In the context of sustainable construction, the demand for low-carbon building materials has increased interest in raw earth due to its availability and ecological benefits. Bio-based additives such as cellulose fibers and starch are often introduced to enhance mechanical and durability properties, although their impact on hygrothermal performance requires further investigation. This study examined the hygrothermal and mechanical properties of three raw earth adobe brick formulations: a reference sample, one with cellulose fibers, and one with starch. Key properties, including water vapor permeability, thermal conductivity, volumetric heat capacity, moisture buffer value, and elastic modulus, were analyzed. Notably, moisture regulation and air permeability remained excellent across all formulations, with minimal impact from the bio-based additives. These findings underscore the potential of raw earth adobe bricks, with or without bio-based additives, as a viable low-carbon material for sustainable construction.