The advancement of electric vehicles necessitates a rigorous focus on passenger cabin safety, particularly concerning the severe thermal hazard of a lithium-ion battery thermal runaway. Unlike internal combustion engine vehicles, electric vehicles require interior materials that provide superior thermal resistance to slow heat propagation, delay autoignition, and minimize smoke and toxic gas emissions, thereby securing a survivable evacuation window. This paper examines the application of the lumped-capacitance thermal model and the derived thermal time constant (τ) as a foundational framework for evaluating and selecting cabin materials. This approach enables a quantitative, physics-based ranking of materials—including seat composites, sound-deadening layers, electrical insulation, and carpet assemblies—based on their intrinsic ability to delay their own temperature rise under transient heat flux. By integrating materials with a high τ and elevated critical failure temperatures, this study proposes a performance-based material selection strategy. This strategy is critical for extending the safe egress period mandated by standards such as UN ECE R.100 and GB 38031-2020 and is increasingly vital as safety benchmarks evolve toward longer durations. The synthesis of high-inertia materials with traditional fire-resistant insulation provides a multi-layered defense, enhancing passenger protection by functionally delaying the progression of heat, fire, and hazardous emissions into the occupied cabin space.