A two-phase moisture transport model accounting for sorption hysteresis in layered porous building constructions

Building constructions most commonly consist of layered porous materials such as masonry. The moisture distribution and its variations due to changes in the surrounding environment are of special interest in such layered construction, since different materials absorb different amounts of water and exhibit different transport properties. A successful model of such a case may shed light on the performance of various constructions with regard to, for example, mould growth and freeze-thaw damage. We have developed a model for this purpose which is based on a two-phase flow, vapour and liquid water, and which also takes sorption hysteresis into account. The various materials in the layered construction under consideration are assigned different properties, e.g. vapour and liquid water diffusivities and boundary (wetting and drying) sorption curves. Furthermore, we modelled the scanning behaviour between the wetting and drying boundary curves by introducing appropriate material constants. Special properties have to be given for the interface between different materials in the layered construction in the model. In this case it is assumed that while vapour penetrates through such interfaces easily, liquid water does not. The model was developed by carefully examining the mass balance postulates for the two constituents under consideration, together with appropriate and suitable constitutive assumptions. A test example was modelled by using an implicit finite-element code, which implements a modified Newton–Raphson scheme to tackle the strong non-linearities in the present problem. The paper describes the numerical method to enable the interested reader to judge the significance of the proposed technique in solving the coupled set of non-linear equations and easily implementing the proposed model.