Humidity for Long-Term Preservation

Q: How do seasonal changes affect humidity control?

In the cold winter months, indoor humidity in any heated building will be low. The outdoor winter air has high relative humidity, as cold air can physically hold less moisture than warm air. So when this air enters a heated building, its capacity for holding water increases, thus lowering its relative humidity.

Q: Are there humidity control standards or guidelines for preservation environments?

Useful publications and guidelines on this topic include: - The ASHRAE Guidelines on Museums, Galleries, Archives and Libraries - ISO 11799:2024 Storage requirements for archive and library materials - NARA 1571 – Archival Storage Standards

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Benefits

Maintaining a stable humidity is vital for long term preservation of materials and artefacts.

Prevents dimensional change

Consistent internal moisture levels avoids degradation from expansion and contraction.

Avoids corrosion and rust

Correct humidity prevents oxidation in metals and susceptible materials.

Prevents damage from pests

Proper humidity control can deter infestations and subsequent damage.

Humidity for long-term preservation

Top of page Benefits Benefits References FAQs

Managing humidity to prevent dimensional changes

Any hygroscopic material has an internal moisture content that can be impacted by the atmosphere around it. If the air is too dry, moisture will move from the material to the air. If the air is humid, moisture can be absorbed by the material. When preserving objects in places like museums or long-term storage, these fluctuations in moisture content can cause microscopic dimensional changes to the object.

Over time these tiny dimensional changes will degrade an object’s integrity, particularly at the surface where the air has most impact. The result can be warping and cracking on an object’s exterior, loosening of joints, delamination of surface layers and separation of composite materials.

Humidifiers and dehumidifiers are vital to maintain a stable environment for long-term preservation. Understanding the ideal humidity for any given material is essential and once this is understood, any conservation programme must include a humidity management strategy.

Rapid fluctuation more damaging than extremes

When changes in humidity happen quickly, the material exposed to the air has less chance to adapt to changes in moisture content. If dimensional change occurs rapidly, it is much more damaging to the integrity of an object than gradual changes over time.

Leading authorities in the heritage sector recommend humidity in museums and galleries should not fluctuate more than +/-10%RH in any 24 hour period for this reason.

To acheive this, humidity control systems must be able to modulate accurately to changing needs of demand, increasing or decreasing their performance accordingly. Sensors and controls also need to be responsive. If a sensor has a +/-5% accuracy and a humidifier also has +/-8%, the complete drift in humidity control could potentially be +/-13%RH.

Preventing pest infestation damage with dehumidification

Pests such as silverfish and bookworms need a high humidity to thrive and breed. High humidity encourages mould growth, which is a food source, and softens organic materials such as paper and textiles, making them easier to consume.

Using dehumidifiers to maintain a humidity below 55%RH is important for preventing pest infestation. A temperature range of 18-22°C is also beneficial for long-term preservation of materials normally prone to infestation damage, such as books, wooden objects and textiles.

Using dehumidifiers to stop rust and mould

Corrosion risk increases for ferrous metals at humidity above 60%RH. At this level, surface moisture can enable the electrochemical reactions of oxidation. The higher the humidity, the faster these reactions occur, particularly in the presence of pollutants, such as salts that make the moisture more conductive.

The ideal level of humidity for long-term preservation will depend on the materials in question, but typically 35-55%RH is accepted as best practise for mixed collections that include metals.

Condensation occurs when air temperature drops to dew point. At this temperature, the air is saturated and can no longer hold water vapour, so it deposits on to surfaces. For instance, if the air is 20°C and 60%RH, any surface less than 12°C will collect condensation. It is important to understand the relationship between temperature, humidity and dew point for any long-term preservation strategy. Humidity control should be implemented to avoid this, not just to prevent rust but also mould growth on organic materials.

Stable humidity is vital in many industries for preservation

Universities

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Projects and references

ARoS Aarhus Art Museum, Aarhus

Condair DL hybrid humidifiers ensure optimal indoor humidity at The ARoS Aarhus Art Museum, protecting over 1,100 paintings and 400 sculptures.

Steinway & Sons showroom, UK

Steinway & Sons has been renowned for producing the finest pianos since 1853. The impressive showroom in London Marylebone was recently refurbished. Condair's steam humidifiers are playing a vital role in maintaining the exceptional craftsmanship involved in the production of Steinway's famous pianos.

Henry Moore Foundation, UK

Two Condair RS steam humidifiers have been installed, by Ceilite Air Conditioning, at the prestigious Henry Moore Foundation in Hertfordshire. Henry Moore was a renowned artist, best known for his figure sculptures.

National Army Museum, UK

The National Army Museum is the leading authority on the British Army and its impact on society past and present. Condair has supplied and installed three Condair RS resistive steam humidifiers to maintain the humidity around the exhibits and archives.

Paul Klee Museum, Bern, Switzerland

The Kunstmuseum, Zentrum Paul Klee, is located in Bern, Switzerland, in the immediate vicinity of the Schlosshalde cemetery, where Paul Klee's grave can also be found. The building is dominated by curved arches that blend harmoniously into the surrounding hilly landscape. It houses the comprehensive work of the Paul Klee Foundation and reflects the artist™s complexity through the division into three hills. The North Hill is dedicated to practical art education, conferences, and workshops. The Central Hill is used for collection presentation and temporary exhibitions, the South Hill for research and administration. Condair's DL hybrid air humidifiers create an optimum indoor climate for visitors and the sensitive works of art.

Foundation Michalski, Switzerland

An ideal work site for literature and literati - that was the vision that Vera and Jan Michalski wanted to achieve with the Maison d'ecriture. It is located at the foot of the Vaud Jura Mountains, in the Bois Ditsert in Montricher, with a view of Lake Geneva and the Alps. The site was chosen for its scenic beauty and the quiet surroundings that are supposed to have a positive effect on writers' activities and inspire them to reflect. This location has a very special appeal and a unique atmosphere. Architects V. Mangeat and P. Wahlen were tasked with designing the architecture of the object. They created several buildings for the foundation's numerous activities that fit perfectly into the surrounding nature and form a kind of settlement. Individual living spaces for authors, a dining room, a recreation room, a large library for research, a room with modular space for exhibitions and an auditorium for all kinds of events promise numerous encounters and brisk cultural exchange. Condair DL hybrid humidifiers ensure comfortable atmospheric humidity.

Rijksmuseum, Netherlands

The Rijksmuseum is one of the world's finest art museums with over 1.5km of galleries and artworks by Rembrandt, Vermeer and Van Gogh. The Condair DL hybrid humidifier is maintaining the ideal humidity for the preservation of paintings such as The Night Watch, The Milkmaid and The Merry Drinker.

The Louvre Museum

The Louvre Museum is a museum located in the Louvre Palace in the 1st arrondissement of Paris, France. It houses some of the most famous works of Western art, including the Mona Lisa and the Venus de Milo.
With 72,735 m² of rooms and galleries, it is the largest art museum in the world, ahead of the Hermitage (66,842 m²) in Russia and the National Museum of China (65,000 m²).

Various Condair humidity control technologies (Condair DL, RS, ME, DC) help to maintain the ideal room humidity for the preservation of the exhibits and the well-being of staff and visitors.

British Museum, UK

The British Museum first opened its doors in 1759, and is now one of the largest and most important culture and history museums in the world. At the center of the building is a covered square, with the former British Library building at its center. The roof was designed by the English star architect Sir Norman Foster, and is made up of 3,312 glass panes arranged in pairs. Condair steam humidifiers on each of the museum's three floors ensure an ideal indoor climate for visitors and exhibits alike.

Columbus Museum of Modern Art, USA

The Columbus Museum of Art needed precise humidity control to protect priceless artworks and ensure visitor comfort. Condair installed its RS steam humidifiers, integrated with the museum‚ HVAC system, delivering consistent humidity in the ideal 50 ± 5 % RH range. The system‚ design allows rapid response to seasonal and daily fluctuations, ensuring paintings, sculptures, and historic artifacts are safeguarded against cracking, warping, or other moisture-related damage. Easy to operate and maintain, the RS units provide reliable, energy-efficient performance while blending unobtrusively into the museum‚ infrastructure. By maintaining stable indoor humidity, Condair helps preserve the integrity of the collection, supports optimal conditions for visitors, and provides curators with confidence that their environment meets stringent conservation standards.

Mudac Museum, Lausanne, Switzerland

The mudac in Lausanne, designed by the Portuguese architects Aires Mateus, is an architectural masterpiece that houses two museums. The Museum of Design and Applied Arts and the Elysée Museum of Photography. Its architecture consists of two structures connected by a horizontal glass opening, giving visitors the feeling of being outdoors, no matter where they are in the building.

The upper part of the building rests on just three load-bearing points, which is an impressive feat of engineering and makes the upper structure appear to float. RS resistive steam humidifiers from Condair ensure a pleasant and healthy indoor climate and optimum humidity levels for conservation and value preservation purposes.

Preserving art through precise humidity control at the Gustav Lübcke Museum

As one of North Rhine-Westphalia’s most valued cultural institutions, the Gustav Lübcke Museum brings history to life through its renowned collections—from Ancient Egypt to modern art. Condair supports this vital heritage space with precise humidity control that protects sensitive artefacts and elevates the visitor experience.

Condair ensures the ideal humidity to preserve the Jewish Museum's collection

The Jewish Museum Munich relies on a Condair DL hybrid humidifier to maintain a controlled indoor environment that protects its diverse collection of cultural and historical objects. With three levels of gallery space dedicated to permanent and temporary exhibitions, the museum requires dependable humidity management to ensure artefacts remain in optimal condition throughout the year.

Quotes from our customers

Ceilite Air ConditioningTim CheshireApplications Engineer Technical Manager

We required an efficient and effective method of humidification that could safely raise relative humidity (RH) in a large archive store for priceless artwork – vital requirements that were successfully met by Condair. There were some challenges posed by the space to work in and the infrastructure of the building, but the excellent installation team overcame the issues without any complaint.

Would I specify Condair’s humidifiers again? Yes, without hesitation.

Opera North, UKRichard AshtonGeneral Manager

The orchestra spends over 20 hours a week rehearsing and performing here, so it was essential to provide a consistent level of humidity during the times when they use the space. After meeting with Condair, we felt confident that the company understood our specialised requirement, not just the level of humidity, but also the need for reliability, flexibility and cost effectiveness.

Since installing humidification, which we switch on as and when required, our musicians have felt much more at ease regarding the safety and performance of their instruments.

Steinway & Sons, UKCraig TerryManaging Director

Incredibly rare and expensive wood is used in some of our models – but actually any type of wood is susceptible to changes in environmental conditions. Dryness is the enemy of pianos so humidity is really important. We need 40% to 50% humidity all the time to protect the wood, whether they’re in the showroom or the workshop. Condair’s units are ideal for the job.

And because we rely so heavily on the reliability of the humidifiers, we also contracted with Condair to service the humidifiers at regular intervals. Would I recommend Condair? Oh yes, they’ve supplied great service.

Frequently asked questions

What is the ideal relative humidity (RH) for material preservation?

The ideal relative humidity will depend on the materials in question. A current common standard for material preservation in museums and galleries is 40-60%RH and a stable temperature in the range 16-25°C, with fluctuations of no more than +/-10%RH in any 24-hour period (ref. Bizot).

But to ensure energy efficient climate control, humidity levels should be set for the specific artefacts in question, to minimise energy and water consumption.

How do seasonal changes affect humidity control?

Are there humidity control standards or guidelines for preservation environments?

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Alternatively, if you would prefer a call or online meeting, our team are always happy to discuss possible solutions and offer free technical advice.

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