Thermal Conductivity of Wood

In model making and engineering we use wood as a thermal barrier, hence it is good to understand the thermal conductivity of wood. A typical example is in the insulation of a boiler.

The thermal conductivity of wood is affected by:

  • density
  • moisture content
  • grain direction
  • structural irregularities
  • temperature

Density

Plotting the thermal conductivity across the grain versus the density of the wood shows a strong relationship.

thermal conductivity of wood vs density

As perhaps expected the best thermally isolating woods are cork and balsa.

Moisture Content

For fibreboards [ref 4] it has been shown that at 85% RH the thermal conductivity increases rapidly with increasing temperature. 

Grain Direction

The conductivity of wood along the grain is between 1.5 to 2.8 times the conductivity across the grain.

Temperature

For wood fibreboards thermal conductivity is linearly proportional to the temperature.

References

  1. Çavuş, V., Şahin, S., Esteves, B., and Ayata, U. (2019). “Determination of thermal conductivity properties in some wood species obtained from Turkey,” BioRes. 14(3), 6709-6715.
  2. Yapici, F., Ozcifci, A., Esen, R., and Kurt, S. (2011). “The effect of grain angle and species on thermal conductivity of some selected wood species,” BioRes. 6(3), 2757-2762.
  3. Panel Guide, Version 4, Wood Panel Industries Federation
  4. Troppova, Eva & Svehlık, Matej & Tippner, Jan & Wimmer, Rupert. (2014). Influence of temperature and moisture content on the thermal conductivity of wood-based fibreboards. Materials and Structures. 48. 10.1617/s11527-014-0467-4.

Barrel shape Boiler Heat Loss Test Object

Boiler Heat Loss Test Object

My original idea for measuring the thermal insulation for the boiler was to make a heating disc / puck. A bit like a metal ice hockey puck. Different materials could be applied as a disc to the copper surface. I could then measure the cooling rate of the puck.

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