This work is focused on mass transfer characterization of hygroscopic materials used for insulation, suchas low density fiberboards (LDF). Due to their particular morphology, these panels present a very highmass 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 transientstate. Based on experimental data obtained with an original set-up and relevant simulations performedusing a comprehensive physical formulation, a throughout vision of this question is proposed in the presentstudy. 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 Nc was derived to quantify the intensity of this coupling. Finally, a practicalexample 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 physicalformulation.