Definition of allowable targets from indoor climate observations in exhibition rooms: the case study of the Rosenborg Castle (Denmark)

The Atmospheric Physics applied to the preventive conservation of cultural heritage plays a key role in the assessment of the climate-induced degradation. Long-term and continuous data are necessary to assess the evolution of hygrothermal variables over time and their impact on the works of art, both movable and immovable. In the framework of the European CollectionCare project (N. 814624), the indoor climate data collected inside the Rosenborg Castle (Copenhagen, Denmark) are one of the most representative in this context. Indeed, thirteen thermo-hygrometers have been located to the six floors of the castle since 2011 at a sampling frequency even below 1 hour. The aim of this contribution is to provide a thorough analysis of the indoor climate trends in exhibition rooms according to the methodologies currently recommended in guidelines and European standards. The analysis focused on Room 7 of the Castle, as hygrothermal data collected in this room fitted the criteria of high quality of time series in terms of completeness and continuity at yearly and monthly basis. In this way, six years of data from 2013 till 2018 were investigated. A comparison of hygrothermal data over the selected years was performed to point out the evolution of seasonal and short-term cycles, especially of relative humidity, i.e. the key climate variable responsible for degradation of organic and hygroscopic materials. Moreover, indoor seasonal trends were assessed with respect to the influence of outdoor climate data, gathered from the European Centre for Medium-Range Weather Forecasts (ECMWF). Finally, a target indoor climate, based on the hygrothermal variability accepted for historical buildings, was defined from the collected data with the aim to analyse the discrepancies among the years from the target indoor climate in order to identify possible risky conditions.