Moisture buffering of compressed earth blocks: Coupled vapour–liquid modelling and effects of boundary conditions, thickness

Hygroscopic materials can moderate fluctuations in indoor relative humidity and keep it within a comfortable hygric range thanks to their moisture-buffering ability. Many studies have been carried out to investigate the moisture buffering capacity of earthen building materials; however, a comprehensive understanding of the coupled vapour –liquid moisture transfer mechanisms and their contribution to the observed buffering behaviour remains limited. In this work, the Moisture Buffer Value (MBV) of a compressed earth block (CEB Thermbatiterre) is experimentally determined following the NORDTEST protocol for three specimen categories defined by thickness and compared with the analytical ideal MBV. In addition, a dynamic moisture transfer simulation is applied to explicitly distinguish between vapour-only transfer and coupled vapour–liquid transfer mechanisms, allowing a more physically based interpretation of moisture buffering behaviour. The model is further used to investigate the influence of initial, boundary conditions, and specimen thickness on the MBV of earthen materials. The results indicate that CEB Thermbatiterre falls within the range of materials classified as having good hygroscopic performance. Vapour diffusion is the principal transport mechanism but cannot alone reproduce the measured MBV: liquid-phase transport contributes a non-negligible additional flux through capillary condensation in the finer pores of the material. The application of Robin boundary conditions consistently leads to lower moisture buffering value (MBV) predictions compared to Dirichlet boundary conditions, highlighting the significant influence of surface moisture transfer resistance on MBV estimation. Furthermore, three moisture-buffering stages (growth, high-buffer and plateau) are identified on the MBV–thickness curve of CEB Thermbatiterre, with the plateau reached at ≈ 25 mm.