Effect of thickness on flame spread behavior of FR-GEPS: An experimental and theoretical study of the critical threshold

The thickness of insulation layer plays a critical yet underexplored role in governing flame spread behavior, particularly for thermoplastic insulation materials. This thickness-induced fire transition directly determines wall fire hazard and is therefore essential to building fire safety engineering. This study experimentally and theoretically investigates the effect of thickness on flame spread over flame-retardant graphite-enhanced expanded polystyrene (FR-GEPS) insulation panels, with the aim of identifying the critical thickness threshold for wall fire spread. Modified Single Burning Item (SBI) tests were conducted on FR-GEPS panels with thicknesses of 5, 10, 15, and 20 cm. Heat release rate, smoke production, temperature evolution, and external heat flux were systematically characterized. The results reveal a clear thickness-dependent transition: panels of 5 and 10 cm exhibited only localized melting and dripping without ignition or fire spread, whereas 15 cm panels ignited and sustained wall flame spread, and 20 cm panels developed such intense combustion that the test had to be prematurely terminated. A theoretical model is proposed based on the competition between the characteristic times for stable upward flame spread and melt-dripping-induced fuel depletion. The model demonstrates that increasing thickness simultaneously accelerates flame spread (by enhancing heat accumulation and reducing back-side heat loss) and retards melt-off (by increasing the combustible reserve), thereby promoting fire spread beyond a critical threshold. The calculated critical thickness of 11.2 cm from the simplified heat transfer model agrees well with the experimentally observed range of 10–15 cm. This work provides both experimental evidence and a theoretical framework for understanding the thickness-controlled fire spread mechanism in thermoplastic insulation materials, offering practical guidance for fire-safety design of building insulation systems.