The present paper is based on showing the work carried out by the authors in an FP7 EU project “REtrofitting Solutions and Services for the enhancement of Energy Efficiency in Public Edification (RESSEEPE)” developing and adapting different numerical simulation tools for the thermal and fluid dynamic behavior analysis of different technologies for retrofitting solutions. Four of the different technologies that are being implemented in four European public buildings have been numerically analyzed in order to have a better understanding of their behavior and virtually analyze their energy efficiency for optimization purposes. Aerogel-based super insulating mortars have been analyzed looking for optimum size of aerogel mixed with the mortar looking for the lower thermal conductivity. Influence of envelope and thermal bridges on vacuum insulating panels have been studied to analyze their influence on effective thermal conductivity. PV panels coupled with ventilated façades have been studied in order to obtain the optimum gap of the ventilated façade depending on working and climate conditions. Finally, new flat plate solar collectors have been optimized by means of different numerical simulation tools obtaining a high efficiency thermal collector for a wide range of working conditions. These different technical solutions have not only numerically analyzed for optimization purposes but also studied for different working conditions under specific climate weathers to be implemented in four different real public buildings as retrofitting solutions. The numerical methodologies, the numerical results obtained, different experimental comparisons and energy reduction expected are analyzed, presented and concluded.