Expanded polystyrene (EPS) is largely used in the building industry. One of its most frequent applications is thermal insulation of buildings. In this case, a material is located at the interface of two different conditions – inside of a building and the surrounding environment. This means that there are different relative humidities on the sides of an EPS panel and, therefore, humidity flows through EPS. Technical norm EN 12086 describes this process as diffusion and suggests the corresponding measurement process in which the measured parameter is a mass flow of water vapor. In this paper we report on such measurements that we carried out for several types of EPS. Our results turn out to vary with the bulk density of samples. Measurements are performed in a climatic chamber with conditions set equal to RH = 85 % and T = 23 °C. The bulk density of samples varied between 15 kg/m3 and 30 kg/m3 and the diffusion coefficient was between 1.13×10−6 m2/s and 0.32×10−6 m2/s.

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