Combined natural convection and radiation heat transfer in a vertical air cavity with a hexagonal honeycomb core of negligible thickness are investigated numerically. The numerical methodology is based on an algebraic coordinate transformation and finite volume technique. Temperature of the honeycomb core wall is determined such that radiation, convection, and conduction energy balances on the core wall are satisfied. The computations were performed forRa = 103−105, for an aspect ratio ofH/L = I, forPr = 0.7, and for a wide range of emissivities. Heat transfer rates by both convection and radiation are presented as a function of emissivity, and the effect of radiation on the natural convection is discussed. The results showed natural convection is affected by radiation although the layer is filled with a transparent gas. This effect is accentuated in the case of a very thin wall.