Temperature and humidity in museums and archives: standards, thresholds, alerts
Conservation guidance has moved away from a single set point toward stable bands — the Bizot Green Guideline asks for 40–60% relative humidity with fluctuations no greater than ±10% per 24 hours. Stability, rather than the target value itself, is the hardest part to maintain and the part that damages collections when it fails.
EnviSensor Team
In short
- The Bizot Green Guideline and the IIC/ICOM-CC declaration set stable 40–60% humidity and 16–25 °C for hygroscopic objects, with fluctuations no greater than ±10% RH per 24 hours.
- The Getty Conservation Institute notes that keeping humidity reliably below 75% avoids mould growth and rapid corrosion; the useful range is bounded by inflection points at roughly 25% and 75%.
- NARA record storage guidance uses bands rather than set points — 10–18 °C and 30–50% humidity for textual records — explicitly balancing preservation with energy efficiency.
- Storage requirements for archive and library materials are covered internationally by ISO 11799; national regulations translate it into binding values and often add permissible daily fluctuations.
- Acidic papers, magnetic media and many films have a usable life of only decades when stored at or above 21 °C.
What conservation guidance requires
For over a decade museum practice has been shifting from a single target value toward acceptable bands. The Bizot Green Guideline states that for many classes of object containing hygroscopic material — canvas paintings, textiles, ethnographic objects — a stable relative humidity in the range of 40–60% is required, with a stable temperature of 16–25 °C and fluctuations of no more than ±10% RH per 24 hours. More sensitive objects require specific and tighter control.
The same range appears in the environmental guidelines declaration issued jointly by IIC and ICOM-CC. Both documents tie it to an energy argument: care of collections should be achieved in a way that does not assume air conditioning or other high energy cost solutions, and blanket conditions should no longer apply.
For archive and library storage the international reference is ISO 11799, which sets document storage requirements for archive and library materials. National regulations typically translate it into binding values — and often add something the international guidance leaves implicit: a permissible daily fluctuation.
Why fluctuations do more harm than a steady suboptimal value
Hygroscopic materials — paper, wood, textiles, leather — absorb and release moisture as conditions change, altering their volume in the process. Where layers of an object respond at different rates or are bonded together, mechanical stresses arise. Those stresses, rather than the humidity value itself, cause cracking, delamination and deformation.
This is why guidance frames the requirement as stable humidity within a band rather than a set point, and why national rules commonly cap how far conditions may drift within a single day.
The proofed fluctuation concept
Stefan Michalski of the Getty Conservation Institute formulated a principle worth knowing for risk assessment: the proofed fluctuation is the largest humidity or temperature swing an object has been exposed to in the past. If future conditions stay within that range, the risk of further mechanical damage is negligible.
He adds an important caveat: the principle covers mechanical damage. Mould risk and chemical ageing follow different rules and cannot be justified by an object history.
Where the safety limit lies
Michalski describes classes of climate control in which the lowest level — option D — reduces to a single requirement: prevent dampness by keeping humidity reliably below 75%. Meeting that condition avoids mould growth and rapid corrosion.
The bounds of the useful range are set by two inflection points on the risk curve, at roughly 25% and 75% relative humidity. Option C permits operation within 25–75% year-round, with temperature rarely above 30 °C. Option B allows ±10% RH and ±5 °C with seasonal adjustment.
Temperature works on a different mechanism — chemical ageing. NARA states the consequence plainly: acidic papers, magnetic media and many films and plastics have a usable life of only decades when stored at or above 21 °C. Lowering storage temperature is the single most effective measure for collections at chemical risk.
Record storage: bands by material type
NARA guidance, in force since 2023, supports moving away from set points toward bands in order to balance long-term preservation of holdings with energy efficiency:
| Material | Temperature | Humidity |
|---|---|---|
| Textual records, paper | 10–18 °C | 30–50% |
| Black-and-white photography, polyester film, colour prints | 10–18 °C | 30–50% |
| Electronic and audiovisual media | 10–18 °C | 30–50% |
| Cold storage (cellulose acetate, colour film) | about 1.7 °C, never below 0 °C | 30–40% |
| Exhibition and work areas | 10–24 °C | 30–55% |
Note that NARA classifies electronic and audiovisual media as low sensitivity, observing that cold storage does not increase their longevity and can cause damage — a useful corrective to the assumption that colder is always better.
National regulations go further by capping daily drift. Polish archival rules, for example, require paper storage at 14–18 °C and 30–50% humidity with a permissible daily fluctuation of just 1 °C and 3%, and mandate daily recording of conditions with weekly review. Whichever jurisdiction applies, the practical lesson is the same: a requirement expressed as a daily fluctuation can only be demonstrated with continuous logging.
How to monitor so that it meets the requirements
- Measure where objects are stored, not in the corridor. Conditions by an external wall and deep inside a store can differ substantially.
- Record continuously, not at points in time. A manual reading may satisfy a daily recording duty, but a permissible daily swing of a degree can only be demonstrated with densely sampled data.
- Alarm on the rate of change, not only on the value. Exceeding the rate of change is an earlier sign of humidifier or air conditioning failure than leaving the range.
- Keep the measurement history and be able to export it. Conservation reporting and funding applications both rely on it.
- Treat 75% humidity as an absolute limit. Above it, mould risk rises regardless of the object history.
Frequently asked questions
What humidity should a museum maintain?
The Bizot Green Guideline and the IIC/ICOM-CC declaration specify a stable relative humidity of 40–60% and a stable temperature of 16–25 °C for objects containing hygroscopic material, with fluctuations no greater than ±10% RH per 24 hours. More sensitive objects require tighter control.
What conditions should an archive store hold?
NARA guidance sets 10–18 °C and 30–50% relative humidity for textual records, with cold storage at about 1.7 °C for cellulose acetate and colour film. Internationally, ISO 11799 sets document storage requirements for archive and library materials, and national regulations translate these into binding values.
Why are humidity fluctuations worse than a steady suboptimal value?
Hygroscopic materials change volume with ambient humidity. When layers of an object respond at different rates, mechanical stresses arise that lead to cracking and delamination. This is why guidance specifies stable bands and permissible daily fluctuations rather than a single set point.
What humidity encourages mould growth?
The Getty Conservation Institute indicates that keeping humidity reliably below 75% avoids mould growth and rapid corrosion. The bounds of the useful range are set by inflection points on the risk curve at roughly 25% and 75% relative humidity.
Does cold storage suit every type of collection?
No. NARA classifies electronic and audiovisual media as low sensitivity and notes that cold storage does not increase their longevity and can cause damage. Cold storage is aimed at chemically unstable materials such as cellulose acetate and colour film.
Sources
- Bizot Green Protocol — Green Guideline (September 2023 refresh)
- IIC and ICOM-CC environmental guidelines declaration (2014)
- S. Michalski, The Ideal Climate, Risk Management, the ASHRAE Chapter, Proofed Fluctuations, Getty Conservation Institute
- NARA 1571, Supplement 2 — Temperature and Relative Humidity Standards for Records Storage
- ISO 11799 — Information and documentation: document storage requirements for archive and library materials
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