This study aims at contributing to the progress of the modeling of the thermal behavior of Building Integrated Photovoltaic (BIPV) modules, a progress that runs way behind the pace of development in other areas of characterization of PV modules, such as the electrical one. To this end, a model based on the physics of heat transfer and optics is proposed, allowing the calculation of the temperature of BIPV modules and their thermal transmittance (U-value) in real operating conditions. The main contribution of this model is to include the impact of some parameters not considered so far, like the solar irradiance and the electrical efficiency. After being validated with experimental data, the model has been used to simulate single and double PV glazing, that compared later with common glazing with the same composition and configuration demonstrate the nonequivalent performance between them, showing differences of about 35% in the U-value under high irradiance conditions. This way, a clear disagreement with the current standard testing procedures occurs, something that should be reviewed. The tool proposed allows the complete study of the thermal behavior of BIPV modules and can help in the development of new specific standard procedures for a more realistic characterization of BIPV modules.