HUMIDITY CONTROL M
aintaining stable indoor humidity levels in museums, galleries,
archives and stately homes is vital for the successful preservation of artefacts. Artworks and antiques contain a variety of materials that are
hygroscopic, which means they can be aff ected by the moisture content of their environment. Elements such as wood, textiles, paper and animal products will release or absorb water to and from the air. Fluctuations in internal moisture content result in small dimensional changes, which lead to the degradation of materials over time. If an atmosphere is too humid, condensation from excess moisture can also lead to damage from fungal growth and mould. Beyond the preservation of materials, humidity
control is particularly important in the heritage sector as exhibits are loaned and insured with a pre-requisite that their environment is closely managed. Humidity, temperature, light and UV levels can all require tight regulation 24/7. If conditions fall outside the required range, benefactors may decide not to permit an artwork loan and insurance companies may not indemnify the exhibits. As a rule, many leading conservation bodies
recommend humidity is maintained in museum environments within the range of 40-60%RH, while minimising fl uctuations of more than 5%RH within any 24-hour period. Rapid fl uctuations cause more damage to materials than gradual changes, as materials have less time to respond to changes in ambient moisture content. Some materials may require more closely managed humidity than others to remain in a stable state of equilibrium with their environment. For instance, preservation of artefacts containing iron will benefi t from an environment with less than 15%RH, whilst conservation of ivories and bone need a higher 50-60%RH. Parchment and vellum typically need tight control of 55-60%RH. So, setting the ideal level in any heritage situation
will depend on the materials in question. When it’s diffi cult to maintain the required environment across a whole room, cabinets or enclosures can be used. This is particularly useful in diffi cult to control areas, like entrance halls or large open spaces. The use of revolving doors and vestibules is also often used to mitigate the outside air entering a building and therefore limit moisture ingress. However, as commercial buildings are required to ventilate to at least 10 litres per second, per person, keeping the outdoors out is not a practical solution for humidity control. The use of technology to manage the indoor humidity is vital in any controlled heritage environment. Humidifi ers are used to raise humidity and dehumidifi ers to remove moisture and lower humidity. When both are used, there needs to be a suffi cient “dead-zone” of control (10%RH) between the set-points of both systems to ensure they aren’t operating against each other. Humidity control systems can either treat air centrally in a building’s air handling unit, or act
26 July 2025
www.heatingandventilating.net
Controlling humidity in museums & galleries
Dave Marshall-George, UK & Ireland sales director at Condair, looks at the complexities of indoor humidity control in heritage sites
locally as in-room systems. The choice of system depends largely on the type of building in question. If a building has ducted centralised air treatment systems, AHU-based humidity control makes sense, as it’s easier to manage. However, many heritage sites are very old buildings, which frequently do not incorporate ducting so multiple, in-room systems can be the only option. As these buildings are open to the public, it is vital any in-room system is tamper-proof, hygienic and robustly safe in its mode of operation. Heritage sites can have large numbers of people entering them in a very short period of time. A bus load of tourists in from the rain, with wet clothes and umbrellas, can add signifi cant moisture to the indoor environment. For this reason, any humidity control system also needs to be capable of rapid response to a control signal, to minimise humidity fl uctuations.
For this reason, resistive steam humidifi ers are a popular choice for museums and galleries with ducted, centralised air treatment. The technology off ers fully modulating output, from 0-100%, and very close humidity control. However, even though this type of humidifi er can distribute steam directly into a room with a fan unit attachment, it is not often recommended for in-room use in public places, due to the presence of high temperature internal components and high voltage electricals. In non-public areas, like archives, it is ideal for in- room use, if no central AHU is available. Mobile evaporative humidifi ers are often
used for in-room humidifi cation. High quality, professional units have tamper-proof controls and can be plumbed-in so are fully automated and easy-to-install in heritage applications. Given the
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