Internal thermal insulation composite system (ITICS) can be an important measure for the energy-saving retrofitting of buildings. However, ITICS may cause harmful effects on the hygrothermal performance of building envelopes. This work investigated the influence of the materials' hygric properties on the hygrothermal performance of a typical ITICS in different climate conditions in China. Two base wall materials, the traditional concrete and a new type aerated concrete, were tested and compared for their hygric properties firstly. The influence of the hygroscopicity of exterior plasters, the permeability of insulation materials and the climate conditions were then analyzed with WUFI simulations. The hygrothermal performance was evaluated with consideration of the total water content (TWC) of the walls and the moisture flux strength, the relative humidity (RH) and the mould growth risk at the interface between the base wall and the insulation layer (B-I interface). The numerical analysis implies that the TWC of internal insulated walls depends mainly on the hygroscopicity of exterior plaster and the wind-driven rain intensity. The upper limits for the water absorption coefficient of exterior plasters used in Beijing, Shanghai and Fuzhou are 1e-9, 1e-10, 1e-10 m2/s respectively. When such limits are guaranteed, a vapour tight system created by using insulation materials with a large vapour resistance factor or adding a vapour barrier can improve the hygrothermal performance of ITICS, especially for concrete walls in cold climate.