The appropriate choice of window structures for a future or renovated building plays a crucial role in terms of the user’s future costs for operating and heating the building. In this paper, the author deals with the analysis of the results measured on the glazing of a plastic window, which is installed in the pavilion laboratory of the Department of Building Engineering and Urban Planning of the Faculty of Civil Engineering of the University of Zilina (Slovakia), where it was tested in standard indoor climatic conditions and real outdoor climatic conditions. During the laboratory measurements, the surface temperature from both the interior and exterior sides of the glazing was recorded at points defined by a regular grid pattern. The data were recorded in minute time steps. The paper will present the effect of surface temperature on the heat flux density passing through the glazing.