Air tightness and RH control in museum showcases: concepts and testing procedures

In museums, Relative Humidity (RH) control is a fundamental action to provide a proper and safe conservation of artworks. Indeed, the majority of collections must be preserved by keeping a RH highly controlled and stable over the time. The tolerable variations, in these applications, are far smaller than those required for the comfort of human beings. Such demanding requirements pose a lot of challenges, if they have to be satisfied by adopting traditional HVAC systems. A solution to these problems is represented by the adoption of passive requirements for museum showcases. This approach allows to realize a “box in box” configuration, where the first and rough climatic control is exerted by a usual HVAC system (which provides a not so strict control of the indoor environment in the museum room), while the “fine and local tuning” of the RH is achieved by means a passive approach into the showcase (e.g. using “buffer materials”). Such approach reveals to be effective only if the showcases have a sufficiently good air tightness. In this paper, authors will discuss the concepts that allows the control of the RH, analysing the behaviour of the showcases and relating their air tightness with the achievable indoor environmental conditions. As it will be highlighted, the air change rates involved in these phenomena are very low and such feature requires suitably adapted measurement procedures. The tracer gas decay method has been used by the authors to assess the air and gas tightness of different showcases installed in the museum. Moreover, the showcase indoor environmental conditions have been measured during a long term monitoring which allowed the capability of controlling the RH to be related with the air tightness properties. Finally, results are critically analysed in order to abstract general recommendations about air tightness and RH control in museum showcases.