This work is focused on mass transfer characterization of hygroscopic materials used for insulation, such as low density fiberboards (LDF). Due to their particular morphology, these panels present a very high mass diffusivity due to the connected gaseous phase together with a very low thermal conductivity. This combination of properties exacerbates the coupling between heat and mass transfer in transient state. Based on experimental data obtained with an original set-up and relevant simulations performed using a comprehensive physical formulation, a throughout vision of this question is proposed in the present study. In particular, we emphasize on: The impressive change in core temperature in terms of magnitude and duration, The great impact of the internal temperature gradient, which slows down mass diffusion, The dramatic error on mass diffusivity value if the coupling is ignored, The possible determination of thermal conductivity during transient sorption tests. A dimensionless number N-c was derived to quantify the intensity of this coupling. Finally, a practical example is proposed that confirms the importance of heat and mass transfer coupling in the case of LDF, and at the same time, proves that this effect can be controlled and corrected by using a relevant physical formulation. (C) 2018 Elsevier Ltd. All rights reserved.