Industrial heat and cooling applications are an essential fraction of the overall energy demand, mainly produced by fossil fuels. Solar thermal energy production can satisfy such a need by adopting the High Vacuum Flat Plate Collectors (HVFPCs) and increasing their efficiency. The absorptance and emittance of Selective Solar Absorbers (SSAs) determine the thermal efficiency of HVFPCs. Being the absorptance already maximized, the thermal emittance of the absorber should be minimized to increase further the operating temperature of the collector and its efficiency. This research aims to reduce the thermal emittance of commercially available Selective Solar Absorber by depositing a thin silver film on the aluminium substrate. So, in this work, the thermal stability of a silver coating has been investigated, and a diffusion barrier layer has been adopted to stabilize the coating performance up to 360 degrees C. The low-emissive layer of Ag and a diffusion barrier of CrOx guarantees a decrease of 11% in thermal emittance at 200 degrees C of commercially available SSA deposited on aluminium. Further emittance reduction can be obtained by depositing a thin Ag film on both sides of the aluminium substrate before the SSA deposition, proving to be a promising way to enhance the efficiency of HVFPCs.