Isothermal moisture flow in building materials:: modelling, measurements and calculations based on Kirchhoff’s potential

The modelling of non-linear, isothermal moisture flow in porous media without hysteresis is considered. Different formulations based on different potentials for the Fickian moisture flow are compared. Kirchhoff’s flow potential, i.e. the integral of any state-dependent moisture flow coefficient, is introduced. The problem with a highly variable flow coefficient vanishes. The theoretical, numerical and computational basis for isothermal moisture transport in building materials using Kirchhoff potentials is described. It is shown that the use of Kirchhoff’s potential has clear advantages compared to models using a moisture flow coefficient. There is a conceptual simplicity. Only the relation between moisture content and Kirchhoff’s potential is used in the internal process, while the relation between Kirchhoff’s potential and any intensive variable is needed at boundaries. Precisely these relations are obtained in measurements, while the flow coefficients require a troublesome derivation. The use of Kirchhoff’s potential simplifies considerably the numerical calculation, since flow coefficients and ensuing interpolation problems are avoided. Complete sets of data for moisture flow and equilibrium are given for five common building materials. In two examples, numerical calculations are compared to experimentally determined moisture distributions.