8 Climate
In order to protect the collections they house, it is extremely important to get the indoor climate of an institution right. Depending on the type of collection concerned, it will be necessary to create certain environmental conditions and ensure they remain constant. The indoor climate must also be considered in terms of the fabric of the building itself, its characteristics and technical facilities. Too high values might cause damage to its structure, as a result of condensation or mold growth for example.
So how can the desired environmental conditions be created? Owing to the interaction with the outdoor climate, without suitable technical systems the indoor conditions would always be subject to large fluctuations, both during the course of a day and across the year. It is seldom feasible to influence these conditions to any significant extent by means of architectural measures or user behavior. It is therefore essential to determine to what extent appropriate technical building services are required to achieve the environmental conditions desired and necessary in the building.
I. Structural aspects
Indoor climate and collections
Indoor climate is generally described in terms of air temperature and relative humidity. Regulating the indoor climate is critical for protecting collections since many materials (e.g. wood, paper, papyrus, steel, iron) interact with the climate. In capillary-porous substances (e.g. plaster, mortar, masonry, wood), the air in the pores assimilates to the indoor environmental conditions, which can lead to either an increase in moisture or to a drying effect. This process is known as sorption and desorption respectively. Depending on the material in question, this can result in swelling or shrinking. In the case of metals, corrosion can occur from a given level of relative humidity. The crucial factor here is not the relative humidity in the room, but the relative humidity at the surface of the objects, which in turn is influenced by the temperature at the surface of the objects.
The AMEV guidelines (guidelines published by the Working Group of Mechanical and Electrical Engineering for State and Local Governments [= Arbeitskreis Maschinen- und Elektrotechnik staatlicher und kommunaler Verwaltungen], www.amev-online.de/Erlasse) provide guidance on suitable indoor climate values for various building uses. Besides relative humidity, the degree of fluctuation of relative humidity in a room during the day or across the year is critical. It is therefore necessary to determine what specific room air conditions are necessary to protect the collection, and how high the relative indoor humidity may be or needs to be, as well as what fluctuation across a day or year is permissible. The book “Sammlungsgut in Sicherheit” by Günter S. Hilbert, published by Gebrüder Mann Verlag, Berlin, provides some useful information on this subject.
Indoor climate and buildings
Besides determining the desired optimal indoor environmental conditions for protecting collections, it is also necessary to establish what conditions will not harm the building, as every building has its own characteristics and technical capabilities. Depending on the thermal protection standard applicable to the building, damage to the fabric of the building may arise if the indoor relative humidity exceeds a given threshold.
If air conditioning is to be installed in an existing building to protect the collection, it is first necessary to check that the building will not be harmed by the proposed climate values. All the building elements that separate air-conditioned rooms from unheated rooms or from the outdoor air must be analyzed and evaluated with respect to their thermal protection. Such analyses can be conducted by specialists in building engineering physics or thermal and moisture protection consultants. If analysis shows that the envisaged indoor environmental conditions cannot be maintained without harming the building, it will be necessary to investigate what measures are possible and necessary to improve the structural thermal protection. In the case of new builds, thermal protection should be designed primarily with the desired indoor environmental conditions in mind. As a rule, the thermal protection requirements for maintaining a room climate are more extensive than the requirements under the German energy saving ordinance.
Condensation and mold germination
A knowledge of some physical engineering interdependences is necessary in order to understand the possible causes of condensation and mold germination. Air comprises oxygen, carbon dioxide and a range of other gases and pollutant gases along with water vapor. This means that air is able to absorb moisture. The quantity of moisture it is able to absorb depends on the temperature of the air. While cold air absorbs only a small quantity of water vapor, warm air can bind larger amounts. The degree of saturation of the air thus increases in line with the temperature.
The chart in Figure 1 illustrates this. The line between the white and the yellow areas represents the line of this saturation humidity as a function of the air temperature. The saturation humidity is the maximum amount of water vapor the air can absorb at a given air temperature.
Figure 1: Saturation humidity of the air as a function of temperature
When evaluating climate conditions, a distinction is drawn between relative humidity and absolute humidity. Absolute humidity describes how many grams of water per cubic meter of air (g/m³) are present in the air. Relative humidity indicates the percentage of saturation humidity reached. Since warm air has a higher saturation humidity than cold air, in absolute terms warm air contains more moisture than cold air at the same relative humidity. In absolute terms, therefore, despite having a very high level of relative humidity, outdoor air with a temperature of 0 °C and a relative humidity of 90% is drier than indoor air with a temperature of 20 °C and 50% relative humidity. If one were to ventilate the room, therefore, the air in the room would become drier in spite of the high relative humidity of the outside air.
To summarize, this means that even if the temperature is low and the relative humidity is high outdoors, ventilating heated rooms will cause the air to become drier.
With respect to condensation it can be seen from the above that if a volume of air with a given temperature and a given relative humidity is cooled, the relative humidity will steadily increase while the absolute humidity will remain constant. The temperature at which relative humidity reaches exactly 100% as a result of cooling is referred to as the dew point.
Indoor dew point temperatures depend to a large extent on what the respective rooms are used for. It must also be borne in mind that as a result of their metabolism, humans constantly give off moisture depending on what they are doing. While in residential settings, moisture is constantly added to the air as a result of activities such as cooking, washing, showering, drying clothes, as well as from plants, pets and aquariums, this does not happen to the same extent in offices or in museums, archives and libraries. During the heating period, moisture is given off only by staff and visitors, for example from damp clothes (coats, umbrellas, shoes). During the winter in particular, it can be assumed that as a result of the natural exchange of air due to drafts or doors and windows being opened, the air in the room will become drier unless some form of artificial humidification is introduced. Here is an example to illustrate this with some figures.
In accordance with Figure 1, the saturation humidity is 4.85 g/m³ at 0 °C. This means that if a volume of air has a temperature of 0 °C and a relative humidity of 100%, the absolute humidity is then equal to the saturation humidity and is 4.85 g/m³. If this air is now heated to 20 °C, the absolute humidity remains the same. However, the saturation humidity changes, as the air is capable of absorbing more moisture at 20 °C than at 0 °C. The saturation humidity is 17.3 g/m³ at 20 °C. The relative humidity is the ratio of absolute humidity to the saturation humidity, which in this case is 28 %. This example illustrates the correlation between temperature and humidity.
A building has certain technical limits, which means that the relative humidity must not exceed certain values otherwise it could cause harm. This is because heat is radiated from exterior building elements that conduct heat, as a result of which the interior surfaces have a lower temperature than the air in the room. If the interior surface temperatures are the same as or lower than the dew point of the room air, condensation can form. The interior surface temperature is determined here by the thermal protection standard of the particular construction method used. Table 1 illustrates this by comparing different thermal transmittance values (U-values) for exterior building elements and the resulting surface temperatures (Toi), relative humidity thresholds (fGr) and the evaluated relative humidity threshold (fGr;0.8).
The U-value states the thermal transmittance expressed as the rate of transfer of heat in watts through one square meter of the structure’s surface divided by the difference in temperature across the structure in degree kelvin. I.e. the better insulated a building element is, the lower the U-value will be. The surface temperatures stated in the second row of the table also illustrate this. It can be seen from the table that the lower the thermal transmittance, i.e. the better the thermal protection, the higher the interior surface temperature of the building elements will be. Given the boundary conditions in question, a U-value of 1.4 W/(m²K) will result in a surface temperature of only 13.6 °C; at 18.1 °C the surface temperature expected at a U-value of 0.4 W/(m²K) is much higher. The higher the surface temperature, the higher the permitted moisture levels. The relative humidity threshold states the value from which condensation can be expected to form on the respective building element. The evaluated relative humidity threshold of fGr;0.8 indicates the level of relative humidity above which a moisture potential conducive to mold germination arises on the porous surfaces of the building elements.
Table 1: Surface temperatures, relative humidity thresholds and evaluated relative humidity thresholds as a function of the U-value of the exterior building element (example)
– Toi: Surface temperature at a room air temperature of 20 °C and an outdoor temperature of -15 °C with unimpeded heat transfer.
– fGr: Threshold for relative humidity above which condensation can be expected on the interior surface given a room air temperature of 20 °C and an outdoor temperature of -15 °C with unimpeded heat transfer.
– fGr;0,8: evaluated relative humidity threshold: Indoor relative humidity above which a moisture potential conducive to mold germination can be expected on the surface of building elements given a room air temperature of 20 °C and an outdoor temperature of -15 °C with unimpeded heat transfer.
The observations on which Table 1 is based are for unimpeded cross sections of building elements and do not take account of any thermal bridging which would normally be expected. Around thermal bridges such as window soffits or exterior wall corners, lower surface temperatures, relative humidity thresholds and evaluated relative humidity thresholds can be expected. The comparison in Table 1 is intended only to illustrate the dependence of the permitted humidity threshold on the thermal protection standard, i.e. on the thermal transmittance (U-value). The U-values of building elements can be determined from planning documents, building component tests or on-site measurements. Specialists in building engineering physics or thermal and moisture protection consultants can also assist.
As well as by the thermal protection standard, the interior surface temperature is also influenced by the heat transferred from the air in the room to the surface of the building element. If this heat transfer is impeded by furnishings (display cases, shelves, cabinets), it can be assumed that the interior surface temperature will be lower. As a result, the temperature may fall below the dew point and cause condensation.
Condensation is not however a prerequisite for mold germination. Elevated levels of moisture in building materials as a result of sorption or capillary condensation are sufficient to create a moisture potential conducive to mold germination. According to the current state of knowledge, it can be assumed that an indoor relative humidity value, referred to as water activity, of AW = 80% relative to the saturated vapor pressure (the saturated vapor pressure is the maximum pressure a volume of air can have at a given temperature) is sufficient for mold to form on the surface of the building element. The saturated vapor pressure at the surface of the building element depends on the surface temperature. With respect to mold germination, it is also necessary to take into account that exceeding a water activity level of AW = 80% for a short time will not necessarily lead to damage. For mold to form, this water activity value must be exceeded over a longer period. Table 1 illustrates the relationship between the relative humidity threshold and the evaluated relative humidity threshold with respect to the presumable risk of mold germination. See also the Pests section for mold growth on collection objects.
Mold can occur not only as a result of the interaction between the indoor climate and the surface temperatures of the building elements, however. It can also be caused by other sources of moisture, such as an inadequately weatherproofed facade, the failure of wall or roof waterproofing, or leaks from pipes carrying water. (See also the Accidents/malfunctions section.)
It can therefore be seen from the above that there are both use-related and structure-related causes of condensation and mold germination on the interior surfaces of building elements.
Use-related causes – when users do not perceive the window to be a “natural indicator” of the room climate. As far as the thermal protection of a building is concerned, windows are usually the weakest link. In comparison with the other building elements, the interior surfaces of window glazing usually exhibit the lowest surface temperatures. If the surface temperature of the glass is lower than the dew point of the air in the room, condensation will automatically form on the interior surface of the glass.
Structure-related causes – when either the glazing is not the weakest link in the building’s thermal protection and consequently does not perform its role as a natural indicator for impermissible environmental conditions or when the thermal protection of the glazing and the intended environmental conditions for protecting the collection are not matched to one another. To avoid this risk, as explained above, with air-conditioned rooms it is absolutely essential to carry out a thermal protection assessment of the individual building elements that conduct heat.
II. Air conditioning
Article 2 of the European Directive on energy performance of buildings EU2002/91/EC defines an air conditioning system as “a combination of all components required to provide a form of air treatment in which temperature is controlled or can be lowered, possibly in combination with the control of ventilation, humidity and air cleanliness”. With reference to this directive and its implementation in Germany through the energy saving ordinance, an “air conditioning system” is defined as follows:
1. Systems with a ventilation function (see Table 1 for air conditioning and ventilation systems)
2. Systems for cooling without a ventilation function (room cooling systems, room air conditioners etc.)
Key: – Not influenced by the system
x Controlled by the system and guaranteed in the room
(x) Influenced by the system but not guaranteed in the room
(Source FGK Status Report 14)
Table 2: Types of air conditioning systems and their thermodynamic function (all ventilation systems include air filters).
Mechanical ventilation systems for conditioning the air in rooms are required if the building structure does not provide sufficient protection for the collection or if other circumstances are present which represent a threat to the collection or impair the comfort of visitors and staff, for example events with large numbers of people, too many visitors in relation to the room space, a high thermal load from lighting, dust and pollutants, fluctuating room temperatures or humidity as a result of ventilation with windows.
New builds and renovated buildings generally have an airtight outer envelope (dictated by building regulations), so mechanical ventilation systems are usually required to ensure the necessary air exchange with the outdoor air. In addition, materials which release pollutants (e.g. solvent vapors) are often used when constructing and furnishing buildings. These contaminants can be removed or mitigated by means of air exchange and ventilation using mechanical ventilation systems in order to avoid damaging collections or harming the health of staff and visitors.
If considering using a mechanical ventilation system, it is first necessary to ascertain what technical services and functions it should perform (filtering, heating, cooling, humidifying and dehumidifying). At the same time, it should be checked if performance and energy expenditure can be improved by means of structural measures or energy-efficient building and system planning.
If they are to work as intended and meet the quality requirements for the lifetime of the air conditioning (HVAC) systems, existing mechanical ventilation systems require competent operation, servicing and maintenance. To ensure the energy-efficient operation of HVAC systems, the German energy saving ordinance (Energieeinsparverordnung, EnEV) also stipulates the correct operation and regular servicing and maintenance of the energy or efficiency-relevant components by suitably competent personnel.
Humidity has the greatest impact on vulnerable objects in collections. It is possible to create a microclimate that is largely independent of the general indoor climate for relatively little outlay. By using hygroscopic materials – such as wood or textiles – and/or by providing salt solutions, liquid mixtures, silica gel and other desiccants, which themselves may not emit any substances, the relative humidity in display cases can be kept within certain limits.
Climate-controlled display cases enable the collection to be protected largely independently of the environmental conditions in the room. Climate-controlled display cases are also ideal for frequently changing room conditions, such as are found in the case of touring exhibitions.
In combination with a change in the room temperature, portable or fixed humidifiers can achieve the required relative humidity to a large extent, provided controlled operation of the appliances is ensured.