Over the last 10 years there has been an increased focus on how internal insulation of the existing building stock affects the hygrothermal performance of the constructions. This study investigates the hygrothermal performance and risk of mould growth in critical locations in internally insulated solid masonry walls when different types of interior paint treatment are applied. The study comprises two parts: field measurements from a large-scale experimental set-up, and a WUFI® Pro simulation study of the effect of paint treatments on different types of internal insulation systems. The focus is on diffusion-open and capillary active insulation systems (autoclaved aerated concrete and calcium silicate). Test walls were applied either with the recommended diffusion-open paint or a widely used acrylic paint. Measurements indicated that the use of common acrylic wall paint does not affect the hygrothermal conditions in internally insulated solid masonry walls considerably, compared with walls painted with the recommended diffusion-open paint. The simulation study did however indicate that in certain situations the use of acrylic paint would be beneficial, while in others diffusion-open paint was preferred.