## 1 Introduction
Both these aims have relevant connections with indoor microclimate conditions, but they are based on different principles: for cultural heritage, well-being means that the item is safe and that its natural aging proceeds at a very slow rate; instead, for people, well-being may be reached within certain temperature and relative humidity ranges, depending on the seasons and personal conditions. To make the microclimate of historical buildings more comfortable for people, HVAC systems are often installed (mainly with radiators or fan coils that control only indoor temperatures). All these systems are used intermittently and are located in specific areas, continually generating microclimate perturbations. For the purpose of conservation, the climate needs to be constant, but for people, it needs to be mild. These factors create a complex situation in which it is necessary to compromise between human comfort and conservation requirements and the need to contain costs [19]. [PERSON]. [PERSON] et al., in their study of Necip Pasa Library in Turkey [20], highlight passive solutions: regarding the HVAC system, higher priority should be given to not disturbing the environmental conditions to which the volumes have acclimatized over time. Instead, in their research, [PERSON] and [PERSON] focus their attention on the importance of long-term monitoring to prevent the deterioration of works of art and verify the capacity of HVAC systems to maintain desired thermo-hygrometric values within operating conditions [21]. The authors, in these cases, agree on giving priority to conservation needs. Another important aspect to be considered in microclimate analysis, both for conservation purposes and for human comfort, is indoor air quality in terms of pollutant concentration. One of the most relevant is carbon dioxide (CO\({}_{2}\)) concentration, whose monitoring is important for two reasons. First, for air quality, because high values make the environment unhealthy. Second, for book conservation, as high values may contribute to making the environment unacceptable, especially when relative humidity is high. For this reason, in libraries and museums, the presence of people can be considered an additional microclimate alteration factor (increase in temperature, hygrometric degree, and pollutants) [16; 22]. In fact, it is necessary to assess the fluctuations of this indoor air pollutant in relation to human presence so that human well-being can be guaranteed without altering the indoor microclimate for conservation purposes. This is an important aspect, and its control is generally delegated to a mechanical ventilation system, but when this cannot be installed, air exchange may take place through natural ventilation systems such as manual window openings (irregularly made by library staff). This is the context in which this paper investigates a multidisciplinary approach to the study of the conservation environment of valuable books housed in historical buildings. The study originates from the need for librarians to investigate the suitability of indoor environments for conservation. In fact, some damage has already been detected on the volumes, such as the darkening and discoloration of many ribs and paper dissolution, which is also responsible for the accumulation of dust on shelves. Adopting a multidisciplinary approach means involving different professional figures (librarians, engineers, architects, restorers) in order to define guidelines for librarians, in which conservation requirements are combined with the need for human comfort. This approach is based on a suitable analysis methodology for libraries in historical buildings in order to study retrofitting strategies and to define zero-cost interventions, such as a protocol for curators to manage manually opening windows and solar shading closing. The case study concerns the \"Sala del Dottorato\" (Hall of Graduates), located on the first floor of Palazzo Murena, a historical building that is the seat of the University of Perugia, Italy.