We present a ID numerical model of heat, steam, and water diffusion across a wall, consisting of several layers of different material. The model is a system of coupled diffusion equations for material temperature, vapour pressure, and water concentration in material pores, accounting for vapour condensation and water evaporation. The system of nonlinear partial differential equations (PDEs) is solved numerically using the finite difference method. The primary objective of modelling is the simulation of the long-term behaviour of the building wall moisture distribution under the influence of seasonal variations in atmospheric air temperature and humidity. (C) 2014 Elsevier B.V. All rights reserved.