A one-dimensional transient formulation is derived to predict frost growth and densification on a cold wall submitted to a moist air flow. The model is based on a local volume averaging technique that allows the computation of temperature and density distributions throughout the entire frost layer according to time. It is particularly shown that the effective vapour mass diffusivity throughout frost should reach values several times larger than the molecular diffusivity, so that the model is in agreement with experimental data. In order to represent this phenomenon, a new expression for the so-called diffusion resistance factor is proposed. Values of an adjustable parameter appearing in this expression are correlated to heat and mass transfer boundary conditions, and to the global rate of densification. Two possible interpretations of the phenomenon are proposed.