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More about indoor climate and preventive conservation

Climate sensitive collection can be found in galleries, museums, storages, restoration studios and archives, here summarized as "the museum environment". Preventive conservation is a process that seeks to prevent, reduce or mitigate the effect of factors that threaten the continued survival of collection. By creating favorable climatic conditions, life time of collections may be prolonged substantially. Damage processes can be divided into mechanical damage (e.g. cracks due to expansion and contraction), microbial damage (such as fungus or insect damage) and chemical degradation (such as fading and corrosion).

 Damaged painting on canvas

For preservation of collection temperature control is generally secondary to RH control, unless specific collection is susceptible to chemical degradation, such as photographs. Because of heating for a comfortable climate during winter time, RH can drop considerable in the museum environment. Depending on the prevailing outdoor climate, long-term values lower than 20% RH indoors may occur. This is because the cold outside air can contain just very little moisture. At higher outside temperatures in summer or autumn, the outside air contains more moisture. Because of this the inside RH can increase to values well above 75% RH during summer of autumn.

 

With a change in surrounding RH, hygroscopic objects desorb or absorb hygroscopic moisture, striving for a hygric equilibrium. At a dropping environmental RH hygroscopic objects desorb moisture and at rising environmental RH objects absorb moisture. The rate of moisture transfer and their influence of this on the object depend mainly on the type of material, the thickness, finish and construction of it. As a result of this moisture release and uptake the material "works", by expanding or contracting. Depending on the duration of the too high or too low RH and the material properties, tensions in the object can occur that may lead to irreversible damages such as a rupture or delamination.

 

If the RH is too high for a prolonged period, objects can be affected by microbiological damage. In this respect also the temperature and the material properties of the object are of importance. At low temperatures insects become inactive and most fungi cannot develop. If the object itself is not a suitable substrate however, mould growth will not be a relevant danger.

 

Many museums are therefore equipped with systems that are able to control the indoor relative humidity, such as mobile humidifiers and dehumidifiers or an elaborate air conditioning system with an air handling unit and ducts. As the climate sensitivity per collection object may vary, the interior climate has to be tuned accordingly. Paintings for example desire an average RH, whereas most metals desire an RH as low as possible. In addition to the exact value of the RH, it is important to prevent large climatic fluctuations.

 

Air conditioning and building physics

Condensation on a single glass pane

By conditioning indoor air, a difference in moisture content and temperature between indoor and outdoor is created. Because of physics these differences will try to equalize, causing heat and moisture transport through the building envelope. Most historical buildings have an uninsulated and relatively air-open envelope. During winter time moisture will migrate through the construction, possibly causing condensation within the construction. If this condensation cannot dry within a certain period, for example mould growth or rotting of wood can occur. Another common problem with humidified historic buildings, is the occurrence of surface condensation on for example single glass panes. Therefore not all buildings are suitable to be air conditioned or at least setpoints should be examined carefully.

 

Balancing the indoor climate

Particularly in historic buildings with a museum function, it is essential to create an optimal climatic balance between preventive conservation, energy use and visitor comfort. This optimum is achieved by the setting of the optimal setpoints of the HVAC-system or optimization of the architectural properties of the building, such as the use of efficient solar shading or optimizing the degree of insulation. Each year many euro’s can be saved just by a more efficient running of the climate system and optimizing the setpoints, without loss of preventive conservation. This however requires thorough research and specific knowledge.

Please contact us in case of any questions, or browse through our reference projects for an impression of our approach.

 

 

Last Updated on Wednesday, 19 August 2015 09:14
 

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