Effect of moisture migration and phase change on effective thermal conductivity of porous building materials

In this paper, a new method for precise evaluation of the effective thermal conductivity (ETC) caused by moisture transfer inside porous building materials was proposed. Moreover, the mathematical models for coupled heat and moisture transfer without and with moisture phase change were established. Based on the interactions among the moisture phase change, temperature field, and moisture field inside the porous materials, a loop iteration calculation method was proposed to solve for the heat flux caused by moisture migration and phase change. By combining the heat flux with the classical evaporation and condensation theory, the additional thermal conductivities (ATCs) caused by the moisture migration and phase change were obtained. In order to obtain the quantitative relationships between the main factors and ATC caused by moisture transfer, different temperature and humidity boundary conditions were used to analyze the ATC. The results show that the ATC caused by moisture migration increases when the temperature difference decreases and the water vapor pressure difference between the indoor and outdoor air increases. The ATC is greatly influenced by outdoor air temperature fluctuations, while the influence of water vapor pressure fluctuations is relatively small. The value for the ATC caused by moisture phase change is mainly determined by the temperature gradient.