New Zealand’s housing crisis, caused by poor insulation, high energy consumption, and lacking thermal comfort, highlights the urgent need for energy-efficient building systems. This study investigates a Fiber-Reinforced Polymer (FRP) wall panel system, which includes phenolic foam rigid insulation in the panel cavities that create 26 mm air gaps for thermal regulation and thermal transmittance. This research primarily focuses on examining the gap between the designed and the actual performance of the system through field monitoring of external walls, using K-type thermocouples, picologgers, and heat flux plates to measure thermal transmittance (U-value), heat flux, and temperature profiles. The results showed a U-value of 0.206 W/m2·K during the observation period, outperforming conventional wall requirement (0.500 W/m2·K) set by H1 energy efficiency New Zealand building code 2022. Furthermore, the findings indicated that the combination of insulation-filled cavities and air gaps worked effectively together: the insulation reduced heat transfer, while the air gaps and insulation collectively contributed to thermal regulation by slowing heat flow and temporarily storing heat, which was gradually released as temperatures dropped, helping to maintain indoor temperatures between 18–25 ℃ for 83.9% of the monitoring period. This study provides empirical evidence of FRP’s dual thermal benefits, such as insulation for resistance and air gaps for thermal buffering, positioning it as a sustainable solution to New Zealand’s housing and energy challenges.