Most European stained glass windows have no protection to reduce the damage due to environmental exposure, and this is the case of the great gothic windows in the Uppsala Cathedral. Currently, the implementation of protective glazing systems is being carried out in order to reduce the damage of the stained glass. In this connection, an analysis of the energy saving potential of the protective system is of great interest. The main obstacle in the implementation of this kind of systems is a social rejection based on the fact that the additional panes perturb the appearance of the historic building. Thus, demonstrating that there would actually be an energy saving, with the corresponding economic saving, would help approving the implementation of the protective glazing system. Measurement data needed for the analysis were available since they were used in condensation studies for the damaging analysis. However, the fact that this data was not gathered thinking of the energy saving analysis and its difficulties, some limitations have appeared in the present study. Accordingly, the needed simplifications imply some uncertainty in the results. However, the reliability of the results and the conclusions that can be drawn from the analysis are contrasted with previous similar studies. Different methods of calculation and analysis will be discussed, and finally results for the heat transfer coefficients through the open air gap and through the whole double paned system will be obtained. Furthermore, an energy saving analysis will be carried out with the winter data for both the case of the case of the Sonens fönster and the case of also implementing a similar system in the rosette of the cathedral. As a result, the conclusions drawn will be that the implemented protective glazing system reduces the heat losses through the windows to less than a third and that savings of about 8700 kWh/year and 13000 kWh/year are achieved in the case of the system being implemented only in Sonenes fönster and also in the rosette respectively. Finally, a rough study of the condensation problem will be exposed and possible matters, along with some advices, of further studies regarding more accurate analysis of both the condensation and the energy saving analysis will be presented. / Church project