Determination of the Moisture Capacity of Porous Building Materials

The moisture capacity, which is required to solve the isothermal moisture transport equation, is generally expressed by parametric functions covering both the hygroscopic and over-hygroscopic regime. The modality or number of analytical functions needed to describe the corresponding pore volume distribution is introduced as an important parameter for a proper description of the moisture capacity or capillary pressure curve. We used the Markov procedure to estimate the parameters. The method allows not only to evaluate the goodness of fit and the wellposedness of the parameter identification problem, but also to determine the optimal location of the experimental data in order to minimize the effect of errors on the estimated parameters. We found that the wetting capillary pressure curve, relevant for many building physics problems of hygroscopic capillary active materials, is preferentially described by bimodal functions or unimodal functions withsufficient flexibility towards the hygroscopic zone. The use of fixed values of relative humidity for determining the limit between hygroscopic and over-hygroscopic regime cannot be recommended as useful a priori information. This limiting moisture content is rather a fitting parameter and is found not to coincide with the knick point moisture content defining the transition from vapor to liquid permeability. The optimal location of the experimental data is highly dependent on the chosen functional model and on the considered material. The goodness of fit only slightly reduces when using a minimal number of optimal data points compared to an extended data set.