American Conservation Consortium
Nationwide Collections Preservation Services
Low-Tech Environmental Improvement Services
Introduction to RH and Temperature Monitoring and Control
In a collection environment, hygroscopic materials such as wood, textiles, and paper always have some water chemically combined within their structure. The moisture content, as the amount of water is called, varies with the ambient relative humidity and temperature. Since the results are often most graphic with wood, it will be used as an example of the effect of fluctuating relative humidity and temperature. When the moisture content falls, wood shrinks across the grain. When the moisture content rises, wood expands across the grain. Wood can be considered to be dimensionally stable at all moisture contents along the grain direction.
Rising relative humidity causes an increase in moisture content and expansion across the grain, with a corresponding shrinkage when the relative humidity falls. Rising temperature causes shrinkage of wood and falling temperature, expansion. Within the environmental range encountered in Museums and historic houses, the effect of relative humidity changes is far greater than that of temperature. For conceptual purposes, a relative humidity change of 10% is roughly equivalent to a temperature change of 60 degrees F. Generally, temperature and relative humidity are inversely related. If the temperature increases, the relative humidity falls and vice versa.
In a totally unrestrained board that is rather narrow, the effect of expansion and contraction caused by fluctuating relative humidity and temperature may be minor. However, if restraint in any form is present, such as attachment to other boards, or if the board is wide, cracking, splitting, warpage, and veneer delamination can result. It is also possible to induce compression setting to wood, a permanent loss of dimension caused by an inability to expand when necessary. Restraint in historic objects is greatest in cross-grain construction - when one wooden member is attached to another and the grain directions are perpendicular to one another.
Wood is not the only hygroscopic (water-absorbing) material in historic collections. Finishes, adhesives, textiles, paper, leather and paint are all affected by fluctuating relative humidity and temperature. See Table 1 for details about different types of materials. Since all of these materials are components of complex objects, such as furniture, especially severe and damaging results can occur when one material reacts differently than other materials. For this reason, maximum preservation of furniture and most other collection items dictates that the relative humidity, and to a lesser degree the temperature, should be kept as constant as possible, not just during the course of a day, but from summer to winter as well.
In addition to problems with fluctuation, high temperatures accelerate chemical reactions. Since the bulk of degradation of historic objects is chemical in nature, high temperatures increase deterioration. High relative humidity levels (above about 65%) promote mold and mildew growth, as well as corrosion of metals. Low RH can lead to embrittlement of some types of materials, such as paper.
Historically, the generally suggested "ideals" for Museums have been 70 degrees F and 50% RH year round, although there are many circumstances where other levels should be chosen. This represents a temperature that is comfortable for humans and an RH level that is a practical compromise between the lows of winter and the highs of summer. It also assumes that specialized HVAC equipment is in use, and the building is specially designed or retrofitted to handle the deterioration risks introduced by environmental control.
The buildings at most small to medium-sized museums are equipped to maintain the temperature within a fairly narrow range, between about 60 and 90 degrees F, by utilizing heat in the winter and no control during the summer. A few have summer air conditioning, particularly in the more southern regions. However, control of the relative humidity is encountered too infrequently. This is primarily because these buildings are operated under the premise of human comfort, rather than that of object "comfort." In the winter, if the temperature is maintained at 70 degrees F, it is common to find relative humidity readings below 10%. During the summer, the relative humidity may exceed 90% for sustained lengths of time. Conditions such as these are responsible for virtually all of the cracking of solid wood, delamination and loss of veneers, distortion and cockling of paper, cleavage of finishes and paint, and mold and mildew growth on historic objects. Object "comfort" would dictate control systems based around more constant relative humidity, with less attention paid to temperature, exactly the inverse of what generally occurs in most smaller Museums.
"Ideal" environmental conditions that are suggested for collections preservation are likely to damage the building. During the heating season, in order to achieve a 50% RH level at 70 degrees interior temperature, it is necessary to add moisture to the air. The warmer the interior temperature and the colder the exterior temperature, the greater the amount of moisture that must be added. Since the amount of moisture in the air inside the building is much greater than outdoors, this moisture migrates through the walls towards the outside. At some point inside the wall, the dew point is reached and the moisture condenses on the wall materials, potentially leading to rotting wood, peeling paint, rusting metal, and spalling masonry surfaces.
Most of this damage occurs over long time periods and may not manifest itself for years. Nevertheless, it is ongoing, and a basic premise is that to ideally preserve collections, it is necessary to consider the building as sacrificial, needing to be significantly repaired or replaced at some time in the future. In a new Museum building, careful design and specialized construction techniques can allow a building to maintain "ideal" environmental conditions with minimal deterioration.
In a historic structure not designed in this manner, however, the building cannot be sacrificial and, therefore, a compromise must be reached that may not be ideal for the collection or the building, but will minimize deterioration to both. Often, this consists of holding the relative humidity within a fairly narrow range and allowing the temperature to fluctuate as necessary to help stabilize the relative humidity. By heating the air less during the winter, the natural RH level will be higher, and adding moisture to the air may not be necessary.
Each collection and building are unique and the specific solution chosen requires careful consideration of the needs of both. Generally, it is best to think of a target range of RH between 35%-60%, allowing the temperature to fluctuate as necessary to keep the RH more constant. Even greater stability of the RH, for example from 45%-55%, decreases the risk of damage to hygroscopic collection materials. Attempting to control both relative humidity and temperature significantly complicates mechanical control systems and makes them far more expensive to install and operate. If funding is limited, concentrate only on relative humidity control to maximize preservation.
The first step in environmental control is preventing moisture access to the building interior. The most useful reading for understanding the movement of moisture is the dew point. This represents the temperature at which 100% RH would be reached if the air was cooled (dew would form), thus the term dew point. This allows different RH and temperature readings in various rooms to be compared to each other in a standard manner. For example, 81 degrees and 49% RH sounds quite different from 73 degrees and 63% RH. However, both have the same dew point of 60 degrees, indicating the actual amount of moisture vapor in the air is the same. By viewing dew points, a better understanding can be gained of moisture migration from outdoors, the basement/crawl space and from one floor to another.
However, hygroscopic materials such as wood, paper and textiles are in equilibrium with the relative humidity (RH) and change dimensionally with changes in the RH. Constant RH is desirable for collections preservation. Since warmer air can hold more moisture, the same amount of moisture vapor in the air (same dew point) will have a lower RH in a warmer room and a higher RH in a cooler room, as in the example above.
Thus, it follows that in a building with a tightly sealed exterior envelope where the interior doors are kept open allowing free moisture movement within the building, the dew points should all be roughly the same, while the RH will vary inversely with temperature variations. The more even the temperature throughout the building, the more even the RH. However, this does not apply to a building with restricted moisture movement, such as closed interior doors, or that is open to the exterior environment (open windows).
Moisture infiltration comes from several different sources, presented below in rough order of importance:
Control of the environment in collection areas must begin with control of moisture infiltration problems. Make sure grading around the building carries water away from the foundation. If necessary, re-grade or install effective gutters. Maintain a tight building envelope to minimize air exchanges with the outside and to prevent direct water access to the interior.
Further improvement of the environment can be effected with mechanical systems. Most off-the-shelf control systems are designed around temperature and not RH control. Therefore, it is important to consult with a competent HVAC engineer who fully understands the concept of controlling the environment for RH stability. Such an individual is not easy to find. Therefore, it is advisable to have a conservator as part of the team to guarantee that all parties fully understand the environmental control goals.
Stability of conditions is important on a daily basis, as well as seasonally. Therefore, generally it is better to keep the RH and T stable day and night, rather than turning up the heat when staff is present (forcing down the RH) and down when they leave (allowing the RH to rise). For this reason, it is not recommended that electric heaters be used in collection areas to temporarily heat them for human comfort. Similarly, air conditioners should be run continuously when needed, not shut off at night.
Opening doors and windows for air circulation and ventilation is not a good idea for collections preservation. This promotes more rapid changes of T and RH, as well as access for dirt and insects. In fact, tightening the windows and doors by the judicious use of weatherstripping and storm windows and doors will help to hold more even RH and T in both the summer and winter. Added benefits are savings in fuel for winter heating and summer cooling, as well as reduced amounts of dust and insects infiltrating the buildings.
Contrary to popular conception, a tightly closed building does not necessarily become unreasonably hot during the summer. In fact, if direct sunlight is blocked, it generally will be cooler throughout the day than one that has open windows. At night it will remain somewhat warmer than outside. Thus, the amount of temperature fluctuation will be reduced, as will be the relative humidity fluctuation. As an example, the Fremont, New Hampshire studio of American Conservation Consortium, Ltd. is a frame building of very tight construction. Windows and doors are kept closed, shaded, and are well insulated. During the month of July when no heating or cooling of the environment occurred, the lowest temperature logged on a recording hygrothermograph was 67 degrees and the highest was 73 degrees. The outside temperature ranged from 54 to 93 degrees. It is unlikely that most buildings could be made as tight as the studio, and this narrow a range would not be achieved. However, a smaller range of fluctuation would result than is present currently.
An alternative worthy of consideration for heating systems is humidistatic control. A humidistat can be substituted for the thermostat on the heating system and set for around 50%. When the RH rises above this level, the heat is turned on, forcing the RH down. Thus, the temperature is allowed to fluctuate in order to more closely control the relative humidity. Clearly, this system can not compensate for RH levels that are too low, but it is not common for RH to fall below about 30% in unheated buildings. Thus, the RH range is kept between about 30% and 80%, instead of 5% and 80%. Allowing the temperature to fall below freezing is not a problem for most collection objects. If active plumbing is present, a temperature over-ride at about 38 degrees can be incorporated to prevent freezing temperatures from being reached. However, the over-ride may cause the RH to fall below desirable levels in very cold weather. Use of a humidistat might not be suitable for some exhibition areas, as the temperature can drop to levels that are uncomfortable for humans. However, it can be very effective for storage areas that are on their own heating system zone.
Generally, in most spaces, dehumidification is needed during the late spring, summer, and early fall. It is possible that some spaces will require dehumidification in the winter as well due to unusual moisture conditions. If the temperature is above about 60 degrees, portable dehumidifiers can be used where a central air handling system does not exist. Two factors are of extreme importance. First, all windows and doors will have to remain closed (this is recommended anyway to help stabilize the naturally-occurring environment) and the building envelope will have to be tightened, or the units will have to dehumidify all of the surrounding region in order to be effective. Second, portable units will have to be emptied conscientiously before they fill and shut off, or a large swing will occur in the RH levels when the dehumidifiers are not working, potentially causing damage to the collections. Attaching portable dehumidifiers to a drain is highly preferable to avoid repetitive emptying.
When temperatures are below about 60 degrees, conventional dehumidifiers will not function properly. Commercial portable dehumidifiers will function down to about 38 degrees. One manufacturer is Therma-Stor Products (1-800-533-7533), who offer the Hi-E-Dry product line.
An alternative for spaces that are below 60 degrees is heating the air slightly to bring the RH down to acceptable levels. A humidistat can be substituted for the thermostat on heating systems or portable heaters and set for around 50% RH. When the RH rises above this level, the heat is turned on, forcing the RH down. Thus, the temperature is allowed to fluctuate in order to more closely control the relative humidity. Clearly, this humidistatically controlled system cannot compensate for RH levels that are too low, but it is not common for RH to fall below about 35% in unheated spaces.
The use of air conditioning to control RH levels may, or may not, be feasible. As the temperature is dropped by the air conditioning, the air can hold less moisture, possibly raising the RH even though some moisture has been removed from the air. In addition, if the air conditioner is too large, it will not run sufficiently long to remove a significant amount of moisture from the air (called short-cycling), and will actually raise the RH of the space. Each specific situation needs to be tested and evaluated to determine if air conditioning can be used effectively to control RH. If so, it can be controlled by a dehumidistat in the same manner as a dehumidifier. If air conditioning can not be used alone to control RH, it can be used in consort with a dehumidifier. On most central air handling systems, dehumidification can be added within the air flow path of the air conditioning system.
Air conditioning also introduces potential problems with moisture gradients between the inside and the exterior of the building, similar to humidifying in the wintertime. This problem is most severe in hot, moist climates. Air in an air conditioned space is cooler and lower in moisture content than the outside air. Therefore, moisture will migrate through the walls from the outside toward the inside. Depending upon the specific conditions, it may reach the dew point inside the walls and condense on the wooden or metal structure of the walls. This can lead to rotting of the wood or rusting of the metal over extended periods of time. Keeping interior air-conditioned temperatures sufficiently high (generally, 78-80 degrees on very hot days) will prevent the dew point from being reached and eliminate the problem.
An alternative method of controlling the RH that can be considered is the use of electronically controlled fans to move air into or out of the building depending upon the difference between the interior and exterior. Such a system is driven by a computer and RH and T sensors. If, for example, the interior RH is too high and the exterior RH is lower, air would be exhausted from the building and outdoor air brought in. However, if both the interior and exterior relative humidities are too high, this type of system can do nothing to bring the RH within desirable levels. The system can work in reverse. If the interior RH is too low and the exterior is higher, outdoor air can be brought in. Fine filtering of air entering the building is required to prevent excessive entry of dust and dirt.
This concept of control is complicated by the fact that RH levels change depending upon the temperature of the air. Colder outdoor air may have a higher RH, but when it is warmed once inside, its new RH may in fact be lower than the interior air. Use of dew points instead of relative humidities eliminates this problem. Then, when the dew point of the air outside is lower than the dew point of the air inside, running the fans will always reduce the amount of moisture inside the building. Due to the complex nature of the required measurements and calculations, a computerized control system is required.
Every building has unique needs and solutions for control of the T and RH. However, in order to make intelligent suggestions on environmental needs and improvements, the existing environment must be monitored for a continuous period of at least a year. A comprehensive program of monitoring the temperature and relative humidity allows thoughtful steps to be taken in controlling the collection's environments. They will contain signals of whether dehumidifiers are needed and when they need to be turned on; if and when air conditioning is required; and may serve to point out problems with drainage, leakage or water infiltration before they would otherwise be noticed. Without monitoring, environmental control efforts are just guesses, with no way of determining if the desired effects are actually occurring.
Data loggers are about the size of a deck of cards or smaller, and contain a battery that gives them an extended life. They take T and RH readings at a user-selected interval, for example, every 15 minutes, and store them internally on a microchip. About once a month (or less frequently if readings are less frequent), the data collected is down-loaded into a personal computer, using the relevant software. Various charts, graphs, and statistics can be generated from the collected data.
The advantage of this system is that the data collected can immediately be manipulated into forms that give useful information. For example, a chart of weekly averages can be generated for a full year cycle. To do this with recording hygrothermographs would require somewhat tedious examination of fifty-two weekly charts and manual averaging and recording of the readings.
An inexpensive data logger is made by Onset Computer Corporation (508-759-9500). This logger sells for about $95 and is quite tiny, being about the size of a small box of matches. Called the Hobo H8 RH/Temp/Light/External, this logger is not quite as accurate as the more expensive loggers, but is fine for the needs of collections within historic buildings. It is an excellent alternative that allows for sophisticated environmental monitoring on a limited budget. As an added bonus, the logger has a light sensor which can be activated to record light levels. Onset also sells other variations on this logger, including one which is weatherproof for outdoor locations.
The design, purchase (of both the system and computers), installation, set-up, and evaluation of monitoring methods is fundable through the Conservation Project grant program of IMLS. The monitoring time can be used as part of the institution's match for an IMLS-CP grant if the project is properly designed.
A complete year of records allows intelligent steps to be taken in controlling the collection's environments. For this reason, assembling the readings should not be taken as an exercise in record-keeping. The readings must be examined and compared, and used as the basis for decision making. They are probably the most important tool to be used in the preservation of the collection, and, in fact, for the preservation of the building itself.
Readings obtained from a monitoring program must be analyzed. If desired, an outside consultant, such as American Conservation Consortium, can be contacted for assistance. As mentioned previously, the goal for RH is 35-60%, with temperature control of relatively minor significance. If tighter RH ranges can be obtained, even less deterioration will occur.
American Conservation Consortium. Ltd.
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