Expanded polystyrene (EPS) is a material commonly used in many different industrial areas. One of them is the building industry in which EPS is used as a thermal insulator, being placed mostly on the external side of building walls. In this setup, EPS lies between two different environments – building walls and the exterior. Consequently, there are different relative humidity on the two sides of an EPS panel and humidity flows through the panel. According to the technical specification EN 12086, the only mechanism of this transport is assumed to be diffusion. Other processes, such as osmosis or surface diffusion, are minor and, therefore, they could be neglected. In this paper we study the diffusion of water vapor through EPS in environments with different relative humidity. Experiments are performed on cylindrical samples with dimensions Ø90 mm×40 mm and density of 14.6 kg·m−3 in the environment with a relative humidity varying from 50% to 85%.

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