The hygrothermal behaviour of bio- and geo-based bilayer wall assemblies is investigated. The highly hygroscopic nature and thermal insulating properties of a sunflower-based concrete make it a promising building material. However, vegetal concretes are usually coated to satisfy aesthetic requirements. Therefore, the finishing layer is taken into consideration when investigating the hygrothermal behaviour of a wall. To improve the heat and mass storage and transport properties, without jeopardizing the general behaviour of plasters, an organically admixed mortar formulation was developed and the hygrothermal properties of each layer (sunflower concrete and plaster) were determined experimentally. The data were included in a coupled heat and moisture transfer 1D-model, which was applied to simulate the hygrothermal behaviour of the sunflower concrete specimen coated with earth plaster. The comparison of numerical moisture buffer values with experimental data for the wall assembly gave satisfactory results in spite of the heterogeneity of the materials and the experimental uncertainty. As expected, the numerical results show that the bilayer structure with the optimised plaster exhibits higher moisture buffer capacities than one with the reference formulation.