Abstract Reducing thermal losses in solar thermal devices is fundamental for enhancing conversion efficiencies, particularly at high operating temperatures. In this work, we consider the benefits of adding an InfraRed (IR) mirror coating to the inner surface of the glass encapsulating a High Vacuum insulated Flat Plate solar thermal Panel (HVFP). The IR mirror helps recover the radiation emitted by the absorber by sending it back to the absorber itself. This mechanism, known as cold-side external photon recycling, allows a reduction of radiative losses and, consequently, an improvement of the panel efficiency. The performance of the structure presented in this manuscript is studied via a thermal model. A detailed discussion on the increasing efficiency is presented, and results are presented by taking into account different parameters, like the mirror transparency, reflectivity and reflection bandwidth, as well as different operating temperatures of the panel. Finally, the annual energy gain associated with the IR mirror is analyzed in the case of three different cities, using historical data, showing that improvement higher than 50% can be obtained at operating temperatures above 300 °C.