The use of biochar-clay composites as internal layer and internal insulation of walls suggests great potential, since the hygrothermal properties of the starting materials complement each other perfectly: the use of clay brings high moisture buffering capability of the composite, while biochar contributes at reducing the effective thermal conductivity. In this paper important hygrothermal properties (thermal conductivity, sorption and desorption isotherms, vapor diffusion resistance factor, liquid water diffusivity, water uptake coefficient and capillary water content) of biochar-clay composites are reported and discussed. Composites of different densities, obtained with different biochar types and volume ratios are compared. It is explored how the material composition affects its hygrothermal behavior.