Transformers are a necessary part for the proper functioning of high voltage systems and high demands are placed on their manufacture process. Moisture is the most common cause of transformer failure, for which the cost of repair can be high. Most of the moisture is contained in the insulation of transformers, which is mainly made out of hygroscopic material based on cellulose, such as insulating paper. For this reason, during the manufacturing process of transformers, most attention is put on the moisture contained in the insulation, the content of which is to be reduced as much as possible. This is achieved by subjecting the transformer to process of vacuum drying, whose duration is often determined empirically. For the mechanism of drying it is necessary to supply the material with heat, so it is useful to know the transfer of heat flow to the transformer insulation, which could later serve in assessing the efficiency of the drying process. As the main objective of this thesis, a mathematical model was developed that includes heat transfer by thermal radiation and convection to transformers in a vacuum chamber during the drying process. In order to get the solutions in the form of transient surface temperature of paper insulation, an implicit method for solving a system of differential nonlinear equations was used in the model. Finally, a model response analysis was performed with respect to different air pressure regimes in the vacuum chamber during the drying process. The thesis also gives a brief overview on the phenomenon of rarefied gases and how it could be taken into account.