The monitoring and control of thermo-hygrometric indoor conditions is necessary for an adequate preservation of cultural heritage. The European standard EN 15757:2010 specifies a procedure for determining if seasonal patterns of relative humidity (RH) and temperature are adequate for the long-term preservation of hygroscopic materials on display at museums, archives, libraries or heritage buildings. This procedure is based on the characterization of the seasonal patterns and the calculation of certain control limits, so that it is possible to assess whether certain changes in the microclimate can be harmful for the preventive conservation of artworks, which would lead to the implementation of corrective actions. In order to discuss the application of this standard, 27 autonomous data-loggers were located in different points at the Archaeological Museum of l’Almoina (Valencia). The HVAC system (heating, ventilation and air conditioning) at the museum tries to reach certain homogeneous environment, which becomes a challenge because parts of the ruins are covered by a skylight that produces a greenhouse effect in summer, resulting in severe thermo-hygrometric gradients. Based on the analysis of temperatures recorded during 16 months, the air conditions in this museum are discussed according to the standard EN 15757:2010, and some corrective measures are proposed to improve the conservation conditions. Although this standard is basically intended for data recorded from a single sensor, an alternative approach proposed in this work is to find zones inside the museum with a homogeneous microclimate and to discuss next the average values collected in each area. A methodology is presented to optimize the application of this standard in places with a complex microclimate like this case, when multiple sensors are located at different positions.

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The quality and quantity of thermo-hygrometric data are essential to carry out an appropriate assessment of the microclimate from a preventive conservation standpoint in those spaces where the artefacts to be preserved are located. These analyses are fundamental for long-term preventive conservation plan to assess chemical, biological or fracture risks. However, many small and medium-sized museums as well as heritage buildings have only a limited amount of historical data, with various problems that hinder the evaluation of microclimatic conditions. Two of the most common problems are short monitoring time periods, usually less than one year, and low sampling rates of measurements. In many of these situations, guidelines such as the European standard EN 15757:2010 cannot be applied because they require a monitoring period of at least 13 months and a minimum sampling frequency of one measurement per hour. In addition to these issues, there are other drawbacks such as missing values or lack of regularity in data collection. This paper proposes a procedure for the reconstruction of historical thermo-hygrometric data using multivariate statistical methods. The methodology allows the arrangement of long historical series of sufficient quality, enabling museums to restore their datasets for further analysis regarding the application of guidelines for preventive conservation. The methodology has been validated on the basis of real data. The application of the European standard EN 15757:2010 is presented as a practical example of the procedure using historical data collected at a partner museum of the H2020 CollectionCare project, together with data currently being collected for some months by a set of wireless sensor nodes.