Thermal and moisture flux in soft fibreboards

Abstract

The thermal conductivity of several wet and dry processed industrial soft fibreboards was determined at temperatures between 10°C and 30°C. Additionally, the thermal conductivity was tested relative to the moisture content ranging from an oven-dry sample to a moisture content of 85% relative humidity. Furthermore, the water vapour diffusion was investigated to determine the resistance factor and the diffusion coefficient under ‘dry cup’ and ‘wet cup’ conditions and the sorptions were tested by means of the sorption isotherms. The thermal conductivity increases with increasing temperature at about 0.45% per Kelvin and with increasing moisture at about 0.17×10−2 W/mK per percent volumetric moisture content. Furthermore, the thermal conductivity depends on the manufacture (dry or wet process) and is influenced by density. The water vapour resistance factor increases and the diffusion coefficient decreases with increasing density. Both factors decrease with increasing moisture content and they are dependant on board thickness, composition and manufacturing. The sorption isotherm is similar to solid wood below about 80% relative humidity but strongly increases for soft fibreboards with binding fibres and additives of ammonium phosphate or sodium borate and boric acid when above 80% relative humidity.