Study on efficient and accurate protocols of measuring sorption isotherm of porous building materials using three-dimensional hygrothermal simulation

The sorption property of porous building materials in the hygroscopic region (sorption isotherm) is an important input to hygrothermal simulation. In this study, we proposed efficient and accurate protocols to obtain sorption isotherms using the static desiccator method. We developed a calculation model for three-dimensional simultaneous heat and moisture transfer in a material corresponding to the measurement specified in ISO 12571. Using an international database of the material properties, we conducted numerical simulations of the sorption process for five types of materials. The evolution of the amount of adsorbed moisture in a specimen and the time to reach equilibrium during the measurement were calculated under various humidity levels and specimen dimensions. According to the simulated results, we improved timing and interval for weighting the specimen to confirm the attainment of equilibrium for each type of a material and for each humidity level from the viewpoint of efficiency and accuracy. Meanwhile, the influence of size of a specimen on the time necessary to obtain the results was quantitatively demonstrated. Moreover, it was demonstrated that the method used for the determination of the attainment of equilibrium provided in ISO 12571 can underestimate the moisture content of materials with low moisture content and vapor permeability, and a policy to reduce risks of the underestimation was proposed.