Determining the Acceptable Ranges of Relative Humidity And Temperature in Museums and Galleries: Part 1, Structural Response to Relative Humidity

Abstract

If one attempts to interpret the temperature and relative humidity (RH) information developed for Smithsonian Facilities Management (or any institution for that matter) it would be most useful to look at the raw data from monitors at individual sites. At the Smithsonian, the current environmental guidelines are 45% RH +/- 8% RH and 70o F +/- 4o F for exhibitions and storage spaces. (Mecklenburg et al, 2004) This simply means it is acceptable to be within a RH and temperature box bounded between 37% RH and 53% RH and 66o F and 74o F. Since the actual data is taken on an hourly basis over a 30 day period one can actually see the excursion events in terms of time and magnitude. This data can be interpreted in such a way that it shows both the actual HVAC system performance and allows a detailed analysis in terms of whether an excursion is actually causing problems for the collections. For example, does a 4 hour depression in RH of 4%-5% outside the allowable bandwidth have a significant impact on the chemical and structural stability of the collection? To answer that question one needs to know what the actual, allowable, RH and temperature ranges are and to examine the time it takes for moisture to enter or leave materials when there is a change in relative humidity. Before discussing the rate of moisture absorption it would useful to review some fundamental issues. This includes how the guidelines were originally established and what were the criteria used? Another fundamental issue is how to interpret the hourly monitoring temperature and RH data.

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