Experimental analysis of moisture transfer and phase change in porous insulation exposed to fire and its effect on heat transfer

Water vapour in porous building materials significantly affects the heat transfer. In addition to the regular humidity, materials, such as gypsum, release water vapour when they are heated, leading to a high resistivity to fire exposure. Although the heat and water vapour transfer through porous building materials at atmospheric conditions is well investigated, no data are available for the effective diffusion coefficient or velocity of the water vapour transfer when the structure is exposed to a thermal load or fire. For this purpose, fire resistance tests were carried out for gypsum/mineral wool constructions to determine the transient heating characteristic and water vapour transfer. It was found that for thin gypsum structures up to 2.5 cm the heating from ambient temperature to 65 ∘C is mainly caused by conduction. Further heating to 100 ∘C is related to the condensation of water vapour instead of thermal heat conduction. Furthermore, a significant decrease of the effective diffusion coefficient of water vapour in the mineral wool from 33.3 mm2/s to 26.7 mm2/s was determined during fire exposure. Based on the measurement the mean velocity of the water vapour transport was determined of being between 1 and 3 mm/s.