One of the precautions taken in building industry in order to minimize the results of increasing energy consumption in the world is energy efficient design of the utilization process of the buildings. In order to ensure energy efficiency in buildings, heat loss through building envelope is tried to be minimized. In Turkey, recently, external thermal insulation applications have been proliferating with the effect of legal obligations and the needs of providing fuel preservation within residences. In this paper, it is researched how hygrothermal performance of the existing building walls would be affected by external thermal insulation applications. In the first chapter of this paper, external thermal insulation composite system (ETICS) applications observed in Istanbul Capa district are dealt with. In this region, it has seen that there are impermeable coatings on the existing wall surface of buildings where external thermal insulation is applied. In the second chapter, changes in water content, relative humidity rate and temperature of different wall types are calculated via simulation method. According to the results, when thermal insulation materials are applied on impermeable coatings on the walls, the moisture amount in the wall layers may reach critical levels in winter and this situation can cause deteriorations in the wall layers in the long term. For these reasons, it is important to handle external thermal insulation applications on existing buildings with a holistic approach and expertise.