Stability
ThermoWood® has improved stability when compared to normal kiln dried pine.
The internal stresses within the structure of timber are reduced when it is treated to the high temperatures of the ThermoWood® process.This reduces the potential for twist and warp. In addition there is a decrease in the equilibrium
moisture content and water permeability.There exists a linear correlation between water intake properties and dimensional stability of the material under changing moisture conditions. Swelling and shrinkage of ThermoWood® is only 50% of the corresponding values of untreated Nordic pine and is in a similar range to teak.
Table 1. Dimensional changes of ThermoWood® compared to untreated pine.
e.g. Dimensional change of 118mm wide ThermoWood® cladding when 50%RH 90%RH
ThermoWood® MC change 5% (5%-->10%)
Width change (%) 5% x 0.17 = 0.85%
Width change in mm 0.0085 x 118mm = 1mm
Durability
Finnforest ThermoWood® has improved durability due to the degradation of hemicelluloses (arabinose, galactose, xylose, mannose).These are the foodstuff for rot-causing fungi and bacteria and in the absence for nourishment they are not able to occupy ThermoWood®.
ThermoWood® has high resistance to most decay fungi.
ThermoWood® is resistant to Longhorn beetles, Anobium punctatum and Lyctus Bruneus.
NB - ThermoWood® is not resistant to the biological growth of algae on its surface.These organisms get their nourishment from the surrounding air and do not cause structural damage. However they are considered in many cases to be an aesthetic problem.Their growth can be prevented by using surface treatments.
Service life
Scandinavian Pine treated with the Thermo-D process can be considered suitable for usage in situations where hazard classes 1-3 (EN-335-2) apply.
General service situations and hazard classes given in EN 335-1 (Appendix 1):
Hazard class 1 Above ground, covered (dry);
Hazard class 2 Above ground, covered (risk of wetting);
Hazard class 3 Above ground, not covered
In normal conditions external cladding is exposed to hazard class 2 or 3
BRE has reviewed data supplied by VTT relating to thermally treated timber. On the basis of this evidence, BRE was able to conclude that data for a test material corresponding to ThermoWood® D pine gave a durability performance equivalent to that acceptable in the UK for cladding, providing a 30 year service life, when following manufacturer’s guidance.
Density
ThermoWood® density is 350 - 480 kg/m3 when its moisture content is 6% (typical for RH = 65%, t = 20ºC conditions). Natural variation will occur between individual boards. Material density is approximately 10% lower than the density of Nordic pine.
Nail & screw holding strength
Nail holding strength values for wire and improved nails for ThermoWood® do not differ from the values of European redwood. However, screw holding strength is about 20% less due to the altered state of the cell wall during the ThermoWood® process.
Emissions
ThermoWood® has a characteristic smell which is caused by the chemical changes that occur during heat treatment. The smell might not be appreciated by all, but several tests have shown that emissions are not harmful or dangerous.
TVOC (Total Volatile Organic Compounds) values are significantly lower than those for untreated softwoods due to the evaporation of most of the terpenes during heat treatment process.
Fire performance
According to EN 13501 (SBI-test) ThermoWood® is in reaction to fire class D. It is possible to treat further with fire protective treatments in order to achieve UK fire rating class 1 Spread of Flame.
Thermal properties
Thermal conductivity of ThermoWood® is reduced by 20-25% compared with normal softwoods. According to VTT tests the thermal conductivity λ10 of ThermoWood® (D, pine) is 0.099 W/(m K).The corresponding value for untreated sawn timber is 0.12 W/(m K).
Colour
The colour of ThermoWood® is affected by the treatment temperature and time.The higher the temperature the darker the appearance.As with all softwoods variances occur and are due to varying densities.When ThermoWood® is exposed to UV-light, it will lose its colour and turn silver grey unless protected by a pigmented surface protection.
Moisture content
ThermoWood® has moisture content of 5-7% (50%RH) when packed at production site.
This level will change according to atmospheric conditions.The Equilibrium moisture content is only half of the corresponding
value for untreated pine (Table 1).
Environment
Finnforest ThermoWood® is PEFC – certificated which ensures the raw material is sourced from sustainable managed forests.
