Highly Tunable Cellulosic Hydrogels with Dynamic Solar Modulation for Energy‐Efficient Windows

Smart windows that can passively regulate incident solar radiation by dynamically modulating optical transmittance have attracted increasing scientific interest due to their potential economic and environmental savings. However, challenges remain in the global adoption of such systems, given the extreme variability in climatic and economic conditions across different geographical locations. Aiming these issues, a methylcellulose (MC) salt system is synthesized with high tunability for intrinsic optical transmittance (89.3%), which can be applied globally to various locations. Specifically, the MC window exhibits superior heat shielding potential below transition temperatures, becoming opaque at temperatures above the Lower Critical Solution Temperature and reducing the solar heat gain by 55%. This optical tunability is attributable to the particle size change triggered by the temperature‐induced reversible coil‐to‐globular transition. This leads to effective refractive index and scattering modulation, making them prospective solutions for light management systems, an application ahead of intelligent fenestration systems. During the field tests, MC‐based windows demonstrated a 9 °C temperature decrease compared to double‐pane windows on sunny days and a 5 °C increase during winters, with simulations predicting an 11% energy savings. The ubiquitous availability of materials, low cost, and ease‐of‐manufacturing will provide technological equity and foster the ambition toward net‐zero buildings. Adapting to the Sun's Blaze: This work describes highly tunable methyl cellulose‐based salt systems that dynamically regulate solar radiation for thermal and light management systems. By undergoing temperature‐induced phase‐change transition, these windows offer significant energy savings, effective refractive index, and scattering modulations. They are also cost‐effective, easy to manufacture, and utilize readily available materials, supporting net‐zero buildings and technological equity.