Humidity and Temperature

Modifying the conditions in buildings

Airconditioning

Airconditioning is the most obvious, but not necessarily the best, method of controlling temperature and relative humidity. The method involves taking air—either fresh air from the outside or recycled air from the inside—and changing its temperature and moisture content.

There are two basic types of cooling airconditioners available.

An evaporative airconditioner works by passing air over a moist surface and increasing the moisture content of the air, raising the relative humidity. This type of airconditioner should not be used unless there is a dehumidifier to remove the moisture from the cool air.

Cooling coil airconditioners work on the refrigerator principle of keeping the air cool and dry. This type of airconditioner should be used with caution, and preferably with a humidifier to add moisture to the air. Monitoring the effectiveness of such equipment is crucial.

If you have airconditioning or you are considering installing it, you should be aware of the following important points:

• the cost of purchasing, operating and maintaining an airconditioning plant is high. If such a financial commitment is possible, seriously consider getting a system which not only regulates temperature but is capable of controlling the relative humidity as well;

• if airconditioning is used to control the environment, it should operate continuously. For example, it is tempting to turn off the airconditioner because of the high operating costs. But the cyclic process of turning it on and off is likely to be more damaging to collections than no air-conditioning at all;

• airconditioning systems have a limited life. They will operate at greatest efficiency for 10 to 15 years; and

• airconditioning systems should be well maintained, otherwise you could experience fluctuations in the environment.

Heating

It is sometimes necessary to heat whole buildings or individual rooms. Generally, heating is used to make people comfortable. This is an important consideration; but you should be aware also that raising the temperature affects the objects in the building.

Heating affects relative humidity. Remember:

• heating a building in an already dry environment could be disastrous, because it will lower the relative humidity;

• heating a building when there is an additional source of water will evaporate more of the water. The relative humidity may remain unchanged or it may alter, depending on the amount of water available and the amount of heat applied. This may not be the effect you're hoping to achieve; and

• if you want to raise the temperature without lowering the relative humidity, you need additional water. Additional water sources could include mechanical devices such as humidifiers or, more simply, dishes or trays of water left to evaporate.

Altering the relative humidity

It is possible to vary the humidity without markedly changing the temperature. Relative humidity can be reduced using a dehumidifier. This is a remedial measure which adjusts a dangerously wet environment.

Dehumidifiers Photograph courtesy of Artlab Australia.

Moisture can be introduced to the air using either a steam generator or an ultrasonic humidifier. These are remedial measures which adjust a dangerously dry environment.

These machines should not be seen as permanent solutions to your environmental problems. They can be costly to run, they need fairly constant attention and can be bulky and noisy.

The building as a buffer zone

Don't despair if your building is not air-conditioned and you can't afford airconditioning. A well-maintained building of solid construction provides a very reasonable environment for collections.

A building made of thick stone walls or cavity-brick construction with high ceilings provides good insulation against climatic changes. In hot weather, these buildings take a few days to heat up; and then, as the outside temperature drops, they lose heat slowly. Fluctuations occur, but they occur gradually.

Make sure your building is well-maintained, so that it provides the maximum possible seal against fluctuations in the outside environment. This is particularly important if the building is made of light building materials and is in a fairly extreme environment.

Clean out gutters, repair cracks in walls and ceilings, and have leaky roofs checked. This improves the stability of the temperature and humidity inside.

If you are considering upgrading the buildings, remember to insulate walls and ceilings—especially if the building is constructed from light-weight or heat-conductive materials such as fibro and corrugated iron. By so doing, it can not only help to modify the internal environment, but also ensure better preservation of the collection as well as making people feel more comfortable inside.

Use the features of the building

Improved conditions for the storage and display of collections can be achieved by choosing good storage and display sites within the building, and using the features of the building to modify conditions.

The most stable area of a non-airconditioned building is an internal room on the ground floor—because it is buffered against climatic changes.

There are other areas one could choose. Basements are acceptable as they provide a cool temperature. However, they are likely to be damp. On the other hand, an attic would often not be insulated, but would be dry.

If there is an optimum choice, the most sensitive objects should be displayed in an internal room, or at least against an internal wall in preference to an external one.

In warm, humid conditions, such as in tropical areas:

• air flow and good ventilation are important if you want to minimise damage to collections;

• use oscillating fans, with doors open to improve air movement; and

• consider other ways of cooling the inside of the building. For example, install interior blinds on windows to limit the amount of heat coming into the building, install exterior shutters or awnings, or put up shade-cloth. If possible, plant trees around the building, but not too close because this gives insects easy access to the building.

If you are building a museum or modifying an existing building in the tropics, remember that non-airconditioned buildings should have breezeways, if possible.

In arid or temperate zones, when the temperature is extreme outside the building, you can limit temperature fluctuations inside your building by:

• keeping doors and windows closed;

• keeping self-closing doors well oiled, so that they shut quickly and fully;

• installing interior blinds on windows, to buffer against outside conditions;

• installing exterior shutters or awnings, or putting up shade-cloth; and

• planting trees around the building—but not too close because this gives insects easy access to the building.

Encourage people to leave wet umbrellas and coats at the front door—by providing umbrella basins and coat hooks. This will prevent the introduction of excess moisture.

Local climates and microclimates

In the same way that you use the features of a building to modify the conditions within the building, you can use doors, windows, blinds and awnings—to modify conditions within individual rooms.

There are a number of other steps which can be taken to create and modify microclimates within the building.

Layers of storage

When storing important objects, give them their own microclimate by providing layers of storage as this provides some protection against climatic extremes and fluctuations, even when conditions in the room or building are difficult to control.

Microclimates also exists within glazed frames, display cases and boxes.

A display case is a box with transparent sides into which an object is placed. The walls of the box isolate the object to some extent. Passive control systems can also be set up within the box.

Silica gel is the most common material used to control relative humidity in display cases. It can be pre-conditioned to achieve a desired humidity level; the quantity placed in the case depends on the volume of the case.

Other materials, such as Nikka pellets and Artsorb, operate on the same principles as silica gel.

Cheaper alternatives which can be used include clean, cotton scraps and shredded, acid-free paper. These absorb excess moisture, but do little to correct a dry environment.

These documents are well protected from evironmental fluctuations by layers of storage—individual sleeves plus a storage box. Photograph courtesy of Artlab Australia

Acid-free wrappers, interleaving, mounting and framing when used individually or in combinations, create layers of protection from extremes of, and fluctuations in, relative humidity. They create small, isolated microclimates in which the relative humidity fluctuates slowly.

The layers of mounting and framing materials buffer these items against environmental extremes and fluctuations. Photograph courtesy of Artlab Australia