using doping elements. i.e., tungsten and magnesium.  The performance of thermochromic smart windows is evaluated by two crucial optical performance parameters: 1) luminous (visible) transmittance (τ_um, for wavelengths ranging between 380-780 nm), which represents the transmission of visible light useful for human vision under normal conditions, and 2) solar modulation ability (Δτ_sol, the difference of solar radiation transmittance τ_sol before and after the phase transition for wavelengths ranging between 300-2500 nm), which represents the modulation of solar radiation energy passing through the window. Currently available thermochromic films possess relatively low τ_lum and Δτ_sol. In addition, the thermochromic performance of VO₂ decrease significantly when exposed to air for several months, a humid environment would also greatly accelerate the oxidation process and turn thermodynamically unstable VO₂ into stable V₂O₅ with no solar modulation ability and thus deterring the practical deployment of VO₂ based smart windows. To address this issue, durability-enhanced VO₂ nanoparticle-polymer thermochromic film, was proposed and fabricated; where the cross-linked and highly entangled poly (methyl methacrylate) (PMMA) chain with molecular weight (~ 950,000) was adopted to block gas diffusion and vapor penetration into the polymer matrix embedded with ~ 3% volume fraction of VO₂  nanoparticles (50 ~ 80 nm). The PMMA-VO₂ film was then coated ~ 50 μm thick transparent and chemically stable biaxially-oriented polyethylene terephthalate (BoPET) sheet using blade coating. The developed film presented a relatively high luminous transmittance of ~ 50%, solar modulation ability of ~17.1%, and a low haze value of ~ 11%. Accelerated environmental tests were performed at 60 ºC temperature, and the relative humidity > 95%. The lifetime of the VO₂ nanoparticles embedded in the cross-linked PMMA matrix with molecular weight ~ 950,000 developed in our work is  ~ 900 hours, which is particularly better than the previously reported values for VO₂ nanoparticles coated by SiO₂ (~ 72 hours, Al(OH)₃ (~ 120 hours), and VO₂ thin film protected by Al₂O₃ (~ 100 hours), and is comparable to the performances of VO₂ nanoparticles coated by Al₂O₃ (> 480 hours) and VO₂ thin film protected by SiN_x (~ 600 hours). Our method provides an easy and yet effective strategy to improve the lifetime of VO₂ nanoparticles, suggesting a promising pathway toward environmentally stable and large-scale deployment of thermochromic films for energy-efficient smart windows. The detailed energy performance analysis shows that the developed PMMA-VO₂ film could greatly reduce the cooling demands in hot climates and improve the thermal comfort in cold climates.