<p dir="ltr">Smart windows, dynamically adjusting optical transmittance, face global adoption challenges due to climatic and economic variability. Aiming these issues, we synthesized a methyl cellulose (MC) salt system with high tunability for intrinsic optical transmittance (89.3%), which can be applied globally to various locations. Specifically, the MC window has superior heat shielding potential below transition temperatures while turning opaque at temperatures above the Lower Critical Solution Temperature (LCST), reducing the solar heat gain by 55%. Such 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. MC-based windows demonstrated a 9°C temperature decrease compared to double-pane windows on sunny days and a 5°C increase during winters in field tests, while simulations predict an 11% energy savings.</p><p dir="ltr">Incorporating MC-based phase change materials in passive solar panels indicated optimized energy efficiency, offering a sustainable alternative. Real-time simulations validate practical applicability in large-scale solar panels. Furthermore, a temperature-responsive sorbent with a dark layer demonstrates an optimal optical and water uptake performance. Transitioning between radiative cooling and solar heating, the sorbent exhibits high water harvesting efficiency in lab and field tests. With an adjustable LCST at 38 ℃, the cellulose-based sorbent presents a potential solution for atmospheric water harvesting, combining optical switching and temperature responsiveness for sustainable water access. Furthermore, the ubiquitous availability of materials, low cost, and ease-of-manufacturing will provide technological equity and foster our ambition towards net-zero buildings and sustainable future.</p>