International building legislation is setting stronger and stronger requirements for the energy performance of buildings. The most recent example is the Energy Performance of Buildings Directive in the European Union. The improved energy performance of buildings can't be achieved by additional insulation or more effective building systems only. A major influence factor on the energy quality is the ventilation technology and also the airtightness of the building. Some countries include in their energy decree already maximum air exchange rates, partly for all building types, partly only for those that include a mechanical ventilation system. Especially for high performance buildings which go beyond the national requirements, the infiltration losses become a significant factor to the energy performance. The paper presents a short overview on the existing airtightness requirements in different European countries and especially for high performance buildings as well as insights in how strong the impact of improved airtightness can be regarding the net, final and primary energy demand of a building.