Climate risk assessment in museums : degradation risks determined from temperature and relative humidity data

The main subject of this thesis is the determination of climate risks to objects in museums on the basis of measured and/or simulated temperature and relative humidity data. The focus is on the quantification of climate related risks for the preservation quality of indoor climate in Dutch museums. Chapter 1 is an introduction of the background of museum indoor climate guidelines. These climate guidelines have developed over many years and are mostly very strict. The Dutch specifications, which differ from the guidelines in other countries, have been investigated. This leads to the research questions that have been examined in this thesis. In chapter 2 the methods of research are discussed. The background of the Dutch building process and the development of systems to improve the indoor climate are provided. 21 museums were selected based on their combination of Quality of Envelope and Level of Control. The measuring method followed in each museum is provided and the analysis tools are discussed. Obviously, the main boundary condition – the Dutch weather – is also described. In chapter 3 the effect of the indoor climate on degradation processes is described. The three most important processes are biological, chemical and mechanical degradation. Indoor climates are described by statistical parameters to provide information such as averages, short time fluctuations and seasonal fluctuations for temperature and relative humidity. The general climate risk assessment method is introduced, which analyzes climate data by determining the percentage of data that fits into each ASHRAE climate class. This method is applied to all museums investigated in chapter 4. It is concluded that the method has limitations: the general climate risk assessment method directly shows risks to objects but risks resulting from a short climate excursion e.g. extreme weather conditions or climate system failure are disregarded. Chapter 5 introduces a new method in which measured or simulated data on temperature and humidity are directly related to risks for objects. Biological, chemical and mechanical damage are assessed for four specific, well defined objects. Now the response time of objects is used to calculate the climate the objects experience, rather than looking at the climate around the objects. Risk analysis determines whether the object is safe, possibly to be damaged or likely to be damaged when subjected to the described climate. This risk analysis consists of 1) comparing climate data to germination and fungal growth isopleths for assessing the amount of fungal growth; 2) determining the average lifetime multiplier in order to estimate chemical degradation; 3) comparing strain in objects to determine whether deformation is elastic or plastic and 4) assessing the number of cycles during a century and determine whether a first crack occurs in the pictorial layer. This specific climate risk assessment method is applied to all museums investigated in chapter 6. Moreover, this method is compared to the previously described general method. It is concluded that this method is easy to use and that it provides more reliable results than the general climate risk assessment method. Chapter 7 uses a computational model to investigate the influence of changes to the building on the preservational properties of the indoor climate. This model calculates the indoor climate for combinations of building type and climate system type, as defined in chapter 2, using weather data. The output is converted into risks to objects, using the methods described in chapter 3 and 5. Firstly, the effect of building adaptations on climate risks and energy use is investigated, such as thicker walls and larger windows. It is concluded that especially the influence of ventilation rate and exterior surface area have a large influence on collection risks. Furthermore, the number of visitors has an effect that cannot be disregarded. Secondly, the influence of set points is researched, both on energy use and climate risks. The climate system tries to achieve a certain target value for temperature and relative humidity: the set point. The choice for this set point value has an influence on the risks for objects. Additionally risks for the building, such as mould growth or wood rot, largely depend on this choice for set points. Simulations in a well defined case show that improving the building envelope (reducing infiltration and increasing insulation) reduces energy use by a factor 8. These changes to the building, however, might interfere with the original character of the building. By making set points for temperature and RH dependent on the outdoor climate – "following the seasons" – and in some cases lower the difference between average indoor and outdoor conditions, energy savings up to 23% are possible. The risks for objects do not increase, on the contrary: in most cases the risk on chemical degradation for objects, which is closely related to indoor temperature, decreases. In chapter 8 it is concluded that a low risk indoor climate can be realized in almost all combinations of Quality of Envelope and Level of Control. In unheated buildings a minimal risk on fungal growth is present; temperature and relative humidity in monumental buildings are usually fairly constant and of little risk. This is also supported by the fact that lots of objects have lasted for centuries in unheated buildings. When heating is added a larger difference in relative humidity between summer and winter appears. This seasonal variation is noticed by most objects (their response time is shorter than a season); moderate risks on mechanical damage are introduced. When also humidification and dehumidification are added, the risks on mechanical damage to objects are reduced. Both local measures and centrally controlled climate systems are able to create a safe indoor climate for the objects. This is, however, dependent on the chosen set points and type of system. Close to a monumental envelope (or even behind double wall constructions) problems might arise due to fungal growth or condensation. Moreover, objects placed close to the envelope might still be exposed to high risks. This effect is largest when a lower Quality of Envelope and a higher Level of Control are combined. A display case or microclimate case usually provides a more stable climate around the object. Risks on mechanical damage are reduced considerably. Even the daily changes caused by a day/night regime or solar radiation seem to have little effect on the objects in a display case. The only exception is when display cases are exposed to heat sources such as halogen lighting: sudden and dramatic changes in temperature might also cause fluctuations in RH. The climate guidelines used until a few years ago were very strict; too strict to apply in most monumental buildings. Risks for most objects are however low. This is a great opportunity: guidelines can become less strict without increasing risks for objects. In addition, there is a large potential for energy savings. Important for risk assessment is also the introduction of new risks by introducing a climate system. A system failure might expose a large part of the collection to a climate that deviates considerably from the normal conditions. This is an extra risk that calls for a proper monitoring and notification system. Safety of objects is for a great part dependent on the human factor: proper management and routine checks are necessary.