Moisture dependent thermal properties of hydrophilic mineral wool: Application of the effective media theory

Abstract

Thermal properties of mineral wool based materials appear to be of high importance because the most widespread practice is using them as thermal insulation boards. Their thermal conductivity is easily found, i.e. data sheets from producers, often including specific heat capacity, but usually only characteristic values for dry state. Exposure to outside climate or any other environment containing moisture can negatively affect the thermal insulation properties of the mineral wool. This is why the presence of water inside the mineral wool is undesirable for the majority of applications; so, they are often provided with hydrophobic substances, whereas hydrophilic additives are seldom used. However, the later combination has good potential for some applications (i.e. desalination of masonries and green roofs). In those cases, mineral wool will work with a certain moisture content, which will change the thermal properties it had in the dry state. On this account, moisture dependent thermal properties (thermal conductivity and specific heat capacity) of hydrophilic mineral wool (HMW) are studied in a wide range of moisture content using impulse technique. The experimentally determined thermal conductivity data are analysed using a several homogenization formulas based on the effective media theory. In terms of homogenization, a porous material is considered as a mixture of two or three phases: solid (basalt fibres) and gaseous (air) phase for the dry state, adding the liquid phase (water) when moisten. The homogenization techniques are first applied to calculate the thermal conductivity of the solid matrix. Then, it dependence on moisture content is evaluated using some mixing formulas. To verify the obtained results, Wiener’s and Hashin-Shtrikman’s bounds are used. As a summary, the application of homogenization techniques can successfully estimate measured data for a highly inhomogeneous fibrous material (i.e. mineral wool), even consuming less time.