A method to develop an insulator window film which is able to filter thermal emission while transmitting the visible spectrum of sunlight is proposed. The proposed film is constructed from engineered metallic nano-spheres randomly distributed in SiO2, as a host medium. The performance of the designed film is investigated using both analytical models and numerical full-wave simulations. The analytical analysis shows that the thermal emission (the wavelengths in the range of 6–16 µm) is suppressed by more than 10 dB when going through the designed film, meaning that more than 90% of the thermal power is filtered by the film. This is while more than 50% of the visible light passes through the film. Similar results are obtained using numerical full-wave simulations. Moreover, to have a more comprehensive study on the ability of our method, the illuminance due to the insulator window film is calculated and compared with illuminance in different places. This comparison shows that the resultant illuminance in a typical room at the distance of 4 m from the designed window is in the range of illuminance required in a typical office room.