BC Archival Preservation Service

The Canadian Council of Archives has recently published Guidelines for Humidity and Temperature in Canadian Archives.(1) This publication by Stefan Michalski of the Canadian Conservation Institute attempts to explain the recent reevaluation of relative humidity and temperature standards in archives and other heritage institutions. Michalski also offers some low, moderate and high cost practical strategies for the long-term preservation of a range of archival materials.

In the last few years the accepted relative humidity and temperature standards have been re-evaluated. Prior to this re-evaluation it was generally accepted that minimal fluctuation of both temperature (+/-2° ) and relative humidity (+/-3 to 5%) was considered optimum for the long-term preservation of collections. Maintenance of these setpoints was often difficult, costly and required sophisticated HVAC systems. Because of these reasons, in part, setpoint fluctuations were reviewed to determine what damage could be anticipated from allowing wider setpoint fluctuations.

The control of both relative humidity and temperature is important because both can have a profound affect on the life expectancy of your collection. A general rule is that with every 5° C increase in temperature reaction rates double. Or to put it another way archival records stored, for example, at 20° C records will have half the life expectancy of those stored at 15° C. The general rule for relative humidity is "half the RH, more than double the life".(2) Of course, there are limits as to the practical control and desirability of greatly reduced temperatures and relative humidity to various media.

A current theory is that as most archival records come to an archives from an uncontrolled environment (an office, historic house, basement) they have already been subjected to wide fluctuations in relative humidity and temperature and so much of the damage that is caused by these fluctuations has already occurred. Because of this, it is now thought that the stringent environmental standards set for the long-term preservation of archival collections can perhaps be somewhat relaxed. The need for control of relative humidity and temperature fluctuations is still required but the allowable setpoint fluctuations could be increased.

Michalski ranks archival records based on their chemical and mechanical stability. Three categories of archival records based on their chemical stability are listed: high, medium and low chemical stability. (For a more complete list see the Information Bulletin).

Mechanical damage to archival records is ranked by comparing high, medium and low vulnerability records against +/- 5, 10, 20 and 40% relative humidity fluctuations. The damage is ranked qualitatively -none to tiny; none- small damage etc. In Table 4 – Estimated Mechanical Risks to Records - it is only at the +/- 20% fluctuation level that small-severe damage is noted. However, as the footnote points out mechanical damage refers to irreversible deformation, fracture, and delamination. This damage is cumulative so that several thousand fluctuations of "small’ can accumulate to ‘severe’.(3)

Michalski has compiled data from a variety of sources regarding the effects of chemical and mechanical actions on archival materials. From this "Guidelines and Strategies – cost-saving and low, low/moderate-cost strategies" were compiled with regard to relative humidity and temperature recommendations.

"Guidelines and Strategies" Summary of relative humidity and temperature recommendations
Low/Moderate Cost Strategies				Moderate/higher Cost Strategies
Record Type	Temp	RH	Storage Required		Temp	RH	Storage Required
Medium stability	If summer > 25° C then A/C to 20° C	Low as possible – don’t overtax system	High value records – moisture proof encl.		Extend winter cond. Year round e.g.. 10° C	Extend winter RH e.g.. 30%	HVAC
High value Low stability	5° C –	40-65% - uncontrolled RH floats	Refrigerated room/Cold storage vault		Cold storage e.g. 5° C +/- 2° C	50% +/- 10%	Refrigerated room/Cold storage vault
Low stability- all but magnetic mead	-20° C	50%	CMI system or other like system - records must be in room with RH less than 50% when packed		-20° C	CMI system or other like system⁽⁴⁾ - records must be in room with RH less than 50% when packed
Records – high vulnerability to RH fluctuations					Cool	50% +/- 10%/week setpoint drift to 60% in summer and 40% in winter okay
Records – require to maintain precise dimensions e.g. aerial photos					18-25° C +/- 2° C in one day	45-55% +/_ 5%

So, how do we interpret this re-evaluation of environmental standards and what does it mean in practical terms to small archives with a limited budget and a "one size fits all" storage environment? Well…..

So in conclusion, I would suggest that it is worth the effort required to read the CCA Information Bulletin 15 - Guidelines for Humidity and Temperature in Canadian Archives. In addition to the practical suggestions offered in ‘Guidelines and Strategies- Cost Savings and low-cost strategies" the comprehensive footnotes provide interesting, useful and essential background information.

(1) Michalski, Stefan.Guidelines for Humidity and Temperature in Canadian Archives, Canadian Council of Archives Preservation Committee Information Bulletin 15, Canadian Council of Archives, Ottawa, 2000.

(4) McCormick-Goodhart, Mark.Methods for Creating Cold Storage Environments, Care of Photographic Moving Image & Sound Collections, Institute of Paper Conservation, Leigh Lodge, 1999.