In this study the ignition model of a PMMA (polymethyl-methacrylate) wall has been proposed in a two-dimensional rectangular enclosure when it is initially exposed to a high-temperature source. The effect of surface radiation has been taken into account. It was found that the transient development of thermo-fluid fields was strongly governed by wall heating due to surface radiation. In particular, the rapid heating of the adiabatic floor made the flow very unstable, creating complex secondary recirculating flows. Depending on the hot source temperature, the ignition process was controlled either by mixing and transport of fuel vapor and oxidizer in the vicinity of the hot wall, or by infiltration of hot air into the region near the PMMA wall.