Hi, I would like to re-define my question from June 5 th. I made an experiment. A bigger surface of rock wool insulation matrix (size 0.4 m x 0.4 m x 0.1 m) was moistened with 45 g of water and sealed with PVC. Then the sealed sample was put into the testing set up. The testing set up consists of two paralel plates, temperature of which can be set. I was changing the temperature of both boudaries (example: T of boundary 1 is 40 C, T of boundary 2 is 20 C for 48 hours, then T of boundary 1 is 20 C, T of boundary 2 is 40 C for another 48 hours) and consequently water evaporated at the boundary with higher temperature and diffused towards the boundary with lower temperature, where the condensation took place (condensation took place also inside the insulation matrix). I measured the temperature ob both boundaries and heat flux at both boundaries. Photo of the speciman and a picture of the testing set up are attached. When the temperature of the boundary with lower temperature was set to sub 0 C (273K), we found that water freezes at about -5C (268K), but ice melts at 0 C (273 K). During freezing of water at the PVC boundary surface, the temperature of the boundary raises for about 2 K. Does anybody know why water freezes at -5C? Subcooling? Something else? The sample was moistened with tap water. Is evaporation – diffusion – condensation proces a kind of destilation? Thanks in advance.