This book was written in order to serve as a guide for teaching, in the energy performance assessment. All calculations were made in accordance with the standards in force in Romania "C 107-3:2005, Thermal performance calculation on construction elements of the building" and "MC001/2006," "Methodology for Calculating energy performance of buildings." As shown by the calculations, thermal insulation is one of the most economical way to reduce energy consumption, especially now that gas prices have risen so much. For a healthy economy, potential future recommendations and guidelines is certainly in the energy save.