An investigation on the effect of latent heat on the hygrothermal performance of earth building materials

A coupled finite element model has been developed to simulate heat and water transfer across earth walls, considering pore water phase changes and the associated latent heat fluxes. The adopted approach simplifies parametric analyses by expressing all material hygrothermal properties as functions of porosity and water retention characteristics. The model is used to assess the influence of pore water latent heat on the passive hygrothermal regulation provided by two infinite earth walls that enclose an idealised room exposed to an external cold, humid climate. The findings indicate that latent heat buffering by pore water in earth walls increases with greater relative humidity gradients between the outdoor and indoor environments. The low vapour diffusivity confines latent heat production to the outer cold wall region where pore vapour condenses. The condensed moisture then flows inward and re-evaporates in the inner region of the wall. Additionally, the phase changes of water crossing the wall interfaces contribute to latent heat buffering, thereby enhancing hygrothermal efficiency. The process of pore vapour condensation and liquid transport intensifies with higher volumetric capacity and diffusivity of liquid water, which in turn increase with greater porosity, steeper retention curves, and larger saturation levels. Hydraulic effusivity is defined as a function of the volumetric capacity and diffusivity of liquid water to measure latent heat exchanges. Large hydraulic effusivity values indicate a greater potential for latent heat buffering. Finally, when compared to conventional concrete walls, earth walls demonstrate considerably better hygrothermal performance, which is mostly attributed to greater latent heat exchanges.