Temperature and humidity in museums

Temperature and relative humidity (RH) are two of the most important factors in collections care. Inappropriate or fluctuating conditions can severely damage objects over time. Biological, chemical, and dimensional changes are just a few of the problems that occur when these conditions are unstable.

This guide will help you:

• Understand the impact of temperature and humidity on objects
• Understand how temperature and humidity affect each other

Humidity is the amount of water vapour present in the air. In a museum setting, relative humidity describes the total amount of water vapour as a percentage at a given temperature. As temperature rises, so does the amount of water vapour needed to reach saturation. 100% RH is saturated air.

Museum collections contain both organic and inorganic material. Both suffer when conditions regularly fall outside the optimal range for their preservation.

Organic materials

Plants and animals contain a high proportion of water, so their products also retain moisture. Materials that absorb and retain moisture are described as hygroscopic. These materials absorb or release moisture until they reach equilibrium with the surrounding air.

Hygroscopic materials include:

• Wood
• Paper
• Cotton
• Linen
• Wool
• Silk
• Parchment
• Leather
• Fur and feathers
• Ivory
• Bone
• Horn
• Organic plastics, for example Bakelite and Cellulose Nitrate
• Organic lacquers, pigments, and binders

Very dry air causes organic materials to release moisture. They can become brittle or distorted and may shrink, warp, split, or crack. Distortion of painted or lacquered surfaces through humidity fluctuations can also release vapours that discolour or corrode other items nearby.

Damp air causes organic materials to absorb moisture. They may swell, cockle, warp, change shape, or lose strength. Dampness combined with poor airflow can activate dormant mould spores and cause fungal growth.

Inorganic materials

Inorganic materials in museum collections include glass, ceramics, metals, synthetic polymers and plastics, minerals, and materials used in past conservation treatments such as biocides, adhesives, and consolidants. High, low, or fluctuating humidity can damage all of these.

Materials with a natural salt content may suffer from efflorescence when the air is dry. Salts in deteriorated glass, porous ceramics, and some archaeological or geological material travel to the surface as moisture evaporates. The salts then crystallise on the surface.

Other effects on inorganic materials include:

• Corrosion in metals
• Stone erosion
• Glass crystallisation
• Stress cracking and crazing in mixed media materials and plastics
• Increased chemical reactions
• Faded dyes and pigments
• Pyrite decay in geological materials
• Byne’s disease in shells and minerals

Physical damage from humidity fluctuations

Frequent humidity changes cause hygroscopic materials to swell and shrink repeatedly. This creates internal stress and damage.

Composite objects are particularly vulnerable, as different materials shrink and expand at different rates. The expansion of one material can force changes in another, causing tension and eventual damage. Look out for this in objects such as skins on drums and paintings on wooden panels.

Moisture can also start or accelerate the damaging effects of air pollutants and other harmful substances on collection objects. Our advice on identifying and reducing air pollution covers this in detail.

Objects are rarely affected by temperature directly, unless they come into contact with a heat source such as a radiator. Fluctuating room temperatures cause indirect damage in several ways.

Uncontrolled temperatures can:

• Cause changes in humidity, damaging sensitive objects through RH fluctuations as rooms warm and cool. This is the main reason to control temperature.
• Speed up chemical processes and biological activity
• Cause certain materials to expand and contract. This is particularly damaging for composite materials where parts expand at different rates.
• Affect the comfort of staff and visitors working with or accessing collections

The humidity of air depends on its temperature. One cubic metre of air holds 10g of water at 10°C. The same volume can hold over 30g when heated to 30°C.

Absolute and relative humidity

Measuring moisture in grams, or absolute humidity, has limited use when monitoring a museum environment. 10g of water feels damp at 10°C but dry at 30°C.

Museums measure their environment in relative humidity (RH), expressed as a percentage of the maximum amount of water the air can hold at a given temperature.

For example:

• At 10°C, 10g of water is the maximum the air can hold, so the RH is 100%
• At 30°C, 10g is about one-third of the maximum, so the RH is approximately 33%

RH changes when temperature changes, for example after the sun sets. This is one of the most important principles in museum environmental control.

Temperature

The recommended temperature for museum gallery spaces sits within human comfort levels of 16 to 22°C.

Consistency matters most. Moderate fluctuations between 10 and 22°C, where temperature rises and falls gradually over 24 hours, are unlikely to damage museum objects. Rooms below 16°C may be uncomfortable for some visitors. Temperatures below 10°C can cause condensation, affect RH, and damage building fabric over time. Temperatures above 22°C may be too hot for visitors and will accelerate degradation in museum objects.

Museum stores typically run cooler than 16°C, as visitors do not access them and collections can be boxed and buffered against RH and temperature changes. Items will need to acclimatise gradually when moving between storage and display.

Relative humidity

For mixed collections, relative humidity should not drop below 40% or rise above 70%.

RH below 40% can cause moisture-sensitive items to become dry and brittle. The upper limit reflects the point at which fungal growth begins, which is at RH over 65%. Reducing RH fluctuation is particularly important.

These are broad recommendations based on mixed collections. Some items may need more precisely controlled RH, or may have acclimatised to specific conditions. Moving items quickly between environments, for example from an unheated store to a heated gallery, can cause irreversible damage. Always consult a conservator for advice on acclimatising items gradually.

Understanding the environmental buffer your building provides is a good first step in controlling conditions and using conservation heating to manage RH cost-effectively.

Scotland’s climate creates particular challenges. When temperatures drop in winter, heating systems switch on and lower relative humidity, sometimes below 30%. In summer, internal spaces can remain humid even on warmer days, with external humidity fluctuating between 65 and 95%.

In a naturally ventilated building in good condition, indoor conditions respond to outdoor conditions. Several factors can affect the temperature and humidity inside a building, room, or display case.

External factors

• Water penetration or rising damp causes high humidity
• Air pollution from traffic
• Sudden weather changes cause dramatic fluctuations, particularly in poorly insulated buildings
• Direct sunshine on metal roofs and glass skylights causes significant heat gain and day-to-night variation

Internal factors

• Poor air circulation or ventilation creates micro-climates that differ from ambient conditions
• Heating systems that run during the day for comfort cause day-to-night fluctuations
• Visitors produce moisture, especially on rainy days, and raise CO2 levels
• In-case and spot lighting create localised pockets of high temperature and low RH
• Radiators or heating units positioned beneath objects cause severe RH drops

Display cases can be designed to create favourable micro-climates. Choose cases carefully to make sure they protect rather than harm collections. Our advice on choosing new display cases covers this in detail.

Continuous monitoring

Monitor your environment regularly to track fluctuations and identify trends. After 12 months of monitoring you will have a good picture of changing conditions in your museum. Aim for 24 months of data to confirm consistency. Use what you learn to decide where and how to display sensitive items, and invest in control equipment where necessary.

Make sure all equipment is calibrated and in good working order. When using handheld equipment, take readings in the same areas each time. Ask a preventive conservator for further advice.

Understanding fluctuations

When monitoring your museum, pay particular attention to regular, frequent fluctuations as these can cause significant damage. Slow changes over 24 hours give objects enough time to adjust. Very gradual fluctuations allow items to acclimatise slowly and cause less damage. Frequent changes that happen quickly cause movement, stress, and fatigue in materials. Occasional very rapid fluctuations within one or two hours are less damaging than sustained frequent ones.

Some types of objects are more affected by fluctuations than others, and the resulting damage can be irreversible or costly to repair. When transporting objects to an area with different environmental conditions, keep them well-packed in acid-free tissue, a box, or a blanket from the room they have acclimatised to. This allows gradual acclimatisation to new conditions.

To reduce the effect of external conditions on the internal environment:

• Carry out comprehensive building and energy efficiency surveys to check the building is in good condition and well insulated
• Maintain the building to act as a barrier against external conditions
• Keep windows closed unless ventilation is needed to reduce heat build-up. Assess the risks to affected collections before opening windows.
• Keep entrance doors closed as much as possible, or install double doors, revolving doors, or a vestibule. This also reduces energy use.
• Apply solar control film to windows and skylights to reduce the effect of direct sunlight
• Install heaters, humidifiers, or dehumidifiers where needed to maintain a stable environment. Keep all equipment well maintained and regularly checked, for example by emptying dehumidifiers regularly.
• Make sure building control systems complement rather than undermine any environmental systems you have in place, such as conservation heating

Stability of relative humidity is the main aim. Temperature can be allowed to fluctuate moderately in favour of stable RH. Use a humidistat rather than a thermostat to control heating with humidity levels. Where humidity fluctuates significantly, humidifiers or dehumidifiers may be more cost-effective.

When controlling or creating a museum environment, aim for a stable RH that benefits your collection. Maintaining a stable level close to the desired level at all times is better than hitting the exact target only part of the time.

Objects in showcases or on internal walls experience different conditions from those on open display, in stores, or on external walls.

Factor these considerations into your environmental planning:

• Understand how your building responds to environmental change
• Note how many windows or skylights your museum has
• Consider orientation, for example north-facing
• Consider location, for example city centre
• Note what the building is made of and its date of construction, including any later changes
• Identify your heating system and whether it covers gallery and storage spaces
• Estimate what percentage of the building houses collections

How to create micro-climates

In some situations, controlling the environment of an entire building may not be financially viable or necessary. In these cases, create localised environments, or micro-climates, around specific sensitive items.

In the building:

• Locate galleries or stores in the parts of the building with the least climatic variation
• Use window treatments to reduce solar gain and temperature rises
• Keep gallery doors closed
• Build rooms within rooms to create a buffer
• Control the environment with standalone humidistat-controlled heaters, or fit a humidistat to radiators and link to an environmental management system
• Use humidifiers or dehumidifiers

In display cases:

• Choose a display case that buffers fluctuations well, or fit gaskets. Monitor environments in sealed cases using humidity-indicator strips and hygrometers or loggers. Our advice on choosing new display cases covers this in detail.
• Use passive systems, for example silica gel or Prosorb in showcases, to condition the air
• Prevent direct sunlight or heat-producing lights from affecting temperatures

In storage:

• Keep doors closed at all times
• Cover windows to prevent solar gain
• Limit wet cleaning of floors
• Manage output from heating systems, for example at the radiator
• Use museum-standard enclosures, which act as physical buffers and create stable micro-climates for stored items

Use these materials for appropriate storage and packing:

• Acid-free tissue
• Melinex sleeves
• Cotton covers
• Boxes
• Trays with lids
• Cupboards, wardrobes, and chests of drawers

Our advice on storage and display materials covers this in detail.

Maintaining stable temperature and humidity uses energy. Done inefficiently, this adds to your running costs and your building’s carbon footprint. Good environmental management reduces both.

The sector is moving away from rigid environmental targets towards a more flexible, risk-based approach. This shift recognises that many collections do not need the narrow temperature and humidity bands that have historically been the standard, and that chasing exact targets can waste significant energy. Conservation professionals set out this approach in the Institute of Conservation’s guidance note on environmental management and climate sustainability.

Two sector standards support a more sustainable approach to setting temperature and humidity conditions.

The Bizot Green Protocol sets out international guidelines for more sustainable collections care, formally endorsed in 2023 at the first Museum COP. It recommends climate values between 16-25°C at 40-60% relative humidity for many collections, broader than the bands museums have traditionally maintained. These wider ranges matter because holding tight, narrow targets is one of the most energy-intensive things a museum does. Adopting broader bands where your collection can tolerate them reduces energy use and carbon emissions while still protecting objects.

BS 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, is the current British and European standard for collections buildings and stores. It supports an evidence-led approach to environmental conditions, helping you balance the needs of your collection, your building, and your energy use. It replaced PAS 198:2012, which BSI has withdrawn, so make sure any older guidance you hold refers to the current standard.

Practical steps you can take:

• Take a more flexible approach to environmental control in areas or collections less sensitive to tight parameters, or areas that can run at cooler temperatures. A slightly wider RH and temperature range can significantly reduce energy use and carbon emissions. Always implement changes gradually, for example lowering heating by one degree each week, and monitor collections for any adverse reaction.
• Reduce artificial means of altering temperature and humidity where possible. Improving the building envelope, for example through insulation and draught-proofing, is often a lower-carbon approach than running mechanical systems to compensate for a leaky building.
• Use conservation heating, which maintains RH stability rather than temperature comfort. This can significantly reduce energy use compared to conventional heating.
• Consider renewable energy to power any environmental control systems. Our climate action examples from Scottish museums show how other organisations have reduced their environmental impact.
• Communicate openly with staff, volunteers, and visitors. Explain why spaces may be cooler, why blinds are drawn on certain days, or why environmental conditions sometimes take priority over comfort. Public understanding supports better environmental practice.

Always keep conditions within the recommended guidelines set out in this guide. Make any changes gradually, monitor them, and document them.

Although all museums should aim to meet the recommended temperature and RH guidelines, some objects need special consideration.

The guidance below is based on advice from Conservation and exhibitions: packing, transport, storage, and environmental considerations (Stolow, N. London, Butterworths, 1987).

Archaeological specimens

Once removed from a site, archaeological items can become fragile and degrade quickly. For humidity-sensitive items such as wood, leather, and fibrous components, aim for 40-60% RH. Stone, ceramic, and other inert materials need the same range, unless salts are active, in which case aim for 20-30%. Keep iron very dry, below 15% RH. Check the stability of other metals before setting an RH target. Low RH reduces the rate of corrosion in unstable metals. Keep small finds in conditioned containers with humidity strips.

Arms, armour, and metals

Aim for seasonal RH between 40-60%, depending on the condition of the metal, oxide formation, and the presence of organic components. Unstable or corroding metals may need drier conditions, as low as 10%. In these cases use conditioned sealed containers or mount items in sealed frames, and discuss with a conservator. Our advice on caring for metal collections covers this in detail.

Botanical objects

Dried plants, seeds, and other botanical objects are stable between 40-60% RH. Packaging and mounts help buffer against fluctuations. Test by monitoring in boxes with proposed materials before storing.

Ceramics, tiles, and stone

Decide on an RH between 20-60% based on the presence of embedded salts. These items are vulnerable to freeze-thaw damage if stored outdoors. In storage, interleave tiles with tissue to absorb moisture and review regularly.

Coins and numismatic collections

Corrosion products, oxides, and patina all affect the desired RH for coins. Assess their stability and aim for 40-60% RH, though drier for unstable corroding objects. Make sure containers do not accelerate corrosion.

Costumes, textiles, rugs, and tapestries

RH between 30-50% protects most textiles. Silk and wool are more sensitive to moisture damage than cotton or linen. Many dyes and adornments are affected at higher RH levels. Painted textiles are the most sensitive to RH changes. Synthetic fabrics are less reactive but attract dust through static at low RH. Our advice on caring for textiles covers this in detail.

Entomological collections

Keep dried and mounted insects between 40-60% RH. Store in drawers or acid-free containers with lids to help buffer conditions.

Ethnographic items

Bark cloth, basketry, manilla, sisal, masks, feathers, and leather garments all react differently. Aim for 40-60% RH. Support items in display and storage to prevent warping, sagging, or damage to the main structure from heavy elements such as beadwork.

Furniture and marquetry

Aim for 40-60% RH, depending on wood content, grain, joinery, and the condition of surface or barrier coatings. Furniture is particularly vulnerable to seasonal RH fluctuations. Keep RH below 65% to prevent mould. Check inside drawers and the backs of cabinets regularly. Keep furniture away from external walls or maintain some airflow.

Geological material

Keep minerals, rock, and fossil material containing pyrite at a maximum of 50% RH, ideally 30%. Other geological items are stable between 45-60% RH. Isolate any minerals showing decay from the rest of the collection. Our advice on hazards in collections covers this in detail.

Glass

Aim for 40-60% RH. Crizzled or weeping glass needs narrower control around 40% to prevent the condition advancing. RH is less critical for other types of glass. Consult a conservator.

Ivories and bone carvings

Aim for 50-60% RH. Ivory and bone need more precise control than other anatomical collections. Thin sheets, such as those used in miniatures, respond more quickly. Check that items are genuinely ivory or bone, as organic plastics require different conditions.

Lacquerware

Aim for 50-60% RH, although Japanese authorities recommend levels up to 70%. Support loose or heavy items with cotton tape and packaging to prevent loss if adhesive fails.

Leather, skins, and bindings

The stability of leather varies depending on how it was tanned. Aim for 45-60% RH. Pad and support items such as shoes with acid-free tissue to maintain their shape.

Paintings on canvas

In general, aim for 40-60% RH. Unlined paintings and those lined with hygroscopic adhesives are more reactive than those lined with wax or synthetic materials. Our advice on preserving paintings and frames covers this in detail.

Paintings on wood and polychrome sculptures

The right RH depends on the thickness, wood grain, ground, and method of joining sections, within a 40-60% range. Some panel paintings need narrow RH control to reduce warping. Large wood sculptures are particularly vulnerable to seasonal drift. Take extra care when moving these items between rooms or buildings.

Paper and paper products

General guidelines are:

• Parchment and vellum need narrow control between 55-60% because of their high hygroscopicity
• Stretched paper, such as screens and stretched frames, needs narrow RH control in the 45-55% range
• Other paper needs 40-50% RH, although some authorities recommend lower

Our advice on caring for paper collections covers this in detail.

Photographs and films

Aim for 30-45% RH. Gelatin in photography is reactive, as is support paper. Plastic film components are less responsive. Our advice on caring for photographic collections covers this in detail.

Plastic materials

Aim for 30-50% RH. Most plastic materials, including acrylic supports, sculptures, and castings, have slight humidity responses. Thin sheets warp in varying conditions. Plastics attract dust through static at low RH.

Wood

Aim for 40-60% RH for musical instruments, models

For more collections care guidance, find our other advice guides on collections.

The Collections Trust provides a wide range of guidance and advice on collections care.