OBSERVATIONS
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We are often asked whether it is possible to humidify an old building during cold weather to prevent the dryness that otherwise occurs in comfort-heated buildings. Although the theoretical answer is "yes," any attempt to maintain a significantly elevated relative humidity in an old building in winter carries with it such a risk of condensation damage to the building that the practical answer must be "no." The explanation for this apparent contradiction lies in the capacity of air to hold moisture. Warm air can hold more water vapor than cold--a lot more. When we say, "humidity," we usually mean relative humidity. Relative humidity is a measure of how much water vapor (moisture) is contained in a given quantity of air at a particular temperature, compared to the maximum that it can hold. In other words, a relative humidity of 50% means that the air contains half as much moisture as it can possibly hold at that temperature. Warm air can hold more moisture than cool. This means that as the temperature of the air rises, the relative humidity will drop (so long as no more moisture is added). Our problem, though, begins when the temperature of the air drops. Even if no water vapor is added, the relative humidity will rise. If the temperature drops sufficiently, it will reach saturation, or 100%. At this point the air contains as much moisture it can. If the temperature continues to drop, some of this water vapor will have to be gotten rid of. The process by which water vapor leaves the air is condensation. Water vapor becomes liquid water. When this happens outdoors, we get fog or precipitation. When it happens in a building, things get wet that shouldn't, wood rots, metal corrodes, and we have problems.
This mechanism can be pictured on a chart like the one above. The vertical lines represent temperature in degrees, Fahrenheit, the horizontal lines show absolute quantities of water vapor in the air, and the curved lines indicate relative humidity. The important thing to notice is how the relative-humidity lines get steeper from left to right (as the temperature goes up). The warmer the air gets, the more moisture it can hold, and that moisture capacity rises faster than the temperature. This begins to explain why it's nearly impossible to keep the interiors of heated buildings from drying out in winter. If the outdoor air temperature is 30 degrees F and the relative humidity is 50%, there is only sufficient moisture in the air to produce about 15% relative humidity once it is warmed to 65 degrees. Adding moisture to the indoor air will raise the relative humidity and reduce drying, but the temperature at which condensation occurs also will rise, increasing the probability that condensation will occur inside the building. New buildings can be designed to accommodate elevated relative humidities without damage, but it is simply not practical to maintain the relative humidity inside a heated old building high enough during cold weather to prevent dryness without risking serious damage to the structure. Back
to the top The conventional wisdom holds that in order to safeguard the collections it houses, the interior environment in a museum should be maintained at about 70 degrees F, and about 50% relative humidity, year-round, with only small fluctuations allowed. For most house museums, this is an impossible standard--Quite apart from the expense of installing, operating, and maintaining a climate-control system that can meet these criteria, these conditions will cause condensation during cold weather that will quickly destroy the building. During the past decade or so, conservation scientists have begun to question this "standard," which turns out not to be based on experimental knowledge. Perhaps surprisingly, temperature variation appears to have less effect on objects than do changes in relative humidity, although excessively high temperatures and freezing can both be harmful. To no one's astonishment, different materials turn out to be affected differently by fluctuations in humidity. Oil paintings, for instance, become brittle and paint can crack and flake off at low humidities, while paper molds and cockles at high humidities. Sensitivity to humidity variations differs according to many factors. For instance, painted or varnished wooden chests may respond to relative humidity change in a matter of days when they are open, in weeks when closed tight, but only in months when they are kept closed and full of textiles, which act as humidity buffers. Recent work at the Canadian Conservation Institute suggests that a variation in relative humidity of about ten percent above and below a central level of 45% to 55% presents a low risk of mechanical damage for almost all organic objects acclimated to mid-range humidity. The risk gradually rises, until at variation of about forty percent above or below the middle range, conditions will be destructive to most organic objects, if maintained long enough for the object to react. An interesting result of some of these discoveries is the careful use of heat--controlled by humidistats, rather than thermostats--to modulate the indoor relative humidity rise that accompanies the arrival of cold weather. Since personal comfort is not the goal, actual indoor temperatures can fall quite low, making this, therefore, a problematic approach for a building that is to be occupied during cold seasons. For buildings that receive occasional cold-weather use, the most conservative approach appears to be to make the heated periods as short as possible, to minimize the time that objects have to respond to the changing humidity. Vol. XXVIII No. 3 of the APT Bulletin is devoted to the topic of museums in historic buildings, and contains excellent and well-documented summary of the history and current state of museum environmental thinking. APT/ Box 3511/ Williamsburg/ VA 23187. Back
to the top WHAT IS YOUR MUSEUM'S COMPETITION? Many history museum organiza tions act as though they believed that their competition was the next museum up the road, or if they're thinking on a larger scale, Old Sturbridge Village, Historic Deerfield, or even Colonial Williamsburg. A friend of mine recently made a remark that makes me think that they're mistaken--he had just returned from a trip that included Williamsburg. "They do it better," he said, "in Orlando." This comment points up an important fact: History museums compete for people's leisure time. This means that they are competing against the New England Patriots, Loon Mountain Ski Area, the beach, television, the Appalachian Trail, the Internet, and yes, Disney World, too. Closely related to the question of why any one should visit your museum is the question, why should anyone support your museum financially? To ask this is not to put your museum down; rather it is the first step in focusing your museum and making it a more important part of its community. What about your museum makes it important? Why should some one visit your museum rather than go for a boat ride or watch a ballgame? Why should someone support it instead of any of a hundred other utterly deserving causes? These are hard questions. Thinking about them can lead your museum into new and worthwhile paths. Back
to the top WHAT'S ALL THIS MUSEUM STUFF, ANYWAY? I have written a fair amount on museum-related topics. Readers who are not involved with history museums might wonder what is the big deal. Museum-oriented preservation architecture can't be all that different from the "real world," can it? I think it is: In museum work, the place of the needs and taste of the users of an occupied building is taken by concerns for preserving the physical object (curatorial) and using it to tell a story (interpretation). Research and literal accuracy assume great importance. In order to work responsibly on buildings in curatorial hands it is important to find out as much as we can of their past, and to understand not only their architectural form and style, but also the social and technological contexts that gave rise to them. "Restoration," after all, means the process of returning a building to its known former appearance. History museums are stewards of objects from the past on behalf of the present and the future. This fact explains the great concern with preservation of historical fabric: The building that is there right now is as real as it's going to get. What we do, no matter how skillfully we do it or how desperately the work is needed, will remove bits of the historical artifact and replace them with new work, gradually altering and eroding the building's integrity. A building in curatorial hands has no "real" 24-hour-per-day user. No one lives in museum space; its purpose is different. Mundane things that are essential to a building that is actually used--a comfortable heating system, functioning kitchens and bathrooms, and light fixtures and convenience outlets--may not be needed, or may even be harmful to a museum structure and its contents. Building codes frequently recognize the special nature of museums' buildings by allowing the use of construction techniques and materials that are prohibited elsewhere. Most notable is the Massachusetts State Building Code's Totally Preserved Building category, which allows to the owners of buildings specifically listed in the Code great latitude in preserving their structures' fabric. All in all, museums are a different world. I believe that by working with museums I become more informed and knowledgeable about all old buildings and broaden my understanding of architectural preservation , and so can better serve all my clients, whether curatorial organizations, institutions, or private individuals. |
