In this paper a numerical simulation of transient heat penetration through a vertical rectangular composite cell, filled with a solid-liquid phase change material (PCM) and air layer, is presented. Inside the composite cell the PCM layer is separated from the air layer by a solid partition of finite thickness. The buoyancy-induced flows developed in both the air-filled layer and the molten PCM zone inside the PCM layer were modeled as two-dimensional laminar Newtonian fluid flow adhering to the Boussinesq approximation. Meanwhile, two-dimensional conduction heat transfer accounted for the unmelted solid PCM region as well as the solid partition. The numerical results for the composite cell with a thin diathermal partition demonstrate that by means of the latent-heat absorption inside the PCM layer, heat penetration across the composite cell can be greatly retarded over an effective duration until a critical instant, around which the melting front of the PCM reaches the partition wall. Such an effective thermal protection duration is found to be a strong function of the modified Rayleigh number, the modified Stefan number, the subcooling factor, the relative PCM thickness ratio, and the aspect ratio of the composite cell. A metry of a shallow rectangular composite cell having a larger PCM/air thickness ratio is found to be preferable for effective thermal protection applications. In addition, the effect of a solid partition of finite thickness and conductivity on thermal protection efficacy of the PCM/air composite cell is examined.