Volume 17, Number 1, Jan 1995, p.24

Some Thoughts on the Recent CAL Press Release on Climate Control for Cultural Collections

Reprinted with permission from The Abbey Newsletter Nov 1994, pp.85-86.

While the research being conducted at the Conservation Analytical Lab (CAL) by Erhardt, Mecklenburg, et al., is extremely important to the conservation field, the conclusions drawn by the Smithsonian's press release ("Work of Smithsonian Scientists Revises Guidelines for Climate Control in Museums and Archives", August 21, 1994) appear to me to be premature. They could easily be misleading to museum administrators who are understandably eager to believe they can save vast sums of money with no ill effects to their collections. Indeed, as conservators, many of us have innumerable important projects we would like to see funded, and if there really is an imminent windfall here, we would all like to participate in it. Not to do so would be irresponsible.

To maintain the conditions described in the press release--35% to 65% relative humidity and 52 to 88 degrees Fahrenheit--museums in most climates will still need heating, ventilating, and air conditioning (HVAC) systems with humidification, dehumidification, reheat, boilers and chillers, fans, ducts, and controls (thermostats and humidistats); this basic equipment is required whether or not a stringent 50% ±2% is maintained. It is true that to maintain very tight control, some museums have installed the most accurate humidistats available, which are indeed more costly than those required to maintain a wider fluctuation. Therefore, the research does seem to support the conclusion that some savings on humidistats are likely. However, even with expensive humidistats, the capital cost of HVAC in a recent museum renovation here in Pittsburgh (the Andy Warhol Museum) represented only about 20% of the total construction costs; the costlier humidistats were but a small fraction of that. The press release suggests that HVAC costs may run as high as 50% of construction costs.

Energy Costs and Maximum Safe Excursions

The researchers found that films of rabbit skin glue (a material very common in paintings, for example, and similar to materials present in many other types of cultural collections) can withstand fluctuations of ±15% at a moderate relative humidity of 50% RH, but only ±8% at the lower setpoint of 35% RH (Erhardt and Mecklenburg, "Relative Humidity Re-examined", in Preventive Conservation: Practice, Theory and Research, Preprints of the Contributions to the Ottawa Congress, 12-16 September, 1994. IIC, 1994). If museums adopt seasonally-adjusted RH setpoints, as the press release suggests, then collections are likely to be exposed to RH at the 35% setpoint for many months during the winter; to avoid damage, the tighter range of ±8% must then apply. Therefore, reading between the lines of the research itself, I conclude that our HVAC systems must be designed to maintain RH control--at least for part of the year--at the tighter ±8%, and not at ±15%. A range of ±8% is in fact not very much more than the ±5% that many museums currently aim for as "standard".

If ±8% is in fact our goal, as the research seems to support, albeit indirectly, then great savings are probably not to be had in energy costs, according to a computer-modelling study commissioned by the Getty Conservation Institute (Ayers, Lau, and Haiad, Energy Conservation and Climate Control in Museums. 1988, GCI). This research concludes that the difference in operating cost between maintaining RH at ±2% vs. ±7% is minimal, on the order of 3%. Of course, if the wider fluctuations are safe for our collections, we should take the savings, however minimal, and apply them to more fruitful causes.

The press release suggests that lower winter setpoints are not only just as safe for collections, but also safer for the buildings that house them. This depends, however, on the building. Some modern construction with no vapor barrier in the building envelope cannot even tolerate as much as 30% RH without substantial risk of condensation and damage to the building structure. As for new construction, the cost of installing vapor barriers is not particularly great, and if avoided, would not yield major savings. Of course, to retrofit an existing museum building with vapor barriers would indeed be very costly; some museums may in fact be able to avoid this step if the seasonally adjusted RH setpoints suggested by CAL are indeed safe for the collections. Unfortunately, the lower winter setpoints do not translate into savings on capital investment in HVAC equipment, because the same basic equipment is required to keep RH at 30% as to keep it at 50%.

Reliability of RH Measuring Instruments

At the recent IIC Congress, Jonathan P. Brown presented a paper entitled "Hygrometric Measurement in Museums: Calibration, Accuracy, and the Specification of Relative Humidity." The gist of the study was that RH measuring instruments, even when they are carefully calibrated (which is difficult and therefore rarely the case), are not necessarily giving us a realistic picture of our environments. This could be because an instrument is accurate in one RH range, but not in another, or because even if perfectly accurate, the instrument can only measure RH at one point in the building. Because of widely variable air flow, stratification, and microclimates, the RH measured at any given point may not represent the RH experienced by an object elsewhere in the same room. The author suggests that because of this, stringent RH specifications are irrelevant, because they cannot be reliably measured or confirmed.

My own conclusion from these facts is that we already have far less control over RH than we would like to believe. In spite of our attempts to maintain tight RH control, misguided or not, our collections are probably already experiencing conditions perhaps not unlike those now suggested in the press release, precisely because of the factors outlined in Brown's paper. If we begin to aim for wider fluctuations, our collections are likely to experience an even wider range of fluctuations than we intend, as they already do. Thus, we may believe we are specifying and maintaining an acceptably safe fluctuation, but in fact, at the object level, we may be exceeding the safe limit.

I would guess that conservators in institutions that strive for the best possible RH control still see, as I do, evidence of environmental damage. Try as we might, we seem unable to eliminate cracking, flaking, desiccation, and other signs of deterioration. Many of us try to further buffer our collections from the environment by means of microclimates or housings in an attempt to prevent damage, often with some success. I believe our collections present us with ample empirical evidence that attempting to minimize climatic damage is a worthy cause. It would be hard for me to ignore my experience and defer to the conclusions of the press release, which tell a much different story.

Susceptibility of Individual Items

Before a recent upgrade of our HVAC system, we had limited humidification capability (humidifiers were located in the central air handler only, rather than in the ducts serving each individual gallery zone, as they are now). As a result, winter RH levels frequently fell below 30%. On one of these occasions, a lacquer cabinet on loan from a major museum cracked and then tented and flaked out of control. Within a matter of hours, the object was so seriously damaged that it had to be removed from the exhibition and sent home.

An unwitting museum administrator, acting on the implications of the CAL press release, might make the mistake of compromising humidification and winding up with a system like the one we used to have, believing it to be perfectly adequate. Obviously, it would not be. It is important to note that nowhere does the research itself suggest that RH below 30% for this kind of material is safe. The press release does suggest, however, that equipment used for climate control in museums is probably excessive and unnecessary. It is this suggestion that lays the groundwork for unfortunate misinterpretations and omissions that could turn out to be unsafe for collections.

Furthermore, the lacquer cabinet incident is evidence that even among large classes of objects, there are indeed individual differences in response to RH; the cabinet was the only object, among many similar ones, damaged by the low RH episode. This experience leads me to question the conclusion offered in the press release that because "the [research] is capable of defining the tolerance limits of large classes of materials. . . we don't have to study every single object".

HVAC Systems Protect Collections in Other Ways Too

The press release also omits a discussion of one of the more important aspects of HVAC: ventilation and filtration. There is a body of research demonstrating that air stagnation (which can lead to mold growth), dust, and certain pollutants are unquestionably damaging to many types of collections. If an institution opts to reduce these damaging factors, the resistance of filter sections at the targeted ventilation rates will probably dictate the requirements for fan sizes and the horsepower for fan motors. These are almost certainly more expensive than equipment needed for more routine application, but they have nothing to do with temperature and RH control. By focusing the HVAC question on climate control alone, the press release leaves the impression that the preservation benefits of filtration and ventilation are irrelevant.

In conclusion, there is no doubt that we as conservators and preservation managers must continue to learn from researchers, that we continually examine and when necessary revise our standards, and that we responsibly allocate limited resources where they most effectively benefit our collections. While the CAL research is a good example of how this process can work, the Smithsonian press release seems instead to undermine the validity of the research by drawing conclusions from it that are not supported by the facts. Perhaps the press release will turn out to be beneficial to the field insofar as it generates debate and encourages re-examination of the facts.