CLT - wood panels for passive house construction

If you are interested in building wooden houses If you look outside for information, you'll see that the most common material is CLT (Cross Laminated Timber) - considered by many to be the material of the future. Almost unknown to us, it has been used for some time in European countries to build 7 or 10-storey houses. Used to build passive cell, is an essential contribution to reducing energy costs. We'll be talking more about this material, the pros and cons of using it, tall buildings made entirely of wood and how you can build like this 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 tackle, with 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 may be 3,5, 7 or 9 layers of wood.

These panels are made in a similar way to laminated beams (glulam) except that in glulam the layers of wood have parallel grain. It is, however, the same way of lengthening wooden elements by finger-jointing, and very moisture-resistant adhesives are used.

The material first appeared in Switzerland in the 70s, but industrial manufacturing began in Austria in the 90s. 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 environmental concerns and the trend towards phasing 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're thinking of more serious documentation, you should know that you can also find it under other names: KLH (mostly found in Austria and Germany), cross-lam, X-lam or jumpo plywood.

How CLT panels are made

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

Once the project is finalized it goes to the factory. Here the construction of each individual wall according to the design begins. Everything is computerized and the dimensions are passed on to robots, which place the pieces of timber and glue them together. Once the glue is applied, the layers of wood are transported by cranes and laid on top of each other perpendicularly. The panel is then squeezed to eliminate voids and put into the press which hardens the adhesive using high-frequency currents (CIF).

After gluing, the panels are sanded, shaped, and then all the necessary work is done: the window and door openings are cut, spaces or paths are milled for various installations, 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 that the material is much lighter than a similar brick or concrete wall. The construction itself will be much easier, which means that the foundations don't have to be built like a concrete or brick house, cutting costs.

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

Reduced construction time is another advantage. As the walls are made entirely in the factory, only assembly is done on site. It eliminates reaction time - curing the concrete - or supply time. Andrew Waugh, the architect who built the tallest CLT building in the world at the time - Dalton Works in east London - said construction time had been cut six times and material deliveries had fallen from 800, if the building had been made of 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. Using such walls, determining and solving 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 and when using these panels there are some disadvantages. One would be the thickness of the flooring. For the same strength CLT flooring will be thicker than concrete. In addition, sound insulation is lower and has 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 achieved are 16 m long and 4-5 m wide. To make taller constructions it is necessary to match these elements very well. The existence of a 7-storey building in Norway since 2012 and a 10-storey building in London (since 2015) shows that this is a drawback that can be overcome. What's more, the worldwide take-up of this type of construction shows that even the most skeptical of builders have overcome their misgivings about this innovative building material.

Fire resistance

I have chosen to deal separately with the fire resistance of CLT panels because I am convinced that by now there have been many who have wondered continuously what happens if a fire breaks out. The material is very resistant to burning and, like glulam beams, retains its load-bearing capacity in the event of a fire. The fire only advances through the first layer of wood and stops in the adhesive layer. The formed charcoal layer 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, causing it to collapse. So, not only is there no greater danger with CLT construction, it is actually stronger than concrete.

CLT construction in Romania

I said at the beginning that in Romania the material is little known. However, there are a few such constructions. The one that has drawn most attention to passive houses and the building materials for them is George Buhnici, the former ProTV presenter with a passion for IT. He built his home using the new A wave of architects and builders, young people with a passion for modern products and technologies as well as environmental protection.