This is because thermally conducting moisture replaces the better insulating air in the pores of the materials.
Moisture also enhan-ces the thermal contact between the solid grains of a porous material.
Other processes which from a macroscopic point of point can be treated as heat conduction, and thus driven by temperature differences, are convection and radiation heat transfers that take place microscopically within a porous structure. Thermal transmissivity or apparent thermal conductivity are commonly used terms for the material property associated with these phenomena. When moisture flows in a material without phase conversion, it carries its own enthalpy from one region of the material to another. However, for normal moisture and heat flow rates in building applications, this enthalpy flow can be neglected when compared to normal flow rates for heat transmission, and the rates of potential heat that is carried with moisture that changes phase at the terminals of its passage. Note however, that convection of air most often will have an important effect on the overall heat flow - but that is a different topic.