General Care and Preventive Conservation

by David Grattan and Stefan Michalski

This general introduction to the current approach to controlling ambient RH and temperature in museums is intended for all museum professionals. It is based on the "Museums, Galleries, Archives and Libraries" chapter in the American Society of Heating, Refrigeration, and Air Conditioning Engineers Inc. (ASHRAE) Handbook, a rather technical document intended primarily for engineers designing, maintaining, or operating HVAC systems in buildings that house heritage collections. The approach taken in the chapter represents a departure from earlier more traditional thinking about museum environments, which called for stringent control of RH and temperature. In the current approach, RH fluctuation is linked to measurable damage in artifacts. Certain types of artifacts are much more sensitive to RH fluctuation than others, and it is neither economical nor environmentally acceptable to have very tightly controlled conditions if they are not necessary.

The Role of Temperature and RH

Temperature and RH are directly related:

  • when a volume of warm air is cooled, its RH goes up
  • when a volume of cool air is warmed, its RH goes down

Because of this interdependence, temperature and RH are usually considered together. The damage caused by incorrect temperature and incorrect RH falls into three broad categories:

General Set Points

The target value of temperature or RH that a mechanical system is designed to maintain over time is known as the "set point". However, even the best mechanical systems will produce values that fluctuate above and below the given set point.

The term "set point" can be used in two ways:

  • to refer to the setting of the thermostat or humidistat over a short period of time (hours, days)
  • to refer to the average annual setting of the thermostat and humidistat (because the set points may be adjusted over the year for various reasons, such as energy saving)

Note that the "set point" is often defined by museums as 50% RH with the temperature between 15 and 25°C, although it can also be based on the historic averages. In practice, it may be defined by factors such as the needs of the collection, the performance of the building plus the HVAC system, and the climatic variation in temperature. On the other hand, class of control is defined by the degree of fluctuation in temperature and RH. And it is fluctuation rather than set point that we now strive to control because fluctuation provides the main threat to most artifacts and Class of Control defines the allowable degree of fluctuation.

There are five Classes of Control: AA, A, B, C, and D. Within control levels AA, A, and B, seasonal adjustments are noted separately from permissible short-term fluctuations. For control levels C and D, the wide fluctuations specified could result from either purposeful seasonal adjustments, or from short-term fluctuations, or from both (which is usually the case).

What Best Describes Your Collection?Footnote 1

Collection Type: General museums, art galleries, libraries, and archives (all reading and retrieval rooms, rooms for storage of chemically stable collections, especially if mechanically medium to high vulnerability)

RH and temperature set points: historical annual average for permanent collections or 50% RH with the temperature between 15 and 25°C.

Table 1. How to Describe Your Collection
Maximum fluctuations and gradients in controlled spacesClass of control
Short-termTable footer * fluctuations and space gradientsSeasonal adjustments in system set point
Return to footnote * Short-term fluctuations are any fluctuations less than the seasonal adjustment; however, some fluctuations are too short to affect some less-sensitive artifacts and those that are enclosed.
±5% RH
RH no change.
Up 5°C and down 5°C.

Precision control, minimal seasonal changes to temperature only.

±5% RH

Up 10% RH and down 10% RH.
Up 5°C and down 10°C.

Good control, some gradients or seasonal changes.

±10% RH
RH no change.
Up 5°C and down 10°C.

Good control, seasonal change to temperature only.

±10% RH
Up 10% RH and down 10% RH.
Up 10°C (but not above 30°C) and down as low as necessary to maintain RH control.

Control, some gradients plus winter temperature setback.

Within range 25–75% RH year-round.
Rarely over 30°C, usually below 25°C.

Prevent all high risk extremes.

Reliably below 75% RH.

Prevent damp.

A Note of Caution

Environmental conditions affect objects in many ways. Some objects are vulnerable in conditions that may not affect other objects at all. Some attempts to improve conditions for an object might actually affect it adversely. For example, moving an object from a poor environment to a theoretically better one might cause severe mechanical damage. There is simply no one-size-fits-all pattern for good environmental control strategies, nor is there likely to be, no matter how good HVAC engineering becomes. Understanding how environmental factors affect collections helps conservators make consistently good choices.

Table 2. Collection Type: Archives and libraries (storage of chemically unstable collections)
Set pointMaximum fluctuations and gradients in controlled spaces

Cold storage:
40% RH

±10% RH

Cool storage:
30–50% RH
Even if achieved only during winter setback, cool storage is a net advantage to collections as long as damp is not incurred.
Table 3. Collection Type: Special metals
Set pointMaximum fluctuations and gradients in controlled spaces

Dry rooms:
0–30% RH

RH not to exceed some critical value, typically 30% RH.

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