This study examines the thermal stability and insulating performance of materials intended for the cable sheath of the Remote Automatic High-Energy Flare (RAHEF) Ignition System. Thermal characterization was performed using Differential Scanning Calorimetry (DSC), Thermogravimetric Analysis (TGA), Differential Thermal Analysis (DTA), and Coefficient of Thermal Expansion (CTE) measurements, adhering to ASTM standards. Testing parameters were further refined through electrical resistivity tests and a Box–Behnken response surface methodology. The materials investigated included composite blends of castable refractory cement and refractory clay (CRC), pure castable refractory cement (CR), Portland cement–sharp sand composites (PCS), mica tape (MT), fiberglass sleeves, and conventional PVC. Among these, the CRC composite exhibited exceptional thermal resilience, retaining 86% of its mass up to 993 °C, along with low thermal conductivity and favorable CTE values that indicate structural stability. In contrast, PVC insulation showed significant degradation, losing over 97% of its mass by 488 °C. Both fiberglass sleeves and mica tape demonstrated excellent thermal stability, while PCS exhibited moderate performance. Ultimately, the CRC composite and fiberglass sleeves emerged as the most promising candidates for the RAHEF ignition cable sheaths. Their combined thermal endurance, electrical insulation properties, and structural durability position them as strong options for long-term use in high-temperature flare environments.