Fibrous materials and gas-foamed plastics provide some insulation due to the low thermal conductivity of air but are not transparent. Here we show for the first time that a nano-spaced polymer film consisting of hollow silica nanoparticles (HSNPs) dispersed in a polyurethane (PU) matrix provides good thermal insulation and is transparent. The HSNPs, having a silica shell and nano-sized hollow interior, are formed by removing the core from a core-shell structure prepared by sol-gel reaction of silicon alkoxide (TEOS). One advantage of using HSNPs is that a quasi-vacuum state in the nano-space is formed when the size of the space is close to the length of the mean free path of the air molecules in the space. Heat therefore tends to be transferred along the silica shells of HSNPs instead of through other materials in the film. Another advantage is that there are many defects in silica shells with low apparent density that are prepared using controlled sol-gel reaction conditions. The phonon-defect scattering in the silica shell suppresses heat conduction despite of high thermal conductivity of silica. Dispersed HSNP flocculation in the film is the key to both transparency and good thermal insulation. (C) 2015 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.