Items made from organic material, such as paper, wood and textiles, will react to changes in humidity and physically change shape. Repeated change as the relative humidity fluctuates will cause structural damage to the item. Additionally, being organic there is a high risk of mould formation at higher humidity levels.

With inorganic items, such as iron or steel, the rate of corrosion (rust formation) is directly proportional to the humidity level. The rate of corrosion can be up to six times greater at 90%RH compared to that at less than 50%RH.

The ideal humidity  will be dependent on two aspects, the objects being stored and the temperature range of the store. For organic items, such as paper, wood and textiles, the ideal will be between 35 and 60%RH within a temperature range of 13 to 20°C. For inorganic items, like metal, the lower range of 35 to 50%RH would typically be ideal. However some metal artefacts may require humidity levels below 20% to prevent corrosion.

Heating alone will not keep an archive dry. Whilst it will certainly reduce the relative humidity it does not reduce the absolute humidity. Absolute humidity is the mass of water per unit volume of air.

To “dry” an archive it is necessary to reduce the absolute humidity by physically reducing the quantity of water in the air. This can only be achieved by use of a dehumidifier. Heating alone has a much higher running cost.

Use of a dehumidifier will typically result in a 60% reduction in energy consumption compared to direct space heating.

There are two main types of commercial dehumidifier - condensing and desiccant.

Condensing dehumidifiers operate well provided the store temperature is above 20°C and typically less than 35°C. Humidity levels above 45%RH up to 95%RH are achievable with this technology. Low relative humidity, less than 40% is more difficult to achieve with condensing dehumidifiers. This type of system has an overall control accuracy of around +/-10%RH. Condensing dehumidifiers either require a drain connection or daily maintenance to empty the condensate tanks.

Desiccant dehumidifiers have a much wider range of operation in terms of temperature, -30°C to +40°C and relative humidity 0-100%. They can achieve much lower relative humidity levels, at less than 10%RH if required, and provide a much more stable overall control, typically +/-2%RH or better. There is no need for a drain connection but a ducted wet air vent is required, to exhaust the moisture externally away from the area being treated.

This will depend on lots of factors, including the type of materials in the archive, the geographic location, the size of the area, acceptable tolerance, temperature and volume of air exchange. Small mobile evaporative humidifiers are economic but have a low control accuracy, high maintenance and are not suited to large areas.

Electrode boiler steam humidifiers are a more professional solution, either as part of a ducted HVAC system or mounted directly in the room. They typically offer control accuracy of +/-7.5%RH. Resistive element steam humidifiers have a higher capital cost, but provide +/-3% control or better, and have lower spares and maintenance requirements.