Moisture-safe retrofitting and energy optimization of facades in existing buildings may be challenging, especially for buildings with preservation-worthy or historical facades. Internal insulation changes the hygrothermal conditions of the original wall and may cause several problems, like biological growth and masonry deterioration. This paper explores the use of moisture-adaptive membranes as an additional moisture control measure for safe retrofitting with internal insulation. The results are based on WUFI® Pro simulations. Parameters like minimal and maximal vapour diffusion resistance, relative humidity (RH) range at which the membrane is most diffusion-open and -tight, indoor humidity class, orientation, and insulation type are varied and evaluated with a theoretical mould growth model. Results indicate that the adaptive membrane should already be highly diffusion-open at around 75% RH and highly diffusion-tight in periods with low RH (up to around 40%). Results indicate that this would ensure a moisture-safe construction in humidity classes 2–4 using mineral wool. When using organic insulation materials, there might be a risk in humidity class 4.