Volume 15, Number 2
Apr 1991

The Editor Endorses Low RH

My considered opinion on the issue of low versus high relative humidity for storage of collections is that low RH is best. All the research I have seen or heard of favors a low RH for extending the longevity of library and archival materials... much lower than the 50% typically and traditionally recommended by museum conservators. Even photographic film, a composite material made up of dissimilar layers of materials, lasts much longer at 20-30% RH, four to 10 times as long for acetate and nitrate film.

On the other hand, conservators and other nonresearchers in libraries and archives (and museums) agree neither with me nor each other, recommending RH levels all the way from 40% to 60%. I believe this is because some are following museum practice, even though libraries do not have many museum-type problems, e.g. cracks and warping in ancient boats. The wide range of RH recommendations in the literature suggests to me that some practitioners and advisors are reading the research, while others are not, or do not know how to interpret it.

Perhaps some controlled field observations on this question would give everyone a better sense of the magnitude of the factors involved, and build a more compelling case for low RH. Bill Barrow, where are you now when we need you?