Public buildings represent an important opportunity to depict suitably how energy efficiency may impact on the two most crucial aspects: management costs and thermal comfort. In this regard, many of the secondary schools in Portugal are facing the structural decay of their facilities. Among all, EB23 Escola Conde de Oeiras, built in 1982 in Lisbon district, must enhance its current energy performance in order to guarantee acceptable environmental conditions to students and employees. To accomplish that, it needs to define a number of measures that could provide immediate and permanent effects. With this premises, the present study aims to identify a systematic approach to assess indoor environmental quality related to the implementation of adequate energy efficiency measures for Escola Conde de Oeiras.