Microclimate in museums and archives: relative humidity & temperature limits, silica gel, Art-Sorb, Prosorb, data loggers, anoxia

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September 28, 2026

On September 28, 2026

Relative humidity (RH) and temperature are the two environmental parameters that most affect the long-term preservation of collections. In an overly humid environment fungi grow and corrosion accelerates; in a very dry one, organic materials shrink and crack; rapid fluctuations cause mechanical stresses in wood, paintings, ivory and composite objects. In this guide we summarise the limits recommended by international standards and guidelines, and explain how passive microclimate control works in practice: silica gel, Art-Sorb, Prosorb, humidity indicators, data loggers and anoxia.

Basic concepts

Relative humidity and temperature

Relative humidity expresses how much water vapour the air contains compared with the maximum it can hold at a given temperature. Because the capacity of the air increases with temperature, in a closed space, when the temperature rises, RH falls – and vice versa. That is why heating a building in winter dries the air, while a cold wall or a basement storeroom may reach local condensation.

Hygroscopic materials and "equilibrium"

Wood, paper, leather, textiles and ivory exchange moisture with the environment until they reach equilibrium. Every change in RH translates into swelling or shrinkage. What stresses objects most is not so much a particular value as the large and rapid fluctuations, especially if they exceed the range to which the object has already "become accustomed" (the so-called "historical" or "proven" fluctuation, according to EN 15757).

What limits apply today

The older idea of a single, strict "50% ±5% RH and 20 °C" for all collections has been replaced by a more flexible, risk-based approach. The main reference points are:

Bizot Green Protocol (2014, with input from IIC and ICOM-CC): for most hygroscopic materials, a stable RH in the range 40–60% and a temperature of 16–25 °C, with RH fluctuations of no more than ±10% in 24 hours. Narrower limits are set for sensitive objects on a case-by-case basis.
ASHRAE Handbook – chapter on museums, libraries and archives: defines control classes (AA, A, B, C, D) with different permitted short-term and seasonal fluctuations. For example, class AA allows short-term fluctuations of ±5% RH and ±2 K with no seasonal change in RH, while class A also allows a seasonal shift of the set point.
EN 15757:2010: recommends setting limits based on the historical climate to which the object has adapted (seasonal moving average and permitted deviation).
EN 16893:2018 and ISO 11799: requirements for archive and library storage areas, where lower temperatures are generally preferred for long-term preservation.

Special categories of materials

Metals: low RH, often below 40%; archaeological iron with active chloride-driven corrosion requires significantly lower values.
Mixed collections with organic materials: moderate, stable RH, without extremes.
Photographic materials and film: low temperature (especially for colour materials and cellulose acetate) and moderate RH, according to the ISO 18900 series of standards.
Mould risk: increases significantly when RH remains above about 65–70% for a prolonged period.

Passive control in display cases and boxes

In a well-sealed display case, the small air volume allows the RH to be stabilised with buffering materials (buffers) without mechanical equipment. Success depends on three factors: the airtightness of the case (air exchange rate), the amount and type of buffering material, and its correct preparation (pre-conditioning).

Silica gel

Porous silicon dioxide that absorbs and releases water vapour. There are two main uses: (a) as desiccant, dry, to keep RH low (e.g. for metals); (b) as buffer, pre-conditioned to a specific RH, to even out fluctuations. Ordinary type "A" silica gel is effective mainly at low RH, while special museum silica gels are designed to perform in the mid-RH range. Indicating silica gel changes colour when saturated, but some indicators (e.g. with cobalt chloride) are no longer recommended for health reasons; prefer cobalt-free indicators or separate indicator cards.

Prosorb

Museum silica gel designed to absorb and release large amounts of water vapour in the range of about 30–60% RH. Available as beads, sachets and cassettes, pre-conditioned to a chosen RH value, so it can be placed directly in display cases.

Art-Sorb

A high-performance buffering material (in bead, sheet or cassette form) with a high moisture-exchange capacity, especially at medium and high RH. It is usually supplied pre-conditioned at around 50% RH and can be reconditioned many times. It is suitable for environments where the outside RH is high, as in many Greek buildings near the sea.

How much buffering material is needed?

The amount depends on the volume of the display case, the air exchange rate, the difference between internal and external RH and how often reconditioning can be carried out. G. Thomson (The Museum Environment, 1978/1986) gave classic estimates for the ordinary silica gel of the time; modern, well-sealed display cases with high-performance materials need significantly smaller quantities. Follow the manufacturer's instructions for the specific material and confirm with measurements.

Steps for use in a display case

Check the seal of the display case (joints, openings, gaskets). Without a good seal, no buffering material will be sufficient for long.
Choose construction materials that do not release acids, aldehydes or sulphur compounds (Oddy test). Barrier films and suitable tapes/silicones reduce emissions from wooden or MDF parts.
Pre-condition the buffering material to the desired RH (or obtain it pre-conditioned).
Place it in a separate, ventilated compartment, so that it does not come into contact with the objects and can be replaced without opening the display area.
Monitor with a data logger or at least a humidity indicator card.
Recondition when readings show that the RH is drifting from the desired range.

Measurement and recording

What isn't measured isn't controlled. The main options are:

Humidity indicator cards: economical, rough check (accuracy of the order of ±10% RH). Useful inside boxes, barrier bags and during transport.
Digital thermo-hygrometers: instantaneous readings and, on some models, alarms and dew point.
Data loggers: continuous recording of temperature (and RH, depending on the model) at regular intervals. The data are analysed to identify fluctuations, seasonal trends and malfunctions.

RH sensors drift over time; schedule periodic calibration or checking against a reference (e.g. with saturated salt solutions or compared with a calibrated instrument). EN 16242 describes procedures for measuring humidity for cultural heritage and EN 15758 for temperature.

Anoxia: storage and pest treatment without chemicals

The anoxic method is based on reducing oxygen to very low levels inside a sealed enclosure made of oxygen barrier film (e.g. Escal, multilayer aluminium films). It is used for two purposes:

Pest treatment: at oxygen concentrations below about 0.1–0.3% for several weeks, insects are killed at all stages. The time required depends on the species, temperature and RH – the literature (e.g. Selwitz & Maekawa, Inert Gases in the Control of Museum Insect Pests, GCI 1998) usually states from 2–3 to several weeks.
Long-term storage: for materials sensitive to oxidation (e.g. certain metals, rubbers, pigments), combined with RH control.

How it is applied

Calculate the air volume inside the bag or chamber and choose oxygen absorbers of suitable capacity (with a safety margin).
Choose the type of absorber: iron-based absorbers need moisture to work and may raise the RH slightly; some systems (e.g. RP-A) absorb moisture at the same time and are intended for metals, while others (e.g. RP-K) do not alter the RH and are intended for organic materials.
Seal with heat or suitable clips and place oxygen indicator visible from outside.
Monitor the indicator and the exposure time; record temperature and RH.

Oxygen absorbers give off heat during the reaction; position them so that they do not touch the objects.

Security

Read the SDS of desiccants and oxygen absorbers. Silica gel dust irritates the eyes and respiratory tract; use gloves and, for loose material, a dust mask and goggles.
When reconditioning by heating, follow the manufacturer's temperatures and work in a ventilated area.
In large anoxic chambers or in nitrogen/argon systems, low-oxygen air is dangerous for people (risk of asphyxiation). Ventilation and an oxygen detector are required in the workplace.
Mouldy objects are isolated and handled with suitable personal protective equipment.

Frequently asked questions

What is the "ideal" relative humidity?

There is no single ideal value for everything. For most mixed collections a stable range of 40–60% is acceptable, with rapid fluctuations limited. Metals need lower RH, while some sensitive objects need narrower limits.

How does Art-Sorb differ from ordinary silica gel?

Museum buffering materials (Art-Sorb, Prosorb) are designed to exchange large amounts of moisture in the mid-RH range, so they stabilise a display case more effectively. Ordinary silica gel is more efficient as a desiccant at low RH.

How often should I replace or recondition silica gel?

When readings show that the RH in the display case is drifting from the desired range. In well-sealed cases this may mean once a year or even less often; in leaky ones, much more often.

Is a thermo-hygrometer enough or do I need a data logger?

A thermo-hygrometer shows only the instantaneous situation. A data logger records fluctuations day and night, seasonal trends and incidents (e.g. air-conditioning failure), which are essential for documentation and for loan requests.

Is anoxia safe for all materials?

It is one of the safest methods of pest eradication, as it uses neither toxic chemicals nor extreme temperatures. Some pigments have been reported to change in an anoxic environment under light; for painted objects, consult the literature and keep the bags in the dark.

Why do display-case materials need an Oddy test?

In a closed display case, pollutants released by wood, adhesives, paints or textiles accumulate and may corrode metals or attack sensitive materials. The Oddy test checks whether a construction material is safe to use near objects.

Products mentioned

Categories: Microclimate Measurement & Monitoring · Passive Microclimate Control · Microclimate Measuring and Logging Systems

Standards and bibliography

EN 15757:2010 – Conservation of cultural property – Specifications for temperature and relative humidity to limit climate-induced mechanical damage in organic hygroscopic materials.
EN 16893:2018 – Conservation of cultural heritage – Specifications for location, construction and modification of buildings or rooms intended for the storage or use of heritage collections.
EN 16242 – Conservation of cultural property – Procedures and instruments for measuring humidity in the air and moisture exchanges between air and cultural property.
ASHRAE Handbook – HVAC Applications, chapter "Museums, Galleries, Archives and Libraries".
Bizot Group (2014). Bizot Green Protocol / IIC–ICOM-CC Declaration on Environmental Guidelines.
Thomson, G. (1986). The Museum Environment, 2nd ed. Butterworth-Heinemann.
Selwitz, C., Maekawa, S. (1998). Inert Gases in the Control of Museum Insect Pests. Getty Conservation Institute.