In Portugal, the external thermal insulation systems (ETIS) are nowadays a current technical solution in residential buildings, contrasting with the rarely used internal thermal insulation systems (ITIS). In this paper, a quantitative analysis on ITIS is done based on three hygrothermal façade requirements: interstitial condensations, thermal bridges, and temperature variations across the external wall. Computer simulation was used as research tool. The selected software applications were in compliance with the available European standards, particularly for the interstitial condensations and thermal bridges. The effect of distinct external climate conditions was evaluated, taking also into account the typical indoor hygrothermal characteristics of Portuguese residential buildings, namely temperature and ventilation rate. The results of this study quantitavely shows, for the analised hygrothermal requirements, that the ITIS option should not be disregarded at design stage, provided that some proper measures are guaranteed.