Tell Me What You Want: Specifying Storage Conditions for Collections

When new or renovated collection storage areas are planned to be built in a library, museum, or archive, it is necessary for the institution to decide on environmental requirements (temperature and relative humidity conditions) for the new spaces. Usually a team of professionals is given the task to work together and, in general, it is easy to get agreement on the general goals for this process:

  • Obtain the maximum ‘preservation quality’ in storage areas to ensure longevity for collections.
  • Minimize capital and operating costs.
  • Ensure minimal inhibition of staff workflow and productivity.
  • Be environmentally responsible in choices of materials and energy use.

Once past the generalities, however, the specifics raise many questions that are difficult to resolve. As the process unfolds in real life, administrators, curators, architects, design engineers, mechanical contractors, building operators, and conservators have their own concerns. Ideally, each professional contributes expertise to an overall satisfactory result that makes sense to each participant and meets as many of the general goals as possible. Unfortunately, the ideal is not often reached.

Architects and engineers often ask the client in the design process for a specification of environmental conditions in the form of a target temperature and RH, with specified tolerances around the targets. The conservator and curator representing the client institution are expected to know or to easily determine what the ‘ideal’ condition should be for each collection space. Architects and engineers are often shocked not only at the lack of relevant, accepted standards, but the general absence of off-the-shelf guidance on how to create a museum and library environment that is suitable for particular collections. Unfortunately, conservators are often equally surprised at the shortage of directly relevant standards.

Some references do exist. There are standards or guidelines for practice developed by organizations such as National Archives and Records Administration (NARA), National Information Standards Organization (NISO), International Organization for Standardization (ISO), American Society of Heating, Refrigeration, and Air Conditioning Engineers (ASHRAE), and British Standards Institution (BSI) concerning various aspects of environments for cultural institutions. While these standards can be helpful in some circumstances—especially when ‘outside authority’ is needed to convince reluctant parties to the process —more often than not they disappoint both designers and institutional staff who are looking for simple, direct answers.

Certainly many climate system design precedents exist, some of them are innovative and energy-efficient. However, simply copying designs created for other buildings may not be feasible since local conditions and collection contents vary widely from institution to institution. Some precedents are also unworthy of imitation, and it can be challenging to know whether or not a system has had a successful outcome for collection preservation over the long term. The fact is that environmental specifications in the form of temperature and relative humidity (RH) targets and ranges are often adopted without a deeper analysis of their cost, contribution to preservation quality, sustainability, and operating simplicity.

A Very Short History of Temperature and RH Targets

Beginning around the time of World War II, major art museums became more aware of the risks of excessive dryness and dampness with paintings and furniture. The conservation profession began to emerge and practitioners wrote about the necessity of controlling the environment to improve preservation of collections. Control of RH between 40-70% or 50-60% was advocated to minimize the risks of dryness and dampness to fine art collections. There was a concern about embrittlement at RH levels below 40% and mold growth above 70%. Because the focus of concern was the effect of RH on fine art materials, the importance of temperature was diminished, and it did not seem to matter if the 'ideal' temperature conditions were crafted to be comfortable for humans. (Later research has shown that cool temperatures are essential for some materials and beneficial to many.) Gradually, steady RH near 50% at temperatures comfortable to humans (68°F or 20°C) came to be regarded as the best conditions possible and the self-evident basis for assessing environmental quality. Everyone likes simplicity, and everyone likes to be comfortable. Unfortunately, these recommendations were based more on practicality than research and analysis.

Although early conservators recognized the gross oversimplification that an 'ideal' like this represented, a number of circumstances helped to shape the popular acceptance of these concepts. Despite continued research on the effects of environment on collection materials, climate specifications were strongly influenced by human comfort needs and mechanical system limitations. Monitoring of conditions, if done at all, resulted in handwritten logs of temperature and RH or weekly charts made by hygrothermographs. Without the power of computers to do extensive calculations or condense a whole year of data onto one graph, derived statistics were nearly impossible to achieve. Weekly monitoring enforced a short-term perspective that worked against perceiving longer-term, more significant trends such as seasonal variation. Determining whether lines were flat or targets were achieved was about all one could learn from monitoring in this way. Deviations from 'ideal' ranges were easy to spot and by definition undesirable. 'Flat lining' (nearly unchanging temperature and RH) was seen to be the best for all collections, and had the added advantage of being easy to demonstrate to administrators and facilities staff.

Research-Based Guidelines for Preservation Environments

Making the best choice of environmental conditions for a collection begins by knowing the nature of the collection objects themselves, and from there deciding what forms of deterioration will be of primary concern. Targets and ranges (70°F +/- 2°F, 50% RH +/- 5% RH for example) are never ideal for the preservation of all collection types or materials. Targets cannot help determine the degree of risk or benefit that environmental conditions bring to collections. Cool temperatures matter more to a library or a photo archive than to a furniture or decorative art collection. Tight control of RH matters more to a furniture collection than a library. A rare book storage area with human comfort temperatures but tight RH control would be very good from the mechanical point of view but poor from the chemical point of view. Considering the great variety of many collections, it is obvious that no one condition would be equally good for everything, and that choices have to be made based on the primacy of certain objects and forms of deterioration.

Environmental specifications for collections that are vulnerable to chemical change could include:

  • Make it as cool as you can while avoiding mechanical (below 20% RH) or biological damage (above 65% RH)
  • Keep summertime dew points as low as possible

Environmental specifications for collections that are vulnerable to biological decay could include:

  • Minimize risk by avoiding high RH at moderate temperatures
  • Keep excursions above 65% RH to a few days or less
  • Keep summertime dew points low

Environmental specifications for collections that are vulnerable to mechanical damage could include:

  • Keep excursions below 20% RH or above 70 % RH as short and infrequent as possible
  • Keep wintertime dew points from being too low and summertime dew points from being too high

One helpful aspect of these guidelines for preservation environments is that all the modalities of decay can be modeled and measured. Image Permanence Institute (IPI) has developed algorithms that can start with temperature and RH data and calculate the risks and benefits to collections posed by chemical, mechanical, and biological decay. These algorithms, called Preservation Metrics, have been used successfully in many institutions to manage and plan storage environments. IPI has produced both hardware (its Preservation Environment Monitor® and PEM2 dataloggers) and software (Climate Notebook®) for this purpose. Metrics allow genuine risk assessment and mitigation approaches to be used with preservation environments because they yield quantitative estimates of the rate of the major forms of decay. Using Preservation Metrics during the storage climate specifications process allows you to calculate the degree of risk or benefit to collections over time, and to weigh this information against other factors such as cost and sustainability.


Share Information

It is very important that engineers and architects understand the rudiments of deterioration mechanisms and explore with their clients the underlying issues affecting collection preservation. Conservators, curators, and preservation specialists must understand the rudiments of HVAC operations and explore with architects and engineers the design of a safe operating envelope that maximizes preservation quality while being kind to operating budgets and to the planet. Finally, everyone in the specification process should keep the building operator, who must make complex designs and equipment function the way it was designed, informed about the broad role that mechanical system design and operation plays in the long term preservation of collections.