Frøslev Heatwood / Thermowood

Frøslev Heatwood is heat-treated wood - the wood goes through a process during which it is heated to a high temperature while steam is added. The result is an incredibly dimensionally stable end product that is resistant to rot and fungi. The process is 100% free of chemicals, and Frøslev Heatwood therefore carries the Nordic Swan Ecolabel.

The method behind heat-treating wood is nothing new - quite the contrary. Already back in the Viking Age, wood was treated with fire and water to make it more resilient and durable. The treatment is not to be confused with heat treatment of e.g. wooden pallets for the food industry where the temperature does not get anywhere near the same degrees.

Frøslev Abodo

Frøslev ABODO is the new PREMIUM member of the Frøslev Heatwood family which was launched on the Danish market in 2019. The wood originates from New Zealand where it comes from sustainable plantation forestry.

Heat-treated at high temperature

As with all other Heatwood wood, Frøslev ABODO is modified at a high temperature with added water vapour in the processing. The wood gets an incredibly beautiful and warm, brown shade that will eventually patinate into a greyish shade. The wood can also be continuously surface-treated with a colour-pigmented wood preservative to preserve the beautiful shade.

Frøslev ABODO differs from the other Heatwood variants by being knotless ‘in principle’. This means that the profiles are generally knotless - but as wood is a living material created from nature, a few, minor knots cannot be excluded. The product has an exclusive look with a uniform surface.

Use Frøslev ABODO for horizontal or vertical façade cladding in construction or for terrace boards as an alternative to hardwood. Fitting terrace boards with concealed assemblies using Frøslev SUPRO Kliksystem (click system) is a great choice. 

At Frøslev, we process the profiles ourselves, and no possibility is left out - from our standard profile range as well as from the architect’s/building owner’s customised profile. 


Frøslev Heatwood - Pine & Spruce

Frøslev Heatwood pine and spruce is the classic choice of thermo-modified wood and already exists in many buildings today. Frøslev offers both variants for exterior cladding and for terrace boards.

Only the best raw material is used from trees from the northernmost, Scandinavian latitudes. The trees here are slow-growing which results in narrow, annual growth rings and excellent durability. The difference between pine and spruce is particularly evident in the number and size of knots. Pine wood has larger, scattered knots while spruce is characterised by smaller and darker pin knots.

Furthermore, Frøslev Heatwood pine and spruce also carry the Nordic Swan Ecolabel!

Learn more about the ecolabel >

Frøslev Heatwood -

Ash used as a thermo-modified wood is in increasing demand for modern construction. Of course, Frøslev is also in a position to deliver within this area. 

See the unique construction of the floating activity house in Nordhavn in Copenhagen. The building has an indoor sauna, outdoor façade cladding and a catwalk built by Frøslev Heatwood Ash.

Heatwood Ash is a rather exclusive choice, and ash is characterised by being knotless in principle. The result is a beautiful and calm façade.


Image: Graph of the process

The production process of Frøslev Heatwood is based on the use of high temperatures and water vapour preventing the wood from burning. The process removes sugary substances from the wood, and the cellular structure is altered. Rot and fungi can therefore not settle in the tree, and the finished product becomes very dimensionally stable. Frøslev Heatwood is very suitable for use in the Nordic climate, especially during humid seasons.

Frøslev Heatwood is a very sustainable product as no chemicals are used at any point during the process. Moreover, energy consumption during production is very low.

The process can be divided into three steps.

Phase 1 - Increase in temperature and kiln desiccation

The air temperature in the oven is rapidly increased by using heat and steam until it reaches a temperature of about 100 °C. The wood reaches the same temperature. The temperature is then gradually increased to 130 °C, and kiln drying takes place. Steam is used as a moisture barrier to prevent the wood from splitting. The vapour also causes cellular structure changes in the wood.

After this phase, the moisture content of the tree is reduced to almost zero.

Phase 2 - Intensive heat treatment

During the intensive heat treatment phase, the air and wood temperature is increased to between 185 and 225 °C. The highest final temperature depends on the requested use of the wood.

When the requested final temperature has been reached, the temperature is kept constant for 2-3 hours. Steam is used to prevent the wood from burning and splitting and continues to be part of altering the cellular structure of the wood.

Phase 3 - Cooling and adding moisture

During the final phase, the temperature is reduced by using a water spraying system. The moisture content of the wood is raised to just above 4%, so work can still be done on the wood.


Heat-treated wood is available in two versions: Class D for outdoor use where the wood is heated up to 212 °C and Class S for indoor use

Frøslev only sells Class D for outdoor cladding and terraces. The profiles are not processed till after the heat treatment.

Quality & patination

Images: Frøslev Heatwood as new and after 3 years of patination

Wood is a natural material, and the wood structure will never be the same in two boards when it comes to knots, colour etc.


The wood goes through a general quality assessment. The impact of one defect is evaluated by assessing the overall quality of the product. The board will be rejected if its overall appearance differs significantly from the general quality requirements. Several maximum defects in a product are allowed if the overall appearance remains acceptable. Boards are generally not sorted after 8 cm from the ends. However, cut products are an exception.


Heat treatment changes the natural wood colour, and a warmer shade is created in which the structure and annual growth rings of the wood are highlighted, giving it a unique appearance. At the same time, heat-treated wood has a very special smell. Over time, the wood will patinate and have a beautiful, greyish shade. To begin with, patination will differ depending on the position of the wood in relation to the sun, wind and the weather in general.


Thermowood/heat-treated wood can be left without further treatment after installation.

Frøslev Heatwood must be surface-treated before or right after installation in order to preserve the beautiful, brown shade. This is done with coloured or pigmented wood preservatives containing UV filters. The wood preservative is applied in a thin layer. 

Over time, small cracks and splinters may appear in the wood when exposed to the elements. This can be minimised with a surface treatment that also extends the life of the wood.

To obtain the beautiful, grey patination, the wood can be left untreated and will simply become more beautiful over time.

Terraces are always built on joists to prevent the terrace boards from touching the ground and to allow air circulation under the finished terrace.