A lightweight aluminium nocturnal radiator, painted with an appropriate paint, was established on the roof of the Department of Environmental & Natural Resources Management in Agrinio, in Western Greece. The dynamic thermal performance of the system during summer months was calculated using an accurate mathematical model, based on the heat transferred from the air circulating inside the radiator to the ambient air. Furthermore, an extensive validation process was carried out. Thus, the experimental air temperature values at the radiator’s outlet were compared with the theoretical ones and a very good agreement was achieved. The validation procedure was extended for two different radiator’s paints and it was found that there is a significant impact of the paints’ emissivities on the system’s efficiency. Moreover, the more effective lightweight nocturnal radiator was used to provide space cooling or pre-cooling for the building of the University. Indoor air temperature values of the thermal zone connected with the radiator were compared with those of a similar zone without any cooling system and the results demonstrated a remarkable effectiveness of the system.