The glue-ability

Gluing of ThermoWood can be done with most wood adhesive systems. However, the moisture repellent nature of the wood can lead to longer pressing time for water based adhesives and the slightly acidic material can influence adhesives dependent on specific pH-values for curing. 

The glue-ability of heat treated wood has been studied with 1 and 2 component PVAc-glues, 1 and 2 component polyurethane glues (PU), resorsinol-phenol glues (RF) and emulsion polymer isocyanate glues (EPI). The glue-ability test was done according to DIN 68603. The strength of the glue line was determined according to EN 392 (block shear test). The moisture durability was done according to the delamination test EN 302-2. The penetration of the glue to ThermoWood was studied with a microscope. The glue-ability is dependent both on the treatment time and temperature. The increase of time and temperature decreased the shear strength of the glue line. The reason for this is the brittleness of the material. It also explains the high wood failure percents (90 - 100 %). The breaking of the glue line happened from the wood not from the glue. The penetration of the EPI-glue to heat treated wood material was high which partly can have an effect to the strength values. EPI-glue is mildly alkaline and also the long several hour cold pressing time can help the glue's penetration. 

Experiences from one glue lam mill using heat treated pine as raw material were good. Both MUF- and RF-glues worked well. Normal production parameters (pressing time, pressure etc.) were used. Finger-joints were made with MUF-glue. 

From a gluing perspective the results are better with lower temperature treated wood. When working with PVAc glue the water content in glue should be minimised. Heat treatment process closes the wood cells, absorption of the glue and water into the wood becomes more difficult and takes longer. Some PVAc glues can cause problems in substantially prolonged drying times due to the requirement of the water to penetrate in the wood, e.g. the hardening of the glue is based on the water absorption into the wood. Chemical hardening glues allow normal drying times.

All tests carried out with PU-glues have been successful, but it has to be kept in mind, that the hardening reaction of PU needs water. The water can be absorbed either from the wood material or surrounding air.

To fasten

The following instructions are VERY important and apply especially for all ThermoWood treated Spruce products. 

The ThermoWood cladding or decking should be produced and then fixed so that the year ring positioning is such that the inner (pith face) of the board is orientated away from the weather exposed surface, this is to avoid the risk of year ring loosening.

All Spruce ThermoWood cladding is today produced with the year ring orientation as illustrated in the drawing above. Our standard decking is produced so that the non slip grooves are on the outer surface of the board which means this is the surface which MUST be used as the weather exposed surface. Failure to follow these instructions will almost certainly lead to some year ring loosening.

n practise it turned out that Pine ThermoWood can be planed and fixed with the best side up. Claddings should be fixed with stainless steel nails. Compressed air nail guns work very well and it is possible to fix up to the thickness of the cladding from the end ( a 19 mm board allows for fixing 19 mm from the edge) without the risk of splitting so long as impact is not made with the hammer to the cladding surface. The best shape of the nail is annual ring shank. The head of the nail should not pass much below the surface of the cladding but should not protrude either. For fixing 142 mm and wider claddings, 1 nail on each stud should be used top and bottom, 1/3rd from the bottom of the cladding board.
Deckings can be fastened with nails or screws – both must be stainless steel. The same fixing distance rules from the ends can be applied as with cladding. However, when screwing – pre-drilling at the ends is recommended. Two fixings per deck board per joist is recommended and the head of the screw should be counter sunk to the deck board. Gaps of at least 4-5 mm should be left between each board to allow the surface water to drain away.

Planing information

Some cupping of the boards will occur during the initial drying phase of the heat treatment process, although further movement after treatment is very limited. The initial cupping needs to be taken into consideration when planning ThermoWood in order to avoid checking. It is recommended to exchange single infeed rollers with two separate narrow wheels where the contact with the piece is at the out edges of the face when planning with the cup round up. Alternatively use a single narrow wheel and turn the piece so that cup round is down. With both methods this enables a flat surface to be formed as the piece proceeds through the planer, reducing the risk of surface cracking and enabling higher infeed roller pressure.


General working is the same as with normal treated softwood, no problems have been reported. There is very often no need for sanding, because after planing or milling ThermoWood has a good surface quality.
For Pine and Spruce ThermoWood parameters should be as for hardwood. Sanding is easy and the sanding paper is not becoming clogged up by resin. However from recent tests of sanding using rotating sanding belt machines – it was reported by the manufacturer that the sand paper became worn twice as quickly as when sanding normal pine.  The dust has small particle size, which is to be taken into account for dust removal. It is recommended that masks be used while sanding.

Surface treatment

Our knowledge is based on the personal experiences of our ThermoWood customers and on recommendations given by the manufacturers of finishes and coatings.  New substances and methods of surface treatment are developed constantly. Herewith we give you the most up-to-date information which is available at present. Heat-treated wood can be finished in the same manner as normal, untreated wood. When treating the surface of heat-modified wood, the following factors should be taken into consideration:

  • The absorption of finishes is generally slower, and there are more variations in permeability in different parts of the wood.
    In long-term use, there are clearly less cosmetic problems caused by resin, because a great percentage of resin has been removed during the heat-treatment process. As a consequence, knots might not necessarily require special treatment when finishing a surface
    The wood’s improved dimensional stability decreases peeling and cracking of the coating in changing conditions
    The cell tissue on the surface of heat-modified wood must be opened as usual before treatment in order to ensure and optimize fixing.

Finishing heat-modified wood is very similar to finishing normal kiln-dried wood. This means that any requirements that have been set for standard wood protection in a specific climate can usually be applied to heat-treated wood as well. All stages of heat-treatment are ecological, and the finished goods are safe, hygienic, and ecological, both in use and after. The manufacturers of ThermoWood products insist that the use of heat-modified wood and all its applications are harmless for people, animals, plants, and the whole environment.

In some weather conditions, the colour of heat-treated wood fades and turns gray. If the surface of the wood is not finished, it might crack in the same manner as the surface of untreated wood. Cracking can, for instance, be triggered by water penetrating the wood through the cross-cut ends. It may also be a result of the sun’s ultraviolet radiation. This tends to produce micro cracks on the surface of wood. UV radiation also causes the colour of wood to fade.

The fire resistance

TW has to be treated against fire and looked case by case. The retardant can be applied in the market by pressure impregnation or spray systems depending on the requirements. As with all wood species that have low moisture absorbency – more time is needed to get the required results, such is the case with Western Red Cedar as an external cladding.