CLT - wood panels for passive house construction

If you are interested in building wooden houses and look outside for information, you will see that the most common material is CLT (Cross Laminated Timber) - considered by many to be the material of the future. A material almost unknown to us, it has been used for some time in European countries for the construction of 7 or 10 storey houses. Used in the construction of passive cell, makes an essential contribution to reducing energy costs. We will continue to talk about this material, the advantages and disadvantages of using it, tall buildings made entirely of wood and how you can build with it in Romania.

What is CLT

It is a building material made by gluing several layers of wood perpendicular to each other. The resulting panel is similar to a tacklewith the difference that instead of veneer sheets there are timber panels. As with plywood, the number of layers is odd. Depending on the desired final properties there can be 3,5, 7 or 9 layers of wood.

The manufacture of these panels is similar to that of laminated beams (glulam) with the difference that in glulam the layers of wood have parallel grain. It is, however, the same way of extending wooden elements by finger-joining and very moisture-resistant adhesives are used.

The material first appeared in Switzerland in the 1970s, but industrial production began in Austria in the 1990s. It wasn't very successful at first because of the prejudices associated with building with wood: construction with low resistance over time, to damp, fire or insect attack. Growing concern for environmental protection and the trend to phase out carbon-intensive technologies have brought the material back to the attention of builders, who are gradually discovering its potential and advantages in terms of quality, strength, durability and cost-effectiveness.

If you are thinking of a more serious documentation, you should know that you can also find it under other names: KLH (a name found mainly in Austria and Germany), cross-lam, X-lam or jumpo plywood.

How to get CLT panels

Manufacturing CLT and building with it is different from conventional building materials. The material is not manufactured and sent to building material depots where we take it to build the house. The process starts with the design of the house and all the necessary calculations of strength, partitioning, sizing, etc. As with prefabricated frame houses, first there must be the design.

Once the project is completed it goes to the factory. This is where the construction of each individual wall starts according to the project. Everything is computerised, the dimensions are transmitted to the robots that lay the pieces of timber and glue them together. After the glue is applied, the layers of wood are transported by crane and laid one on top of the other perpendicularly. The boards are then squeezed to eliminate gaps and put into the press that cures the adhesive using high-frequency currents (CIF).

After gluing, the panels are sanded and shaped, then all the necessary work is done: the window and door gaps are cut, spaces or paths for various installations are milled, etc. The walls of the house become huge Lego pieces that are packed and transported to be assembled on the foundation built in the meantime.

Why build with CLT. The main advantages of using the material

The big advantage is thatthe material is much lighter compared to a similar brick or concrete wall. The construction itself will be much easier, which means the foundation doesn't have to be built like a concrete or brick house, reducing costs.

Very high strength and stability. The perpendicular laying of the layers and the elimination of wood defects before gluing make the panel have these qualities. The possibility of obtaining walls with a different number of layers makes it possible to obtain thicker walls - load-bearing walls - or thinner walls - partitions. The very high load-bearing capacity of the load-bearing walls allows the elimination of the load-bearing pillars and the creation of wide open spaces, as well as greater design flexibility.

Reduced construction time is another advantage. As the walls are made entirely in the factory, they are only assembled on site. This eliminates reaction time - concrete curing - or supply time. Andrew Waugh, the architect who built the world's tallest CLT building at the time - Dalton Works in east London - said construction time has been reduced sixfold and material deliveries have dropped from 800, if the building had been concrete, to 111.

Capacity of thermal insulation very good.It is well known that wood is a very good insulating material. In addition, it is naturally warm, keeping a very pleasant atmosphere inside. The use of such walls, determining and resolving the points where heat can be lost, results in a passive house whose energy costs are almost non-existent.

As with any material, there are weaknesses

As there is no perfect material, there are also some disadvantages to using these panels. One would be the thickness of the flooring. For the same strength the CLT floor will be thicker than concrete. In addition, the sound insulation is lower and needs to be improved with special materials.

Limiting the uninterrupted length of the panel is another disadvantage. The maximum dimensions at which panels can now be obtained are 16 m long and 4-5 m wide. To make taller constructions requires the matching of these elements to be very well done. The existence since 2012 of a 7-storey building in Norway and a 10-storey building in London (since 2015) shows that this is a drawback that can be overcome. In addition, the worldwide momentum of this type of construction shows that the reluctance to use this innovative building material has been overcome even by the most sceptical of builders.

Fire resistance

I have chosen to deal separately with the fire resistance of CLT panels because I am sure that up to now many have wondered continuously what happens if a fire breaks out. The material is highly resistant to burning and like glulam beams, retains its load-bearing capacity in the event of a fire. The fire advances only in the first layer of wood stopping in the adhesive layer. The layer of charcoal formed acts as protection and the wood does not lose its strength.

Unlike wood, iron (including that in reinforced concrete) loses its strength at over 700 ºC and no longer has the strength to support the construction, which collapses. So, not only is there no greater danger in CLT construction, but on the contrary, the construction is stronger than a concrete one.

Construction with CLT in Romania

I said at the beginning that the material is little known in Romania. However, there are a few such constructions. The one that has attracted the most attention to passive houses and the building materials for them isGeorge Buhnici, the former ProTV producer and IT enthusiast. He built his house using the new wave of architects and builders, young people who are equally passionate about modern products and technologies and environmental protection.