Cross Laminated Timber.

While traditional balloon frame houses acquired a bad reputation for their flimsiness and vulnerability to fires, an engineered wood product called Cross-Laminated Timber (CLT) can perform as well as concrete or steel without taxing the environment.  

CLT is a panel composed of wooden boards glued together. Each layer is perpendicular to the next, which gives the panels structural rigidity in both directions. It's similar to plywood, but with thicker lamination. The boards vary from 1.5 cm to 5 cm thick and 6 cm to 24 cm wide and can be obtained from young, fast-growing trees in sustainably managed forests, where for every tree that is cut down a few are planted in its place.

CLT was already available in the early 1990s in Austria and Germany, but had a slow adoption rate in the beginning. It gained popularity more recently due to the green building movement, and the list of "world's tallest timber buildings" is always growing. The structural viability and cultural acceptability of timber skyscrapers has already been established and demonstrated by projects such as Mjøstårnet (18 floors, Norway), Hoho Wien (24 floors, Austria) and Brock Commons (18 floors, Canada).

Fast.

Assembling timber structures is a lot easier and faster than those made of concrete or steel. Construction time can be reduced by a third when using timber, and it takes fewer workers – mostly carpenters – and smaller lifting equipment. That lowers the building cost, while also reducing the inconvenience caused by the construction site.

For an extension project atop a five storey building in downtown Reykjavík, architect Michael B. Erichsen of Urban Arkitektar chose a combination of CLT and glulam: "We had to remove the old roof and basically build a new house on the 5th floor". By using a lightweight structure he had no need to worry about damaging the apartment below or reinforcing the original building's structure to hold the new load.

The client was so pleased with the outcome that he commissioned Michael's firm to design housings for the electrical substations that his company produces. The 7x14 m cubicles are assembled in CLT in Reykjavík and trucked to remote locations in the countryside. "It's a smart solution that saves the expense of bringing staff and building materials to remote locations across the country", says Michael.

Regulation.

One of the main obstacles for broader adoption of CLT is the building code, which often imposes limitations driven by misconceptions about its fire resistance. Large pieces of wood don't catch fire very easily, as anyone who has ever tried to start a bonfire will know.  

When wood burns, its surface chars and the carbonised layer helps protect and maintain the structural integrity of the core, which remains cool for a predictable amount of time. This is why large timber sections can often be left completely exposed, where non-combustible materials such as steel would require special fire protection to prevent them from melting and collapsing unpredictably.

While EU fire regulations don't apply to Iceland, the local code allows up to three-storey residential buildings in CLT with interior walls exposed. For staircases and hotel rooms the walls have to be fireproofed with a membrane such as Holzprof or covered with plasterboard.

Green home. 

Architects Kristin Brynja Gunnarsdóttir and Steffan Iwersen set out to build their own sustainable home outside Copenhagen. In addition to energy efficiency, they were particularly focused on creating a house that breathes, and which wouldn't require the typical plastic vapour barrier. CLT elements are air-tight but not vapor-proof, so they help regulate the ambient humidity by absorbing or releasing moisture as needed.

In CLT they found a long-lasting material that is healthy, simple to use and precise, but also easy to adjust. They were able to leave the interior timber surfaces exposed, treated with a pigment to avoid the yellow colouration that occurs in ageing wood. The house was erected in two days.

Cost.

According to Ragnar Jóhannsson at Idex – the distributors of Stora Enso wood products in Iceland – he promotes CLT because of speed, not cost. While the material itself can cost more than other alternatives, that expense is easily offset by savings in construction time, labour, machinery and the period during which the developer's investment is exposed to Iceland's high interest rates.

Because wood is five times lighter than concrete, it is easier to transport and the foundation work is simpler. The millimetrically accurate prefabrication of panels results in much less waste on the site, a safer and more pleasant environment for the workers, and a quieter construction process.

All of the machinery needed to produce CLT is already available in the country in companies such as Límtré Vírnet, which produces glulam in Flúðir – with imported wood.

According to Björn, "we could start producing CLT with Icelandic wood tomorrow". Skógræktin and Nýsköpunarmiðstöð Íslands are working on certification and standardisation of wood grown in Iceland. Once that's cleared, Icelandic wood will be legally ready for use in construction and for other durable products.

As mass timber gains popularity all over the world, the big producers like Stora Enso and KLH are working over capacity. A local CLT industry would not only reduce Iceland's dependence on imports, providing the country with a certain level of self-sufficiency, it is also the ultimate way to build sustainably – creating buildings made from trees that grow right in our backyard.  

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(part II)

Adhesives.

In spite of the unquestionable virtues of timber as a construction material, there's some controversy around the adhesive that holds together the boards of engineered wood products. Ideally, CLT and glulam (another type of laminated timber) elements would perform similarly to solid wood in every way. Engineered wood has the potential to last as long as the massive logs that have been standing since medieval times.

A paper published by the Division of Fire Safety Engineering at Lund University (Sweden) in 2018 states that "It’s not entirely clear how glued engineered timber elements actually perform in fire, and any deviation from the fire behavior of solid wood could mean serious complications." One of the main concerns is the risk of layers of wood peeling off due to fire exposure weakening the glue, a phenomenon called delamination. But the researchers also concluded, after a series of standard tests in a furnace in Stockholm, that delamination can be avoided by choosing the right adhesive.

The most commonly used adhesive is PUR (one-component polyurethane) which is formaldehyde free, but not necessarily sustainable. Polyurethane production involves high temperatures, fossil fuel based raw materials and chemical additives. And if burned, it produces toxic fumes that can cause respiratory diseases.

It's possible to find bio-adhesives obtained from plants and animal wastes or wax, which are 100% recyclable and leave zero carbon footprint. These products still haven't been adopted by the industry, as they tend to be weaker and more expensive than PUR. But researchers speculate that within a few years it will be viable to use employ bio adhesives in CLT.

The reusability and recyclability of timber mixed with glue is somewhat limited, since it's nearly impossible to separate the wood from the glue. Typically, the waste from CLT production is processed into wood pellets for energy generation. Depending on the concentration of adhesive, the incineration process requires sophisticated filters and high temperatures to ensure the destruction of any possible pollutants. A more sensible and ecological way to recycle CLT is to convert it into construction products such as wood fiber insulation, chipboards and wood bonded panels.

Glueless engineered timber.

At Made Expo Milan 2019, one of Europe's most significant construction fairs, an entire sector was dedicated to sustainable buildings, which was unsurprisingly dominated by wood products. In addition to the already established CLT and glulam manufacturers, a few companies from the Alpine region showcased their adhesive free timber products. If CLT advocates are successfully working towards a more sustainable and healthy built environment, these companies are taking a step further towards maximal sustainability.

Ligna Construct, a small South-Tyrolese company established in the Bolzano province in 1894, developed Bio Xlam. Their wood products are made of native spruce sourced in PEFC certified forests from within 100km of the production facility. The lumber boards, treated without any chemicals, are held together by brackets made of galvanized carbon steel, in a low energy consumption production line.

Because there's no glue layer, the walls breath more and are better at regulating moisture and avoiding mold formation. The panels are slightly thicker than their CLT equivalent, which gives them higher thermal capacity, thus requiring less insulation. The extra mass also results in better fire rating. Finally, freedom of movement of the individual boards due to the mechanical connection results in very good static behavior in case of earthquakes.

"From the region, for the region" is the slogan of  Massiv-Holz-Mauer, which has production facilities at various locations in Europe, embracing the "local" aspect as an important component of sustainability. Similarly to Bio Xlam, MHM walls are made of dried softwood without any chemical treatment. The design differs in that the boards are held together by fluted aluminum nails.

Because the boards are not tightly compressed together like in CLT, the structure is even more permeable to vapours and achieves insulation properties that surpass regular solid wood. The manufacturer also claims that the massive monolithic construction of MHM shields up to 95% of high-frequency radiation (mobile network, television, radio), which contributes to the physiological comfort perceived in a wooden house.

Nur Holz is the technology that went the longest way to achieve a 100% wooden structure. Developed by Rombach Holzbau, the boards are held tightly together by giant wooden screws, creating panels that are visually and structurally similar to CLT, but more environmentally whole. Wall panels vary in thickness from 12.5cm to 35cm, while at 17.9cm to 20.8cm ceiling and roof panels can span up to 6m and are commonly used as a finished surface.

Nur Holz elements are made with slowly grown spruce, fir and beech from sustainable forests in the Schwartzwald. The trees are harvested only in winter months, in the days before the new moon. This is when the sap is at its lowest, and the lack of sap prevents insects and fungus from infesting the wood and thus increases its durability. Once cut, the trees are left to dry for months in open air, which prevents cracking, warping and splitting of the wood. This technique has been used for thousands of years, and results in a completely organic, high-performance material.

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As the world becomes more environmentally conscious, and as climate change becomes more evident everywhere, sustainable construction methods are rapidly gaining popularity. A plethora of new materials labeled "eco", "green" and "sustainable" flood the market every year. But maybe it's the humble timber, this low-tech product that has been around for millennia, that holds the answer to a future-proof building culture.