of the University of Hamburg. By comparison, one cubic meter of spruce wood weighs about one fifth of a cubic meter of reinforced concrete, ie concrete reinforced with reinforcing iron.

The compressive strength of wood is equal to that of reinforced concrete. In spruce wood, for example, it is 24 Newton per square millimeter, which corresponds to a load of almost three tons on the surface of a stamp. And Baubuche even manages 60 Newton per square millimeter - as much as high-performance concrete. "Compressive strength is crucial to stacking the projectiles," says Norbert Rüther from the Fraunhofer Institute for Wood Research WKI in Braunschweig.

The Curtain Place in London's trendy suburb of Shoreditch under construction © Will Pryce

The fact that reinforced concrete has become the building material of choice since the early 20th century has good reasons. It can be cast in forms that were previously unthinkable with wood, to curved walls and facades about. It can also be used to prefabricate building modules, which later only have to be assembled together on the construction site and which made possible the prefabricated construction of the previous century. But above all, it seemed to be the answer to the fires that, over the centuries, have been eating their way through cities all over the world, destroying pure wooden houses as well as walled wooden beams. Reinforced concrete itself does not burn. For example, the fire in the aftermath of the 1906 earthquake in San Francisco led to a massive US construction boom on reinforced concrete structures, with other countries soon following suit.

But at last the building material has rapidly caught up with wood, and the constructions of timber construction have also become more sophisticated. "Building with wood is building the future," says Andreas Krause confidently.

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What can you get out of the building material wood?

Wood has two major weaknesses: unfortunately it burns very well, and it is not water resistant. When wood dries moisture, it swells up, which weakens its stability, or fungi nest to decompose it.

The risk of fire can be reduced on the one hand, by using very thick beams as supporting structures. Because wood behaves in a fire "good-natured", as Norbert Rüther formulated: It burns slowly from outside to inside. As a rule of thumb, 0.7 millimeters of wood burn per minute. If you want to be sure that a beam retains its load-bearing capacity even after half an hour's firing, then you have to lay it on both sides by 2.1 centimeters thicker than would be necessary for the construction. Steel, however, loses its strength as soon as it melts at a single point, as the collapse of the World Trade Center in 2001 showed. In the new, very large wooden houses massive support beams are installed, which have a cross-section of one square meter.

Researchers at ETH Zurich and the Swiss Materials Testing Institute Empa have developed a radical approach to making wood fireproof: they artificially petrify it. For this purpose, the wood is impregnated with a solution of calcium chloride and carbonic acid dimethyl ester. If caustic soda is added, a chemical reaction occurs, which deposits lime in the cell structure of the wood. The result: The wood fibers burn as good as no longer. Unlike chemical fire retardants, which are used to prepare wood, the lime-treated wood is easily recyclable. The incorporation of silicon dioxide in the wood has a similar effect. "That works very well in the lab," says Gustav Nyström, head of Empa's Cellulose and Wood Research Laboratory, "but it's not yet ripe for industrial applications - the costs are still too high."

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With interventions in the cell structure, wood can also be made water-resistant. Researchers at the University of Hamburg, for example, introduce gaseous polyethylene - a harmless plastic - into the interstices of the wood cells in pressure chambers, the so-called lumen. There, polymer compounds are formed that keep water out of the wood while providing protection against fungal attack. A similar effect exists when preparing wood with acetylene. Then groups of molecules in the wood that attract water transform into water-repellent ones. Wood that has been prepared for this purpose has already left the research laboratories: in Sneek, the Netherlands, a wooden bridge has been constructed from corresponding beams without further protective coatings, carrying up to 60 tonnes of heavy goods traffic.

The ceilings and walls of the Brock Commons Tallwood House in Vancouver, Canada, are made of plywood boards. © Michael Elkan / Acton Ostry Architects & University of British Columbia

In the construction of the timber has also progressed. Similar to the concrete walls of panel construction, wall panels are prefabricated today with wooden frames in which windows, insulating material and shafts for building services are already installed. These wooden panels can then be assembled together on the construction site.

Some architects also work in the so-called solid construction method. Here, the modules consist of solid cross laminated board panels. In this wood layers are glued together so that their fiber direction is alternately aligned at right angles to each other. The advantage: the wood can not warp. Because the cross-glued fibers fix the board in the desired shape.

For the architect Andrew Waugh, cross laminated timber is "the massive building material of the future". He designed the ten-story Dalston Lane office building in London's East, built in 2017 almost entirely out of such solid wood panels and wooden girders. "Timber construction has to be simple, stackable, so you can build higher," says HoHo Vienna architect Richard Woschitz. He and other experts assume that at 84 meters, the potential of wooden high-rise buildings is far from exhausted.

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How Expensive is Building with Wood?

As a building material, reinforced concrete is for the time being much cheaper than spruce wood, which is the most important building material for timber construction in Europe. Nevertheless, the new wooden buildings are hardly more expensive than conventional buildings in terms of overall costs. The reason: Timber construction is much faster. If reinforced concrete is used, it is necessary to estimate four weeks per filling until the concrete is dry and hardened. Although this can be speeded up by drying the heaps with great effort and, in a sense, it makes you feel like a wet T-shirt. But this procedure devours money and above all energy. Under one year of construction conventional reinforced concrete structures are hard to realize, says Andreas Krause from the University of Hamburg. "For wooden buildings, however, you only need one week per floor." And while there are always delays on conventional construction sites, for example because the weather does not play along, architects and construction companies can plan wood construction very precisely - and even more so if walls and ceilings are delivered as prefabricated modules.

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What does timber construction contribute to climate protection?

Unlike reinforced concrete, wood is not just a renewable resource. It also contains CO₂ from the atmosphere, from which a tree gains the carbon for molecules such as cellulose or lignin when it grows. As a rule of thumb for the climate protection effect applies: Every ton of wood, which is taken from the forest, saves compared to mineral building materials on average two tons of CO₂ emissions.

The Cube in London, a ten-story, 33-meter-high residential building, was also constructed of cross-laminated timber. © Jack Hobhouse

It is true that energy is needed for felling the tree and processing it into boards and beams, as well as for treating it with other materials that make the wood firmer and more water-resistant. Again, CO₂ emissions are incurred. All in all, however, every timber construction is still a contribution to climate protection. In HoHo Vienna, for example, 4350 tons of wood are used - this corresponds to about 2,700 mature spruces - which saves as many tonnes of carbon dioxide in the end. Incidentally, it only takes an hour and twelve and a half minutes for the wood pulp consumed at HoHo Wien to grow into the large Austrian forest population. For Austria, one cubic meter of wood grows in one second.

So far, timber construction in Germany accounts for only 18 percent of the total construction sector. What one would gain in the greenhouse gas balance, shows a study of the Ruhr University Bochum. Assuming that 55 percent of detached houses and 15 percent of all multi-family dwellings have been built in timber since 2016: By 2030, at least 23.9 million tons of CO₂ would be saved. This corresponds to about 80 percent of the emissions that German domestic air traffic would cause in the same period. The Bochum group around Annette Hafner comes to the sober demand: "Overall, a massive increase in the timber construction rate is now necessary."

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Is there enough wood at all?

Now one might ask: Would not the next depletion of nature be initiated by a massive shift to timber construction? Researchers at the University of Cambridge have taken the big bill in the example of a three-room apartment in wooden module construction in Växjo, Sweden. This apartment contains as much wood as is taken in a year on an area of ​​up to ten hectares of commercial forest. If all Europeans moved into such an apartment, 30 percent of the European forest area would be enough to harvest the amount of lumber needed for that purpose.

Many environmentally-motivated people, on the other hand, prefer to leave the forests to themselves, because they care about climate and nature conservation. Forest researchers disagree. A self-relinquished forest in temperate latitudes such as Central and Northern Europe would be outdated after three decades, says Johannes Welling from the Thünen Institute. He then binds less CO₂ from the atmosphere than a young forest. "In Europe, increasing use of wood for construction over the next 30 years is unlikely to cause the forests to become over-exploited," Welling said. Klaus Hennenberg from the Öko-Institut also believes that more wood construction makes sense. "All in all, this does not have to lead to greater logging." Currently, about 25 percent of the trunk wood in the forest is used for energy production in this country. But that hardly saves greenhouse gases, if one counts in the balance sheet the carbon storage in the forest. "Here, a combination of less firewood, more lumber and more extensive forest management can bring a good interaction between climate and nature conservation," recommends Hennenberg.

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