The current contribution describes the experiments performed in an outdoor test stand for so called "double-skin-facades" at the Technical University of Munich. The purpose of the investigation was to determine the time and local averaged overall heat transfer coefficients for solar radiation augmented turbulent mixed convection flows in transparent vertical channels. The external plate of the vertical channel is formed with horizontally oriented ventilation gratings in the external glass facade, thus providing openings for the air circulation within the gap. In the course of the research, the distance between the external and the internal glass fagade was measured for three gaps of 0.3, 0.6 and 0.9 m wide. Using the pressure compensation method, the effective mass flow rates for a given box-window, which is the smallest functional unit of all known double-skin-facade systems, was measured by means of a ventilation duct installed at the outlet of such a box-window. Finally, the experimental data was reduced in terms of an average Nusselt number as a function of an average Archimedes number for several gap distances.