The energy consumption requirements for buildings have been increased several times over the last few decades. The original external thermal insulation composite system (ETICS), installed around the year 2000, is no longer suitable nowadays and the buildings need to be renovated. One way of renovating a façade is to add an additional layer of external thermal insulation on the top of the original insulation and to create a new ETICS system. However, the question remains as to how such a multi-component system will change the moisture performance of the external wall, and how to avoid moisture condensation problems. This numerical case study examines the potential moisture regime of the selected external walls in Slovak conditions, identifying how this regime changes after the addition of insulation. The original ETICS systems considered in this study were typical of Slovak construction in the 1980s, consisting of a cement-based plaster, adhesive layer, insulation (usually expanded polystyrene or mineral wool), and finishing layer. The same representative ETICS configuration was used across all simulated cases, based on commonly applied materials at the time. The reference time period for the ‘original’ ETICS condition is approximately 1985–2000. The numerically simulated moisture distribution in the external envelope material components is used for the estimation and prediction of moisture content in of such additional insulation. The dynamic moisture behavior performed by the numerical simulation is compared with the simplified Glasser model for the design and evaluation of the moisture performance of external envelope compositions, which is still primarily used in the engineering practice. This comparison shows the possible consequences of the assumptions of the incorrect simplified heat and moisture models, especially under future climate conditions.