This paper, the first of two, presents a conceptual model of moisture concentrations in a building cavity. The model is comprehensive and general considering air infiltration, vapour diffusion and material hygroscopicity under non-steady state conditions. The resulting linearised coupled differential equations are analytically solved to study the case of long term cavity moisture behaviour. Dimensionless parameters and algebraic formulae are presented describing all important moisture performance parameters for a non-condensing cavity. Two primary time constants are identified, and a third,which governs the drying rate, derived. This allows the identification of three drying regimes based on cavity tightness. Some general design recommendations are given.