In this paper, a new methodology for determining the moisture diffusion coefficient of transformer solid insulation is presented. Unlike classical methodologies, the proposed one does not require to measure the evolution of the moisture distribution inside the material under test, but the overall moisture evolution is measured, which is easier to do from the experimental standpoint. In addition, the methodology includes an optimization process based on genetic algorithms, with an objective function that incorporates a moisture diffusion model solved by the finite element method.