CLT elements are also called cross laminated timber, and are wooden elements consisting of several perpendicular layers of wooden elements. The many layers are glued together, which results in enormous strength, as it is very dimensionally stable and easy to handle and process on the construction site.
In fact, CLT is such a stable timber element that its properties are often compared to concrete, which is why the elements are widely used in both tall and large constructions around the world.
Cross laminated timber is also being used more frequently in Denmark, but, according to Martin Løvstrup Jensen from Pålsson Arkitekter A/S, there needs to be more focus on how we use the elements in the industry:
“In Denmark, we have a tradition of using wood as a building material. But we have been challenged when it comes to building physics, both due to stricter requirements for energy consumption, efficiency on the construction site, tight schedules, economics and climate changes that lead to an increase in rain.
Here, the understanding and respect for the former and latter are probably the most important parameters for the use of organic materials in general. Because in relation to concrete elements which are inorganic, long-term moisture exposure of organic materials will usually have a negative impact on the material.”
CLT elements are increasingly popular in the construction of both residential and commercial properties, as the stability and strength of the element makes it usable in the majority of building-parts, from construction of decks and walls to ceilings and roofs.
CLT elements are also used increasingly as a load-bearing structure in both single- and multi level buildings, as CLT is a strong element compared to its own weight. Working with CLT elements is also characterized by a high degree of prefabrication, which further contributes to an efficient construction process, as it makes it faster to carry out e.g. assembly work.
In addition to being an attractive material to work with, there are a number of benefits related to climate and sustainability, which also make the CLT elements an attractive building material. It is estimated that timber constructions will be able to reduce the climate impact of the construction industry by approximately 50 % compared to building materials such as steel and concrete, Martin Løvstrup Jensen explains:
“We need to consider organic materials such as CLT elements as more sustainable alternatives to e.g. concrete elements. The organic materials can either be used as a hybrid, where wood-based materials can partially fill the need, or as a direct alternative, depending on the shape and function of the building. However, it has to be assessed on the basis of an overall assessment of whether there is an advantage in relation to the construction technology, building physics, environment, economics and the processes of the construction firm.
An example could be traditional multi-level construction, where concrete elements usually make up a large part of the building. Here, with the advantage of e.g. the total CO2-account, a significantly larger proportion of wood is added, using CLT elements.”
Wood is an organic material and is therefore able to absorb and release water and moisture from its surroundings - including from the air around it. The varying uptake and release of moisture can result in a number of moisture-related issues such as deformation, mold growth, rot and cracking in the elements.
If you work on the basis of an in-depth understanding of materials and good building practice for organic materials, you can, however, avoid the issues related to organic materials:
“We need to work much more with organic materials such as CLT elements. Organic materials do have some challenges, especially if handled in the same way as one is used to handle inorganic materials.
When working with organic materials such as CLT, it is crucial to have an understanding of the material and how to use it based on the specific situation,” explains Martin Løvstrup Jensen.
Video: Example of CLT construction by CLT Denmark, timelapse, 2019.
If the CLT elements are produced, delivered, assembled and operated appropriately from factory to construction site and during maintenance after the building has been taken into use, moisture related issues are rare. Although CLT elements might seem like an ideal material, it is important to remember that the element is made out of wood, which is an organic material and therefore sensitive to moisture. This must be taken into account when using it in construction, as there is otherwise a risk of moisture damage and cracks in the CLT elements:
“Cracks in the timber is a problem, as it causes a breach in the building’s tightness plan. This is especially pronounced in the traditional use of CLT elements, where the elements and the joints in the climate shield form the density plane. Joints can become quite leaky, as the cracks are usually smaller, but many, and occur at the ends of the elements, where the wood absorbs and releases moisture significantly faster than the remaining part.
The cracks occur as a result of moisture and temperature related deformations, where the wood expands and contracts due to varying moisture conditions. Therefore, it is important for the developer to know the moisture history of the CLT elements e.g. through monitoring the moisture content from factory to construction site. If the elements are produced or exposed to unacceptable moisture conditions for a longer period, an increased risk of cracks in the CLT elements must be expected,” explains Martin Løvstrup Jensen.
In addition to cracks, the risk of mold growth and rot increases by an increase in moisture exposure. This does not mean that you should refrain from using sustainable materials. However, it does mean that it is important to have a proper moisture strategy in place when using organic materials such as wood in construction projects.
Martin Løvstrup Jensen highlights, among other things, moisture monitoring as an important part of the strategy when working with wooden elements such as CLT:
“The developer should set stricter requirements for documentation from both the construction firm and manufacturer of the elements, in the form of stricter supervision and self-regulation, which should be defined in the moisture strategy of the construction project.
This could e.g. be stricter supervision from consultants, that the control plan of the construction firm is expanded with specific control points, and that the element manufacturer adapts and presents its production control. This ensures proper moisture monitoring and documentation of the elements from factory, during transportation, to assembly and fitting, and all the way to delivery.”
Video: Example of CLT construction by Settle, Finland, 2017.
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