Dynamically Adaptive Wrinkle‐Structured Light‐Regulating Films for Energy‐Efficient Buildings

Energy‐efficient buildings utilize smart windows with switchable optical properties to minimize energy consumption for lighting and cooling. However, most existing dynamically switchable materials depend on external energy sources, such as electricity, for activation. In this work, a moisture‐responsive light‐regulating film is presented composed of a cellulose nanofibril (CNF) layer and a polyvinyl alcohol (PVA)‐glycerol layer, fabricated through a scalable casting method. The film harnesses moisture‐induced differences in modulus and swelling behavior between its layers, allowing the CNF layer to form a wrinkled surface under dry conditions that flattens upon moisture exposure. This results in a regulation of the sunlight, achieving tunable light diffusion for daylight harvesting, thermal regulation, and privacy control. Compared to conventional glass, the light‐regulating film reduces indoor temperatures by up to 8.4 °C on sunny days in hot summer while maintaining sufficient light on rainy days. Simulations indicate potential energy savings of up to 7.0 MJ m −2 annually for a typical medium office building in Miami, a city in the U.S. This work offers a sustainable and energy‐efficient solution for modern building applications.