This paper takes place in improving the energy performance assessment of cob buildings, by evaluating the variability of its hygrothermal properties at the material scale, related to the traditional construction process. For so, we proposed and analysed data to handle the variability of the hygrothermal properties. The specimens were manufactured using a moulding method representative of on-site cob wall manufacturing process, for three plants species (hemp shiv, flax yarn and hay stalk) and three fibre content (0, 1% and 3%). Using non-destructive tests and statistical analysis, the random variability of cob composites hygrothermal properties (density, thermal conductivity, specific heat capacity, water-vapor permeability, moisture buffering value and sorption isotherms) was found as well as the variability distribution. It has been shown that the variability of properties is sensitive to the nature of plant fibres specie and the fibre content. Using the variability indicators, it has been found on thermal conductivity, a low coefficient of variation of 2.88% for 1%-flax fibred mixture (lower than unfibred material) and a high one for 3%-hemp composites of 10.88%. The variability of sorption isotherms was usually found to be high at lower humidity loads. It has been shown that increase fibre content stabilizes the variability of properties. Moreover, some evolution trends of the variability according to mixes was proposed; two parameters were found: the first, either FCmax highlighting the fibre content for which the maximum of variability was achieved or FCmin for the opposite; the second is FCres highlighting the residual variability, for high fibre content. The distribution of properties were found to be generally centered.