One of the most common uses of expanded polystyrene (EPS) is the thermal insulation of buildings on the external side of building walls. Thus, EPS separates two contrasting environments (the walls and the surroundings), so that it is exposed to different relative humidities on both its sides, causing the presence of a humidity flow. A flow of the humidity through EPS may be described by diffusion, while other transport mechanisms are assumed to be negligible. In this work, we study the diffusion of water vapor through several types of EPS of different densities, namely, 15.0 kg m−3, 19.5 kg m−3, 25.6 kg m−3, and 29.4 kg m−3. We show that the water vapor diffusion coefficient increases with the air humidity in a linear fashion in all four types. In fact, the rate of increase varies from 6.09×10−7 m5 kg−1 s−1 for density 19.5 kg m−3 to 2.45×10−6 m5 kg−1 s−1 for density 29.4 kg m−3.

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