Murals and painted statues in grotto temples are often created on cliff faces or cave surfaces, forming a multi-layered structure composed of rock, earthen plaster (clay), and pigments. Environmental factors such as air temperature, humidity, solar radiation, and rainfall induce significant heat and moisture transfer within these porous materials, leading to possible damage and deterioration (e.g., cracking and flaking) on the surface of these valuable artifacts. However, limited knowledge of the hygrothermal properties of these heritage materials has hindered the understanding of the mural painting material deterioration, as well as the development of preventative conservation methods (to predict and provide early warnings for potential mural deterioration). In this study, laboratory experiments were conducted on earthen plaster samples from the Mogao Caves to investigate their hygrothermal behavior, including isothermal moisture sorption and water vapor permeability (WVP). Five groups of samples were prepared with different soil-to-sand ratios (pure soil and 1:1 mixture) and with the addition of plant fibers (none; hemp, wheat straw or cotton at 2% wt). As a result, significant differences were found in the properties of mural samples based on composition. A higher ratio of sand decreased moisture absorption capabilities, while plant fiber addition increased these properties. Both conditions had a limited influence on water vapor permeability, but the increase by fiber addition is higher. These findings enhance the cognition of mural materials with different components, provide essential data for coupled heat and moisture calculations, and deterioration risk assessment tools, contributing to the preservation of precious mural paintings.