Today window panes are usually coated with at least one metal or metal oxide layer to prevent heat energy of the light spectrum from propagating to the other side. This has given problems regarding radio propagation through windows, which might be utilized as a part of a buildings IEMI protection. This paper reports the results from measurements of the shielding effectiveness of a selection of modern window panes before and after irradiation with high power electromagnetic waves. The shielding effectiveness measurements are made in a nested reverberation chamber covering the range 1 – 18 GHz; both before and after high power irradiation at 1.3 GHz. The results show that the shielding effectiveness of window panes may be severely impaired due to thermal stress effects on the coatings during the irradiation, depending of the type of coating.