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Humidity Control

This section details a few caveats about control technique.

Control of humidity requires the capability to sense and increase or decrease humidity in the system. Control is complicated by the energy exchange accompanying water phase changes as humidity is increased or decreased so the requirement for simultaneous control of temperature and humidity poses special problems

Topics of interest

Passive Microclimate Control

Buildings “rely entirely on the natural stability provided by their structure and geometry. The hygrothermal inertia depends mainly on the thickness of the walls and their composition, the interiors and the air exchange rate. The building will ameliorate the diurnal fluctuations in temperature and relative humidity (rH). The indoor climate in a building with only passive control is thus decided by the building itself and the outdoor climate.“ [1]

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Humidity Control

Humidity control is performed by releasing water vapour to the air if the RH is too low (humidification), or by removing water vapour from the air if the RH is too high (dehumidification).

There are two methods of humidification:

  • Injection of steam into the air
  • Evaporation of water or water mist

Dehumidification is a commonly used method for decreasing high values of relative humidity. There are two methods for dehumidification:

  • Condensation – requires a bucket or a drain to collect and remove condensate
  • Adsorption - requires an air duct to remove moist air

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Humidistatic Heating

“Humidistatic heating, or conservation heating, is the concept of heating a building in order to keep the relative humidity below given limits. The temperature is continuously adjusted and not controlled to a constant set point. Humidistatic heating has been used for many years to maintain a moderate relative humidity in historic houses in winter. It is a simple and robust climate control strategy. The functionality depends on building characteristics, hygrothermal inertia, insulation, air tightness, the air infiltration rate and the temperature control.“ [1]

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Equal-Sorption Humidity Control

“One of the main tasks of preventive conservation is to prevent moisture sensitive materials from anisotropic swelling or shrinking caused by changes in the absorbed moisture content… Although the moisture content compensation is dependent on particular material properties, it has been shown that the difference can be neglected for most types of organic hygroscopic materials.

The stabilisation of moisture content in the objects is implemented by humidity control. “

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“The results presented here can provide guidance for adjusting the climate control strategy … the methods presented are generally applicable and could be used for any building in any region.

Passive climate control would not be acceptable by any standard. The variations in rH are too large, both in the short and long term, and there are long periods of excessively high rH. Humidistatic heating should only be implemented if there is a need for human comfort. Auxiliary dehumidification would be needed in the summer to avoid uncomfortably high temperatures and to keep rH at safe levels.

Equal-sorption control with dehumidification and humidification gives the most stable conditions for the moisture-sensitive objects (wooden panel paintings). However, the difference when compared to conventional humidity control is relatively small.

Natural climate fluctuation control provides rH stability in the short term, but seasonal variations and high levels of rH are problematic. In this case, the control algorithm needs additional conditions to address long-term variations and high levels of rH… as the different approaches have different control targets, there are no optimal solutions based on quantitative data.

The primary options would be equal-sorption control, humidity control and an augmented natural climate fluctuation control.

Preventive conservation can be described as a process that seeks to prevent, reduce or mitigate the effects of factors that threaten the continued survival of collections.“ [1]

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