To address critical energy issues in civic structures, we have developed a novel concept of optical thermal insulation (OTI) without relying on a conventional thermal intervention medium, such as air or argon, as often used in conventional window systems. We have synthesized the photothermal (PT) materials, such as the Fe3O4 and Fe3O4@Cu2−xS nanoparticles, that exhibit strong UV and near-infrared (NIR) absorptions but with good visible transparency. Upon coating the inner surface of the window glass with a PT film, under solar irradiation, the inner surface temperature rises due to the PT effect. Subsequently, the temperature difference, ΔT, is reduced between the single pane and room interior. This leads to lower the thermal loss through a window, reflected by the U-factor, resulting in considerable energy saving without double- or triple-glazing. Comparing with the Fe3O4 coatings, Fe3O4@Cu2−xS is spectrally characterized with a much stronger NIR absorbance, contributing to an increased PT efficiency under simulated solar irradiation (0.1 W/cm2). PT experiments are carried out via both white light and monochromic NIR irradiations (785 nm). The parameters associated with the thermal performance of the PT films are calculated, including PT conversion efficiency, specific absorption rate (SAR), and U-factor. Based on the concept of OTI, we have reached an optimum U-factor of 1.46 W/m2 K for a single pane, which is satisfactory to the DOE requirement (<1.7 W/m2 K).