Setting Standards for Conservation: New Temperature and Relative Humidity Guidelines Are Now Published

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CCI Newsletter, No. 24, November 1999

Setting Standards for Conservation: New Temperature and Relative Humidity Guidelines Are Now Published

by Stefan Michalski, Manager, Preventive Conservation Services

The lack of universally accepted standards for environmental parameters has been a problem in conservation for many years. It has been a common occurrence for engineers to arrive at a museum planning meeting with one set of guidelines, only to meet with museum staff who have a completely different benchmark in mind. This results in lost time, confusion, and frustration for everyone involved! A set of mutually agreed upon standards could not only simplify the planning process, but ultimately offer better protection to our precious collections as well.

In 1995, I was invited to participate on a technical committee1 that would write a new chapter (focussing specifically on museums, libraries, and archives) for the handbook of the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE). A great deal of debate had just taken place in the conservation profession on the implications of current research for such environmental guidelines, so it was a good time to capture a consensus and place it in this crucial handbook (which is a 'bible' to all Canadian and American mechanical engineers). The ASHRAE handbooks are revised on a three-year cycle, and our committee agreed to have the chapter ready for the 1999 edition. At CCI we were happy to centre our efforts on this collaborative project, rather than a revision of our old Technical Bulletin on the subject.

I was the lead author on the sections concerning 'why guidelines are necessary' and the target specifications for temperature and relative humidity (RH). To address the issue of permissible fluctuations for mixed collections, we adopted a range of acceptable levels (AA, A, B, C, D) to replace the old magic numbers. The table of specifications describes the risks to collections at each level as clearly as present knowledge allows.

Figure 1
An example of one of the figures I developed for the ASHRAE chapter. This is a set of "isoperms" (lines of constant lifetime) for archival materials that deteriorate rapidly by acid hydrolysis. Instead of the usual axes of temperature and relative humidity, the isoperms have been placed on the fundamental design tool of mechanical engineers, i.e. the psychrometric chart. Also shown is a line of constant equilibrium moisture content for most paper and gelatin-based records (the heavy dotted line that deviates slightly from the 50%RH line).

Level AA2 is essentially the old magic numbers, but it is no longer presented as the 'ideal'. It is described as "No risk of mechanical damage to most (sic) artifacts and paintings." Unfortunately, due to a typographical error on my part, the word 'most' crept in (subconscious caution no doubt), but it will have to be deleted in the next edition. Furthermore "Some metals and minerals may degrade if 50% RH exceeds a critical RH."

Level A3 fluctuation control is described as "Small risk of mechanical damage to high vulnerability artifacts, no mechanical risk to most artifacts, paintings, photographs, and books."

The distinction between level AA and A is an attempt to capture a common consent in the field. Although researchers and practitioners argue over the details, it seems fair to say that popular opinion supports level A as a general optimum, while at the same time recognizing that level AA offers a small but plausible decrease in risk to some collections.

Level B is similar to A, but permits unlimited winter setback in temperature, as needed by seasonal museums. Level C simply limits conditions to within 25-75% RH year round, with temperature not to exceed 30°C in summer. Most historic houses will do well to reach this. Level D stipulates only that humidity not exceed 75%RH, while still noting dampness as one of the most important conditions to avoid in a collection.

For general museums, galleries, libraries, and archives, the suggested RH set point is the familiar 50%. Temperature set point is given as "between 15°C and 25°C"; however, for every class of fluctuation control based on this set point, the description of risks indicates that "Chemically unstable objects unusable within decades." Thus the reader is continually reminded of the role of cool or cold storage set points elsewhere in the table.

Another aspect of the chapter addresses various issues on 'how to achieve desirable standards'. Bill Lull took the lead on writing this section, with assistance from Lou Kelter and Alexander Zhivov. We included short summaries on the role of display cases and humidistatic control, so that engineers would understood their role in humidity control. There is a section on building envelope issues (led by William Rose) as these invariably determine what is feasible in environmental control. We abridged Ernie Conrad's 'control potential' classification scheme4 for seven types of building, and suggested which level of fluctuation control is feasible for each. The chapter, therefore, provides guidance not only for large new museum projects, but also for small historic house museums and sheds.

The whole chapter has been through the revision and voting approval process of the ASHRAE organization. Thanks to the hard work of all committee members (especially our experienced and supportive chairman) and to the editors of ASHRAE (who granted us a lot of leeway in deadlines at the final stages), the chapter is now finished! "Museums, Libraries, and Archives" is Chapter 20 in the 1999 ASHRAE Applications Handbook.5 It includes recommendations for both temperature and relative humidity. Unfortunately, a well-supported section on pollution and filtration could not be prepared in time for the 1999 deadline, but this will be added for the 2002 edition.

When Lou Kelter first announced our intention to have the chapter ready for the 1999 edition at a meeting 3 years ago, the publications committee of ASHRAE burst into spontaneous applause. We hope that everyone is as satisfied with the final result.

  1. In addition to myself, committee members included: Chairman S. Louis Kelter (of Kelter & Gilligo, P.C.); William P. Lull (of Garrison/Lull Inc.); and William B. Rose and Alexander M. Zhivov (both from the University of Illinois).
  2. ±5%RH, ±2°C, no seasonal RH change, up 5°C, down 5°C seasonal possible.
  3. ±10%RH, ±2°C short-term, no seasonal RH change, up 5°C, down 10°C possible or
    ±5%RH, ±2°C short-term, seasonal RH change ±10%RH, up 5°C, down 10°C seasonal possible.
  4. Conrad, E. A table for classification of climatic control potential in buildings. Landmark Facilities Group, Inc., Norwalk, CT, 1995.
  5. Chapter 20, "Museums, Libraries, and Archives," in 1999 ASHRAE Applications Handbook (SI edition or I-P edition). American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., 1791 Tullie Circle, NE, Atlanta GA 30329, USA.