Exposure to particulate matter and dust, apart from worsening respiratory and long-term cardiovascular problems in humans, also facilitate water condensation on object surfaces, which subsequently makes them more susceptible to damage [21; 22] In addition, illuminece should not exceed specific levels, as it poses a threat to exhibits such as embrittlement, yellowing, and overall weakening [23]. Therefore, stable optimum microclimatic conditions and improved indoor air quality should go hand in hand with energy efficiency when planning a renovation strategy in museums. Nevertheless, most historic buildings were not designed initially to function as museums, and their original technical specifications do not always meet the rigorous requirements for artifact conservation. In addition, historic buildings are usually protected by country specific laws on cultural heritage; thus, interventions that jeopardize their architectural quality are not always allowed [24]. Thus, in order to control heat transfer and moisture fluctuations and to achieve adequate levels of thermal comfort and improved indoor air quality in an energy efficient manner, interventions should not be invasive, whereas their heating, ventilation and air conditioning (HVAC) system as well as any intervention on the building's envelope should be carefully designed [25; 26].Ongoing climate change is not only a threat to the integrity of buildings, affecting their degradation rates due to the severity of weather phenomena, but also calls for retrofits in order for buildings to sustain optimum thermal comfort conditions [27]. Especially in regions such as the Mediterranean zone, where heat waves are expected to become more intense and last longer [28], museums should provide shelter not only to employees and their thousands of visitors, but to their artifacts as well. Retrofitting approaches and upgrade measures in historic buildings and museums have been implemented with success during the last two decades within the Mediterranean and in other regions [29]. [PERSON] et al. presented a replicable method for renovating a historic building in Italy by combining both active and passive energy efficiency techniques, which led to average energy savings greater than 64% for heating and cooling [30], while in another work, an integrated approach for the restoration of heritage buildings by means of combined passive and active retrofit strategies was proposed [31]. Interestingly, [PERSON] et al. conducted a pre- and a post-intervention measurement campaign on eight museums across Europe, in which various measures to improve the microclimatic conditions and the buildings' overall energy efficiency were adopted. As shown in the results, the internal thermal comfort conditions were improved while also respecting the specific requirements of the exhibits, and on the other hand, adequate amounts of energy were conserved, with savings ranging from 39% to 77% [32]. This paper aims to contribute to the previously discussed framework by undertaking an indoor evaluation strategy as an upgrade measure in a historic building located in the Mediterranean zone (Athens, Greece) to not only enhance the already existing literature concerning IEQ and thermal comfort regimes but also to provide useful data for the development of a state-of-the-art energy retrofitting protocol for museums hosted in historic buildings.