<p>Smart windows that modulate solar radiation by changing their optical state in response to temperature stimulus are developing as promising solutions towards reducing the energy consumption of buildings. The market adoption of such systems has been slow due to the barriers in scalability, cost, as well as complexity in their integration into existing systems. Aiming these features, we have proposed a retrofit smart window design based on the temperature-responsive polymer Methyl Cellulose (MC). The system utilizes a sustainable, earth abundant and cost-effective cellulose based thermo-responsive material to transform existing windows to a thermally dynamic smart window system. The observed optical change of MC from transparent to opaque state is dependent on temperature and is triggered by the thermodynamic mechanism of reversible coil-globule transition, which results in a stable performance of the proposed device. Its solar modulation ability was studied using ultraviolet-visible- spectroscopy. Effect of MC concentration and various salts on the optical performance were investigated. It was found that the transition temperature the polymer can be tuned by varying MC concentration and by adding salts to the system. The tunability of transition temperature is a function of the concentration of salt and the type of anion in the salt. It was observed that the transition temperature of the window can be tuned between to , allowing a wide range of control over switching temperature. Controllable LCST, low freezing point, sustainable base material, scalable production, low cost, retrofit system makes them ideal candidates for smart window applications. </p>