A physical approach to the modeling of hygrothermal transfers in bio-based construction and textile materials

Bio-based construction materials made of wood or plant pieces or fibers enhance occupant comfort due to their moisture-buffering capacity and may reduce energy demands for heating or cooling. These properties are attributed to exchanges between water vapor and bound water (i.e., water absorbed inside the solid phase, while vapor occupies the pore space), along with associated heat transfers. A similar problematic exists for natural clothes, made of vegetal of animal fibers. Understanding and predicting the coupled heat and moisture (hygrothermal) transfers in such hygroscopic porous materials is therefore essential for their optimal selection, appropriate use, and the development of innovative materials. However, current performance analyses often rely on coarse evaluations or macroscopic models that lack detailed physical insight. We propose here a fundamental approach grounded in physical principles and informed by recent investigations of water transport. This approach distinguishes between bound water diffusion, vapor diffusion, and the phase exchanges (sorption/desorption) between them. It leads to a system of two coupled conservation equations (mass and energy), with all physical parameters independently measurable. A similarity analysis then reveals three crucial dimensionless numbers governing the material behavior. Their magnitude indicates that thermal transport generally occurs on much shorter timescales than mass (water) migration. The model also enables identification of the conditions under which strong coupling between heat and moisture transfers arises. Altogether, this approach provides a physically grounded basis for modeling and predicting hygrothermal behavior in bio-based materials. • Development of a general macroscopic modeling method for moisture and heat transport in bio-based materials. • Key dimensionless numbers for hygrothermal behavior are identified and discussed. • Heat propagates faster than moisture. • Moisture dynamics depend on temperature, but not the other way around.