One of the major causes of damage to buildings is the ingress of water or humidity. This project therefore examined the method for determining the water permeation in low-density building materials such as extruded polystyrene and agglomerated (vermiculite) particle board, that are commonly used for thermal insulation. All the laboratories used the same basic gravimetric technique in which the sample is sealed into the mouth of an impermeable "cup", containing either desiccant or a salt solution, which is then placed in an environmental chamber. A constant vapour pressure difference is maintained across the sample and the water vapour permeability of the material is calculated from the steady decrease, or increase, in the weight of the cup.
Excluding outliers, for which explanations could be postulated, the range of the measured water vapour permeabilities remained very large (+/- 15 % for the polystyrene and +/- 20 % for the particle board) despite the fact that the test conditions were rigidly defined. An improvement of the situation would appear to necessitate modifications to the currently used design of the apparatus.
Funding SchemeCSC - Cost-sharing contracts