Energy Performance (EP) of buildings is expressed by indices. They are representative of primary energy use, or final energy use, or carbon dioxide equivalent emissions. The criterions of indices determination, currently used in calculation procedures, don’t actually seem to meet the needs of check energy resources and related environmental impacts. They only measure the quantitative aspects. In practical application we observed a margin of uncertainty in calculation procedures. It results both from political and technical decisions: the relevance on results is greater the more extensive are the boundaries system takes in account. Exergy used as building performance assessment can be an attractive alternative compared to conventional procedures of EP determination. The aim is to give strength and uniqueness to calculation and results. Like EP expression, Exergy Performance can be expressed by an index too. Exergy analysis enables to evaluate the reduction of energy carriers quality levels. The proposal suggests a building evaluation method based on minimization first of building energy needs and then of related energy resources quality level.