In the assessment of the indoor microclimate of heritage buildings, the general goal is to evaluate and improve users' comfort. In our opinion the comfort of users should not come first; the approach should be: "Heritage First", with priority given to the creation of the best microclimate conditions for the preservation of buildings, artefacts, etc. The case study described in this paper concerns an authentic exemplar: the Malatestiana Library in Cesena, Italy, listed in the UNESCO Memory of the World Programme. The indoor microclimatic conditions are exemplary because, since the year of its construction in 1454, its function as a library has remained unchanged: conserving the original book rests, same manuscripts and - most extraordinarily - the same indoor microclimatic conditions. The building does not have HVAC, never. Our hypothesis is that the lack of HVAC has allowed a perfect conservation of the book rests and of the manuscripts. In this paper we describe the results of the monitoring campaign; of how our research led to the discovery that microclimatic parameters change over a long period, as a result of simply opening windows to change the air, involving water vapour and CO2 variation within the air volume of Library. (C) 2014 Elsevier B.V. All rights reserved.