An experimental and theoretical study of the degassing behavior of a poly(ethylene) high-density foam, during storage at laboratory conditions, is presented. This foam is an expanded form of a low-density poly(ethylene), which is produced from its melt, using high-pressure nitrogen gas as the blowing agent, and is the precursor of the fully foamed polymer. Measurements of the mass, dimensions, and density of this material, as a function of the storage time are reported and indicate that the foam degasses at significant rates while its volume exhibits substantial variations. A simple geometric model is described to represent the basic mass transport and volume relaxation processes in a cellular system. The model is applied to the present material and is fitted to the experimental data. Furthermore, it is used to predict the effect of the type of the mechanical behavior of the polymer matrix, elastic or viscoelastic, of the mechanical properties, of the temperature and of the cell volume fraction upon the gas diffusion process in high-density foams.