Evidence on dynamic effects in the water content – water potential relation of building materials

Hygrothermal simulation has become a widely applied tool for the design and assessment of building structures under possible indoor and outdoor climatic conditions. One of the most important prerequisites of such simulations is reliable material data. Different approaches exist here to derive the required material functions, i.e. the moisture storage characteristic and the liquid water conductivity, from measured basic properties. The current state of the art in material modelling as well as the corresponding transport theory implies that the moisture transport function is unique and that the moisture storage characteristic is process dependent with varying significance for the numerical simulation. On the basis of different building materials, a comprehensive instantaneous profile measurement study has been accomplished. Profiles of water content and relative humidity were obtained during a series of adsorption and desorption processes. The data provides clear evidence that the water content – water potential relationship is not only dependent on the process history, but also on the process dynamics. The higher moisture potential gradients were induced, the larger was the deviation between static and dynamic moisture storage data and the more pronounced was the corresponding dynamic hysteresis. The paper thus provides clear experimental evidence on dynamic effects in the water content – water potential relation of building materials. By that, data published by previous authors as Topp et al. (1967), Smiles et al. (1971) and Plagge et al. (1999) is confirmed. Moreover, it is shown that moisture transport processes are well susceptible to dynamic effects already within the hygroscopic moisture content range.