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close this book Prevention and treatment of mold in library collections with an emphasis on tropical climates: A RAMP study
close this folder 4. Prevention
Open this folder and view contents 4.1 Building design and modification
Open this folder and view contents 4.2 Interior modifications in existing facilities
View the document 4.3 Stack maintenance
View the document Literature cited

4. Prevention

There is no question that the prevention of mold growth is much easier when the library environment is controlled, but the technology for such control is expensive to install and maintain. Modification of the environment though less expensive, is not without cost. Care of collections is as important as acquisition and organization, and should be budgeted by every library and archive. Although there are no panaceas, there are a number of possibilities for environmental modification that will enable institutions to reduce likelyhood of mold damage to collections.


4.1 Building design and modification

If one is fortunate enough to be involved in the design of a new building with state of the art environmental controls, there is ample coverage in the library and museum literature, beginning with Garry Thomson's excellent volume, "The Museum Environment"1 which should be an invaluable aid to librarians. No attempt will be made here to retrace this ground. Instead, attention will be focused on the modification of existing buildings and the design of new buildings which will not incorporate environmental control.

It should be noted that a building constructed with the idea that environmental controls will be added at some future date is not a viable option. A building designed in such a way that environmental controls can be effectively installed and economically operated in the future, will most probably be insufferable for both users and collections in the meantime. The low ceilings and closed interiors that make environmental control possible create the worst possible environment in the tropics. By the same token, a building designed to take advantage of natural ventilation makes the installation of a complete environmental control system virtually impossible, or at the very least, astronomically expensive. Decisions regarding the environment must be made early in the planning.

Even without environnmental control, good design can do much to reduce the negative impact of the locale's prevailing climate. There is surprisingly little literature available on building design in the tropics. Vance's bibliography2 lists a scant fifteen pages of references, many 20 to 30 years old. Though the literature is not extensive, working together the librarian and the architect can design a building that will safely house the collection. In building design, it is important that the area's particular type of tropical climate be taken into consideration since the requirements will be different for each of them, though some common denominators do exist.


4.1.1 Location

The various climatic zones discussed in the introduction are important in determining the requirements of a new building, or the best methods for modifying the enviroment in existing structures. Fry and Drew3 provide additional information on the particular variations between continental and island locations which will be helpful in modifying the environment. Only general guidelines can be included here.

In the tropical rainforest climate (Af) where conditions are relatively uniform year round, temperatures are seldom extremely high (usually less than 90 F), and winds are light or non-existent, the major effort should be concentrated on improving circulation and lowering the relative humidity.

In the monsoon climate (Am) the stronger prevailing winds may be used to advantage for improved ventilation and circulation and more resources can be devoted to lowering the relative humidity, especially during the rainy months.

In the tropical savanna (As or Aw), with three distinct climatic seasons, more elaborate systems may be required. During the dry, hot seasons, dust and dirt will be a particular problem. The building must be capable of being closed against dust during this period, while maintaining adequate ventilation in order to prevent the build up of heat in the interior of the building. The problems associated with extremely high temperatures, dust and desiccation may make air conditioning the most important factor in maintaining collections in these conditions. Because there are two dry seasons and a relatively short wet season, mold may be a problem for only a small portion of the year, or not at all. Every effort should be made to prevent extreme fluctuations in RH between seasons. The utilization of natural ventilation in savanna climates varies from that in the Af and Am climates. Okley4 provides several useful diagrams suggesting possibilities for natural ventilation in As and Aw climates.


4.1.2 Structural considerations in environmental modification

Temperature and circulation can be modified directly through building structures. Relative humidity can be modified only indirectly through the effective use of natural ventilation or through technological control which will be discussed later.


East and west walls, which receive the brunt of the morning and afternoon sun should be protected and insulated so that the sun's heat is not transmitted to the interior of the building. The roof, which has a high level of exposure to the mid-day sun, should reflect heat and there should be an attic or ventilation space directly below the roof to provide insulation for the interior of the building.

Double wall construction is an excellent way of insulating buildings in the tropics. Air is an effective insulator, and prevents heat from passing through the outer wall to the inside of the building. In many places in the tropics, the hollow cement block is a staple of the building trade. It provides an economical, though not always aesthetic, building material, and provides adequate insulation for the interior of the building. A true double wall construction is more effective, but considerably more costly. This construction is also used effectively in temperate climates where extremes of cold and heat make long term operating costs for environmental control a primary consideration.

The brise soleil is a variation on the double wall. It may be designed as part of the building, or attached to the facade of existing buildings. Though not as effective as a double wall, it provides protection by absorbing the sun's primary radiation. It also reduces the light levels inside by shading the windows, and allows windows to remain open even in the rainy season. It may cover the entire wall, part of the wall, or in some cases, only windows though this latter construction is considerably less effective in reducing the transmission of solar radiation.

Shading of exposed walls can take a number of other forms, including exterior landscaping with trees and shrubs, the extension of over ranging roofs, and the installation of exterior canopies. The use of interior blinds, curtains or louvres can also reduce the transmission of heat through window glass. Kukreja5 provides a table evaluating the effectiveness of various shading devices based on their reduction of total heat gain, efficiency in ensuring cross ventilation, and the percentage of natural light resulting from that particular form of control.

Glass both transmits and intensifies heat. Large windows in tropical climates can significantly increase interior temperatures, yet are incorporated into many buildings for aesthetic reasons. Ultra violet and heat absorbing films are effective in reducing heat and UV light without obscuring views or lowering the light levels too greatly.

High ceilings are a common feature of older buildings in the tropics and are an effective means of diffusing interior heat. As the warm air rises, it can be pulled out of the building with ceiling or attic fans or through windows placed directly beneath the roof overhang.


In general, buildings in tropical climates should be oriented in such a way as to take advantage of any prevailing winds and designed so that cross ventilation is possible in all areas of the building.

Even buildings designed to utilize natural ventilation will require back-up systems of mechanical ventilation for those times when prevailing winds fail or shift.

Placement of windows is a principle means of assuring adequate air circulation, once the orientation of the building has been determined. Kukreja6 provides excellent diagrams of interior air movement for various window placements. These are useful not only in the design and modification of buildings, but in anticipating problems and determining stack arrangements. He notes that a single window serves no purpose as far as interior ventilation is concerned, and that if modifications are to be made, the best results are obtained by placing windows on opposite walls to insure cross ventilation. Enlarging the outlet window opening results in a definate increase in interior air movement, even when the inlet window remains unchanged. Circulation is also increased significantly by increasing the height of narrow inlet and outlet openings. He also compares the circulation figures for various window areas in relation to floor area, demonstrating that air movement peaks when window openings are equal to 25% of the area's floor space.

Louvred windows can provide excellent ventilation, and are common in the tropics, but are difficult to seal against rain and insects, and their use is best combined with other window treatments. All open windows should be screened with well fitted, fine mesh, fiberglass screens. Placement of the screens on the inside of the windows will facilitate their removal for cleaning, particularly if there is a brise soleil on the outer wall.

If the existing building has high ceilings, the installation of ceiling fans is an excellent investment. Used in conjunction with standing floor fans, or window fans adequate air circulation can be maintained at a relatively modest cost even in Af climates where there is very little natural air movement.


4.2 Interior modifications in existing facilities

In addition to the modifications in building structure which will reduce. the transmission and retention of heat and moisture, modifications can be made in stack and storage areas which will benefit the collections.


4.2.1 Location of stack and storage areas

Because of the high water table common in tropical climates, buildings are usually constructed without the basements and subbasements common in temperate climates. In the event that the building does have one or more levels below ground, every effort should be made to avoid using these areas for either stacks or storage of unused collections. Sub-surface areas are difficult if not impossible to adequately seal, and moisture from the ground will wick through the walls. Even if walls are coated with moisture barrier sealants there is a tendency for moisture and salts to build up below the surface of the coating until the surface of the wall and the coating begin to flake, (a condition known as spelling) exposing the interior of the wall and allowing the moisture to come through into the interior of the building. Adequate ventilation is also difficult to maintain. These factors result in warm, damp, still air and virtually assure the growth of mold.

Even if underground areas are not used for storage, both walls and floors should be sealed as thoroughly as possible to prevent the elevation of relative humidity throughout the building. Frequent inspections should be made to monitor conditions in these areas, and staff should be aware of potential trouble spots in the building.

Sealed interior rooms should also be avoided, unless they can be environmentally controlled by mechanical systems to control both temperature and relative humidity. Such areas should be monitored regularly. In buildings designed with such areas, ventilation may be improved by replacing solid interior walls with a half wall of louvred windows which can provide cross ventilation either naturally or with the use of fans.


4.2.2 Stack arrangement

Stacks should not be placed directly against exterior walls as heat and moisture transfer is greatest there, and circulation will be severely limited. Even a foot of air space between the wall and the stack will improve circulation and prevent condensation of moisture on the wall from creating a micro-climate.

Stacks should be arranged parallel to the air flow, so that the prevailing air movement is across the spines of the books as they stand on the shelves. Stacks should never block air flow from existing windows or ventilation created by fans.

Stacks should be open backed, particularly free standing stacks which are joined at the back. This will improve the ventilation on all sides of the volumes. If strength or stability of the stacks is a concern, cross braces should be used rather than the solid panel supports provided in many commercially available stacks. Compact shelving, however desirable from the stand point of space saving, should be avoided in the tropics, primarily because a micro-environment may be created when the stacks are closed. In addition, the mechanisms for moving the stacks tend to become inoperable in high humidities.

Closed cabinets should be avoided whenever possible. If they are necessary for the storage of microfilm or locked case books both the back and front of the cabinet should be ventilated, or a favorable microclimate should be created in the closed cabinet to counteract the high relative humidity.


4.2.3 Localized environmental modification

In addition to taking advantage of natural conditions in modifying the overall building environment, there are various technological methods for modifying the environment in localized areas within the library. In most collections there are materials which warrant special protection. Rare and valuable works, and items of particular historic importance are often included in this category. There is therefore a tendency to want to create a special area within the library where these materials can be kept secure, and receive the benefits of a more nearly ideal environment. Localized environmental modification may be used in addition to the measures cited above, but should not be considered substitutes for the modification of environment in the building as a whole.

Monitoring Existing Conditions

Before attempting to alter the environment in a specific location, it is imperative that existing conditions be understood. This requires comprehensive monitoring of the existing environment. Information should be available for conditions at given locations at all hours of the day and for all seasons of the year. If air conditioning units are to be installed, it is important to be sure that the lower temperature will not cause an unacceptable increase in the relative humidity, either immediately or during certain seasons.

The most efficient way to monitor temperature and RH is the recording hygrothermograph which provides a 24 hour a day record for seven days at a given location. Several hygrothermographs will be needed, and a schedule should be established for moving them to various locations, so that information will be available for all seasons in all areas of the library. These instruments are relatively inexpensive, $300-500, and are cost effective in the long run. Alternatives include fixed and portable monitors with no recording capability. Thermometers, hygrometers, and hygrothermographs can give readings of temperature, humidity and temperature and humidity respectively, but provide no charts and must be monitored regularly by staff in order to give an overall picture of conditions. These require considerable staff time. Readings must be taken at a variety of locations, at specified times throughout the day (and the night) and the readings recorded in order to create an accurate chart for conditions in each given location.

Sling or motorized psychrometers are necessary for calibrating other monitoring equipment, and can be used for instant readings in problem areas.

Paper humidity indicator strips are of relatively little use in truly tropical climates. They almost always register in the pink (humid) range, and most indicate only very broad changes in relative humidity. Their best use may be in closed cases where a drier microclimate has been created, but they must be monitored regularly in order to be of use.

The equipment recommended above for monitoring conditions prior to an alteration in local environments is also essential in maintaining the desired environment, avoiding fluctuations, and assessing the cause of mold outbreaks should they occur. Acquisition and maintenance of these monitoring devices should be considered as a long term investment in collections care.

Air Conditioning

The term air conditioning, as used in this study, refers to the utilization of individual mechanical units to cool and filter air within a localized area of a building. Central plants which provide environmental control for the entire building are beyond the scope of this work. Air conditioning units fall into two basic categories: evaporative cooling and chilled water cooling.

Evaporative cooling is the simplest and least expensive system, however it is generally not suitable for areas with year round high temperatures and relative humidities.

Chilled water cooling units include a refrigeration device which lowers the air temperature, and a heating unit which warms the air slightly before it enters the room. In humid tropical climates, this procedure is critical, since the air introduced must be above the dew point to prevent an unacceptable increase in relative humidity. A change in temperature of only 1 F will result in a change in relative humidity of 3%. me monitors that control this process are most important. There are various different types of monitors for this system, including wet-dry bulb control, similar to that of a psychrometer, and hair hygrometers similar to those used on hygrothermographs.7 The cost of a refrigeration unit may be double the cost of an evaporative unit, and is considerably more costly to operate in terms of energy costs. There is often a tendency to reduce operating costs by shutting down the heating unit. This inevitably results in severe environmental problems. If the unit is to be purchased and installed, it must be operated properly.

The kind of filtration chosen and the degree of recirculation desirable depend very much on local conditions. Filters should be cleaned or replaced on a regular basis. Not only will this improve the air filtration, it will result in more economical operation of the system. Electrostatic systems should be avoided because they produce ozone which can damage organic materials.8

One other consideration regarding the use of air conditioning units should be mentioned. Air introduced into a particular locale will, like water, find its own level. Cool air entering at or slightly above floor level will remain there while the warm air floats above it. Air conditioning units should be installed as high up in the wall or window as possible to achieve maximum circulation in the area. Stacks and cabinets should be positioned in such a way that they do not block the air flow.


In humid tropical climates, dehumidification may be the most important factor in preventing mold growth. Its strongest challenger is good air circulation, not air conditioning. As noted above, air conditioning may make dehumidification even more necessary. Portable dehumidification units should be available in every library, and for some the installation of a permanent system may be necessary.

The most common methods of dehumidification are mechanical dessicant units are usually restricted to larger, fixed installation systems. They are quite efficient, relatively easy to maintain, and might be considered by institutions with severe, year round humidity problems. A system described by Gates will dry and circulate 1500 cubic feet of air per minute and remove up to 20 pounds (approximately 4 gallons) of water per hour.10 Dehumidification using heated air is, in general, not appropriate in tropical climates, and of the three types is the most costly to operate.

The most effective and economical systems for warm climates are the refrigeration units. Moisture is removed from the air as it condenses on refrigerated coils. Portable units operate on the same principle, and require very little maintenance and energy. Most have very basic internal monitoring devices, and can be set to maintain a given level of relative humidity.

One of the major advantages of dehumidification systems is that they do not require the major duct work that airconditioning systems entail. According to Gates, water vapor will migrate to the point of lowest moisture content.11 Thus, even portable machines can be left in place and adequately dehumidify a room. In large areas, several machines will be necessary.


4.2.4 Creating microclimates in cabinets and cases

A microclimate is any variation from the prevailing temperature and relative humidity of the surrounding environment. It may be either negative or positive in its effects and may occur unsolicited or be artificially induced and maintained.

It may, at times, be necessary to create a microclimate within the larger building environment. This could be occasioned by the nature of the materials, the necessity of protecting valuable items, or by the desire to remove them from a controlled environment and exhibit them in one that is uncontrolled. Microfilm, maps and documents stored in file cabinets are obvious candidates for microclimates in high humidity environments. While the incidence of mold growth can be reduced in the collection as a whole through improved circulation, the closed metal cabinets designed for the storage of microfilm, maps and documents tend to retain moisture, especially if they are not frequently used. By artificially lowering their interior relative humidity a beneficial microclimate can be maintained.

A reduced humidity microclimate can be establised in a closed storage cabinet through the use of dessicants which abssorb moisture from the air. There are a number of products which can be used as dessicants. Two of the most readily available are silica gel (which is available in various grades) and is widely used in the U.S. and Europe, and Nikka pellets (also called Kaken Gel) which is used in Japan and the Far East. Nikka pellets have been found to be more effective than silica gel at humidities above 60%.12 Silica gel often includes a color indicator which turns from blue to pink as moisture is absorbed and indicates when the material has reached its maximum absorption and must be reconditioned.

Before use, the dessicant must be conditioned to 0% relative humidity. This is done by heating the material in an oven to drive off moisture. The pellets or crystals can be reconditioned and used many times without losing their absorption capacity. After conditioning, the dessicant may be placed in the cabinets, either in trays in the base, or in small cloth bags in individual drawers. If the dessicant used is not a color indicator, a hygrometer or indicator strips must be placed in the cabinet to indicate when reconditioning is necessary. Once the cabinet reaches the desired humidity, and an equilibrium has been reached, the dessicant will require reconditioning less often. If the cabinets are used frequently, reconditioning may continue to be required at frequent intervals. The larger the quantity of dessicant used, the longer the microclimate can be maintained before reconditioning is necessary.

There is a great deal of literature available on the creation of microclimates, much of it dealing with the installation of exhibit cases and the packing and shipping of works of art, but virtually all of it is relevant to the control of environments in closed storage cases or other fixed locations. Stolow's recent publication13 contains information on state-of-the-art microclimates.


4.3 Stack maintenance

Routine cleaning and maintenance lack glamour in every library, however it is particularly important where environmental control is lacking. When natural ventilation is used to maintain adequate circulation, dirt and dust is a constant problem. Since these particles are hygroscopic, attracting and holding moisture from the air, and since they often contain nutrients required by molds, the constant cleaning of stack areas is essential in tropical climates. Vacuuming will also reduce, even if only temporarily, the number of spores on materials. In savanna climates, a thorough cleaning just before the rainy season may eliminate outbreaks of mold altogether.

A routine schedule for vacuuming of all books in the stacks on an annual basis (or as often as possible) should be established and maintained. Frequent inspection of the stack areas is also important, particularly little used sections and storage areas.


Literature cited

1. Garry Thomson. The Museum Environment. London, Butterworths, 1978.

2. Mary Vance. "Tropical Architecture: A Bibliography." Vance Bibliographies Architectural Series #A 738. 1982.

3. Maxwell Fry and Jane Drew. Tropical Architecture in the Humid Zone. New York, Reinhold, 1956. pp. 34-36.

4. David Oakley. Tropical Houses; A Guide to their Design. London, Batsfor, 1961. p. 119.

5. C.P. Kukreja. Tropical Architecture. New Delhi, Tata McGraw-Hill, 1978. p. 74.

6. Kukreja, pp. 96-98.

7. F. Hugh Howarth. "An approach to air-conditioning." Contributions to the London Conference on Museum Climatology. Garry Thomson, ed. London, International Institute for Conservation, 1968. pp. 173-180.

8. N.S. Brommell. "Conservation of Museum Objects in the Tropics." Conference on Museum Climatology. Garry Thomson, ed. London, International Institute for Conservation, 1986. p. 145.

9. The Dehumidification Handbook. Amesbury, Mass. Cargocaire Engineering Corp. 6th ed. 1987.

10. Albert S. Gates, et al. "Dehumidification." Deterioration of Materials. Greathouse and Wessel, p. 726.

11. Gates, p. 728.

12. May Cassar. "Checklist for the Establishment of a Microclimate." Canadian Conservation Institute, 1984.

13. Nathan Stolow. Conservation and Exhibitions. London, Butterworths, 1987.