In Europe, about 40% of the energy is consumed in buildings, more than by industry or transport. However, there is a great potential for energy savings in this field, often at little cost. A new European directive and several European standards, including the comfort standard EN15251, were created to develop comfortable and efficient buildings. This paper presents the interaction of this specific standard with the application of energy efficiency regulations. In order to evaluate the impact of the EN15251 application in commercial buildings, a case study was analyzed using the dynamic simulation software TRNSYS-The Transient Energy System Simulation Tool. The building was set according to the standard conditions specified by the national regulation, and the assessment of comfort requirements of EN15251 was verified. It was found that the current comfort requirements of the Portuguese regulation are not sufficient to achieve by themselves the comfort categories specified in the EN15251. About 55% of the comfort hours could not be assured. Furthermore, reaching the main comfort requirements (temperature/fresh air rates) of the EN15251 does not lead directly to the assessment of the corresponding categories. Results showed that the building stayed one comfort category behind from the desired when the correspondent operative temperatures were secured. On the other hand, it was found possible to achieve the comfort categories by increasing the operative temperature ranges, imposed by the standard, about 1 degrees C. This has a negative consequence, which is the increment of energy consumption. However, there is a large room for maneuver to reduce this consumption into acceptable levels according to the EPBD. (C) 2011 Elsevier Ltd. All rights reserved.