Water is harmful for transformers; the dynamic behavior of moisture influences the dielectric integrity and insulation aging in a transformer on operation. In this paper, the determination of the diffusion coefficient of moisture in transformer pressboard is tackled. This work has been focused in analyzing non oil-impregnated pressboard, so the obtained coefficient is useful for studying the processes of transformer drying in factory. Drying experiments were developed in a thermogravimetric analyzer on nonimpregnated samples of pressboard subjected to different drying temperatures. The experimental conditions were simulated by means of a diffusion model implemented by means of Finite Element Method. The comparison between estimated and measured values allows drawing conclusions about the accuracy of the proposed coefficient.