A local kinetics of sorption model: theoretical background and application to the simulation of an ISOBIO demonstrator

The classic models describing the hygric mass transfers inside building materials seem unsuitable in the case of bio-based materials. They are based on the assumption of an instantaneous local equilibrium between relative humidity and water content evolving according to the diffusive fluxes and the sorption isotherms and predict much shorter times of stabilization than those obtained experimentally. A new approach is presented here, it frees from the local instantaneous equilibrium introducing a local kinetics to describe the transformation of water from vapor state to absorbed liquid state and vice versa. In the framework of the European ISOBIO project, a multilayered wall mainly made of bio-based materials has been developed. The different layers have been characterized in terms of sorption, vapor permeabilities and thermal conductivities. The sorption measurements performed on representative samples have allowed to determine the local kinetics constants. Then, the hygrothermal behavior of the test wall has been studied thanks to an instrumented demonstrator (HIVE demonstrator, Wroughton, UK). The recorded measurements are compared to simulations based on the instantaneous local equilibrium model (TMC code) and on the local kinetics of sorption model (TMCKIN code). It appears that TMC very significantly underestimates the dynamics of the local relative humidities variations whereas TMCKIN succeed in predicting this dynamics and leads to results close to measurements.