## 1 Introduction  
Indoor microclimatic conditions play a key role in the conservation and maintenance of heritage volumes, and their control is necessary in order to limit the processes of damage and subsequent restoration work while also increasing their patrimonial value [1, 2, 3, 4, 5, 6]. Library collections consist of a wide range of hygroscopic organic materials (paper, leather, parchment, cloth), which are climate-sensitive and vulnerable to the process of deterioration. This natural process can be accelerated by the environment housing the collections, depending on many factors: indoor temperature, relative humidity (RH), pollutant concentrations, lighting, etc. The principal risks affecting the durability of library collections include mechanical, chemical, and biological mechanisms, as well as photodeteterioration. Several recent studies have investigated microclimate data collected in libraries, evaluating various standards and guidelines for heritage conservation and using various methods for assessing deterioration risks [7, 8, 9]. Italian technical regulations and guidelines concerning cultural heritage conservation, such as UNI 10829:1999 and D.M. 10.05.2001 [10, 11], list optimal and acceptable ranges for temperature and relative humidity, as well as establishing guidelines and methods for measuring indoor temperature and relative humidity values; however, in some cases, when an artefact has acclimatized to specific historical climate conditions over the years, it is advisable to maintain these conditions rather than introduce standard ones [12]. In addition, unsuitable handling of books is frequently responsible for the accumulation of wear and tear on paper, thus reducing the durability of the collections. Moreover, since library materials can shrink/swell as they lose/gain moisture, fluctuations in temperature and relative humidity can be studied as mechanical stressors potentially inducing dimensional changes in library objects [13]. This phenomenon is particularly relevant in historical buildings, especially those not equipped with HVAC (Heating, Ventilation and Air Conditioning) systems or whose envelope is characterized by massive walls with vast hygrothermal inertia. In this case, the indoor microclimate is particularly influenced by the topographic site, the orientation of the walls, the location of the openings and construction materials [14]. Furthermore, for all materials, indoor microclimates should be continually controlled to reduce or eliminate the risk of biological attack by fungi, bacteria, or other parasites. Not only does temperature have a significant effect, but relative humidity can also influence the speed of many deterioration mechanisms of a chemical, physical and biological nature. In fact, biological damage (entomological and microbial), as well as foxing (biological or chemical components), also depend on relative humidity and temperature, which can favour infections and infestations, in addition to deformations, stiffening, ink offset and dehydration. Where the degradation of books and documents in libraries and archives is caused by physical-chemical and biological factors, conservation involves a preventive examination of environmental conditions and a methodology of intervention providing for biological sampling followed by laboratory analyses. A treatment plan may involve disinfection and cleaning by methods suitable for conservation; in fact, when the habitat becomes favourable to pests, microorganisms, insects, or small animals, accurate and frequent cleaning becomes essential. Several works analyse the contribution of architectural features and construction materials to passive climate control [15; 16; 17]. Normally, the features of the buildings that house the collections behave as a filter of the external climate, buffering the external weather fluctuations [18]. However, the purpose of libraries is not only to guarantee the conservation of books but also to be comfortably accessed by guests and the public.