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Summary: Transparent windows that insulate against infrared light and heat entering buildings hold the promise of reducing energy consumption. However, a long-standing challenge for energy-saving window materials is to concurrently achieve a large tuning of refractive index, broad transmission modulation, and near-room-temperature phase transition. Here, we report the insulation of heat and infrared radiation by using a functionally graded transparent nanocomposite consisting of a hollow network-nanoparticle hybrid. We demonstrate ΔT of 6.9°C by combining the light-reflectance properties of metallic nanoparticles and the thermal insulation of hollow shell networks, resulting in an equivalent cooling power of 396 W/m2 under a light intensity of 3 W/cm2. This flexible nanocomposite sheet shows a high transparency of 91.0%, low haze in the visible region, and a modulus of 160 MPa. The gradient nanocomposites combine UV light blocking, visible transparency, and infrared light shielding, potentially opening a new avenue for energy-saving building window applications.