Achieving lower heat release rates (HRR) during combustion is one of the key steps toward obtaining flame retardant materials. UPR thermosets, while mechanically strong and chemically durable, show high HRR upon ignition. While most commercial applications focus on blending of metal oxide or other heterogeneous fillers to reduce HRR, they have significant drawbacks like phase segregation, drop in transparency and other features which disfavor their use in UPRs. Herein, a novel, green technique to generate nSiO₂ in-situ in UPRs is demonstrated. The method is designed such that the precursors act as nucleating agents covalently bonded to the UPRs and as growth fuel for the nSiO₂ production. Apart from major advantages like a uniform phase distribution in the thermoset and transparency, this technique also prevents direct handling of powdered micro or nanoparticles, leading to a safer working environment for the handling of UPRs. The physical, thermal, and mechanical properties analyzed show great promise towards flame retardant composites, as the formed nanocomposite material, with 10 wt% loading of nSiO₂ demonstrates a 41 % reduction in total heat release (THR) and a 52 % reduction in total smoke release (TSR), while retaining optical transmission >90 %. On combination with commercial phosphorus containing flame retardant, ammonium polyphosphate (APP), the composite shows an even greater reduction in THR and TSR, while also being self-extinguishing. These compelling features, coupled with the safe nature of generating nanoparticles in-situ, offer substantial benefits of using this nSiO₂ approach towards HRR reduction in UPR-based thermosets and advocate for their use in commercial formulations.