Thermal diffusion of water vapour in porous materials: Fact or fiction?

The reliable evaluation of moisture transfer in porous materials is essential in many engineering applications, among which building science. One key aspect is a correct description of moisture flow phenomena and their transport potentials. While different issues can be debated in that respect, thermal diffusion of water vapour in porous materials – diffusion driven by temperature gradients – currently stands out, due to the contradictory findings on the topic. Thermal diffusion proponents uphold that, complementary to vapour pressure gradients, temperature gradients equally yield substantial diffusion. Thermal diffusion opponents, on the other hand, assert that these thermal transports are negligibly small. This paper resolves that contradiction. A critical analysis of the investigations supporting the occurrence of thermal diffusion reveals that all are flawed. A correct reinterpretation of all measurements allows concluding that no consistent nor significant thermal diffusion can be observed. This brings these investigations in line with their earlier opponents. This conclusion also agrees with thermodynamics, which confirms the actual existence of thermal diffusion, but also indicates its negligible magnitude. It can in conclusion be stated that thermal diffusion is of no importance for building science applications, leaving vapour pressure as the sole significant transport potential for the diffusion of water vapour in porous materials.