Moisture Migration in Fibrous Insulating Materials Under the Influence of a Thermal Gradient and Its Effect on Thermal Resistance

The thermal conductivity of a high density mineral fiberboard has been experimentally determined as a function of water content, at ambient temperature. The measurements, performed on three different apparatus over a moisture range of 0 to 150 percent by weight, show that the temperature gradient causes a redistribution of moisture which affects the thermal conductivity. The results are strongly dependent on the experimental conditions (for example, the orientation of the specimen). The variation of thermal conductivity with respect to dry state may reach 10 percent for a moisture content of about 30 percent by weight. In practice, the main cause of moisture presence fibrous insulating materials is adsorption. The quantities adsorbed at 90 percent relative humidity are of 2 percent or less (in mass). For this small amount of water the variation of thermal conductivity with respect to dry state is negligible. All the results are discussed with the present problems of standardization mind. Of particular interest are the problems concerning the corrected values of thermal resistances or thermal conductivities of insulants in use (design X value).