In this paper, potential energy savings by implementing different energy saving methods to enhance the building envelope are presented and calculated through transient simulations using TRNSYS software. To do that, a reference building was selected. In order to develop an accurate model, a dwelling of that building was monitored during 3 months, and data obtained in that monitoring study was used to calibrate and adjust the simulation model. The monitoring study as well as the definition of the building model and the different assumed hypothesis are presented in this paper. Then, different energy saving measures (ESM) are defined for roof, façade and windows, and 64 combinations are simulated. Those results, which are evaluated under economic and energy criteria, are assessed using as reference thermal requirements fixed by Spanish regulation (both for new buildings and for building renovations). These results show how energy renovations in buildings involve important benefits not only under an energy or environmental approach, but also considering economic issues. Moreover, they evidence how the Spanish thermal regulation can be still toughened in order to meet European goals led to the mitigation of CO2 emissions. Many thanks are due to the Laboratory for the Quality Control in Buildings (LCCE) of the Basque Government, and to Bilbao Social Housing. Iker González-Pino also wants to acknowledge the financial support of the Basque Government, through the Department of Education, Universities and Research's Personnel Research Training Program BFI-2011-153.