In this study, we investigated phenolic foam (PF) materials with different thicknesses: 10T (PF-10), 30T (PF-30), and 50T (PF-50) based on phenolic foam with phosphorous, melamine cyanurate, trialkyl phosphate, and mono pentaerythritol. The PF materials undergo chemical conversion into carbonized materials during fire exposure. Our heat release rate (HRR) and smoke production rate (SPR) tests revealed that both PF-30 and PF-50 thickness-based PF materials effectively retard flame spreading due to the carbonization layer formed by combining phenol foam polymer and carbon nanotubes. From an economic standpoint, a phenolic foam board with a thickness of PF-30 is recommended. Although PF-50 exhibited slightly superior fire retardation properties, its greater thickness poses practical limitations in construction. Specifically, the material usage for PF-50 was approximately 1.7 times that of PF-30. Therefore, considering fire safety and cost-effectiveness, PF-30 emerges as the optimal choice for fire-retardant applications.