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Low-Tech Environmental Control
Copyright 1997, Marc A. Williams

One of the most important factors for preservation of collections is provision of proper environmental conditions. However, few preservation concepts are as misunderstood. This is due partially to inappropriate target levels of temperature and relative humidity, and partially to a lack of information about how to achieve the desired target levels.

Most museum and historic site administrators realize that sophisticated heating, ventilating and air conditioning (HVAC) systems will provide the desired results, but these are often well beyond their budgets. Very few realize that acceptable environmental control can be provided with relatively inexpensive low-tech solutions. In fact, some preservation procedures cost nothing and actually reduce current expenditures.

The effect of Relative Humidity and Temperature on Collection Objects

In a collection environment, hygroscopic materials such as wood, paper, textiles, paint, and leather always have some water chemically combined within their structure. Their moisture content varies with the ambient relative humidity and temperature. Rising relative humidity causes an increase in moisture content and expansion, with a corresponding shrinkage when the relative humidity falls. Rising temperature causes shrinkage and falling temperature, expansion.

Within the environmental range encountered in historic buildings, 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.

Unfortunately, the amount of expansion and contraction due to environmental fluctuations is not uniform from one material to another. Since complex objects, such as furniture, contain multiple materials (wood, adhesives, finishes, metal, textiles, etc.), especially severe and damaging results can occur. For this reason, maximum preservation of collection items dictates that the temperature, and especially the relative humidity, should be kept as constant as possible, not just during the course of a day, but from summer to winter as well.

Relative humidity and temperature can also cause other types of degradation to collection objects. Table 1 lists common types of deterioration for different types of materials. Unfortunately, the preferred range of RH and T for one type of material may contradict the needs of another material. Since most collections are composed of a whole host of material types, compromises in levels of RH and T may be required.

Table 1 - Possible Damage to Various Materials due to RH and Temperature


Effects of RH Effects of T
Wood High - swelling, warpage, mildew

Low - shrinkage, splits

Fluctuating - delamination, splits

Preferred range: 40%-60%

High, low, or fluctuating - minimal effect

Preferred range: Below 90 deg. F

Finishes, paint High - bloom, mildew

Low - embrittlement

Fluctuating - cracking, delamination

Preferred range: 40%-60%

High - softening, accelerated aging

Low, fluctuating - minimal effect

Preferred range: Below 45 deg. F

Metals High - corrosion/rusting

Low, fluctuating - no effect

Preferred range: Below 35%

High - increases rate of corrosion

Low, fluctuating - no effect

Preferred range: Below 90 deg. F

Glass, ceramics No effect No effect
Textiles High - mold, mildew

Low - possible embrittlement

Fluctuating - possible distortion

Preferred range: 40%-60%

High - accelerated aging

Low, fluctuating - minimal effect

Preferred range: Below 45 deg. F

Paper, photographs High - mold, mildew

Low - possible embrittlement

Fluctuating - possible distortion

Preferred range: 30%-50%

High - accelerated aging

Low, fluctuating - minimal effect

Preferred range: Below 45 deg. F

Leather, skin High - mold, mildew

Low - possible embrittlement

Fluctuating - possible distortion

Preferred range: 40%-60%

High - accelerated deterioration

Low, fluctuating - minimal effect

Preferred range: Below 45 deg. F

Stone No effect No effect
Ivory High - swelling, warpage, mildew

Low - shrinkage, splits

Fluctuating - delamination, splits

Preferred range: 40%-60%

High - accelerated aging

Low, fluctuating - minimal effect

Preferred range: Below 45 deg. F

Target Relative Humidity and Temperature Specifications

General specifications for relative humidity and temperature are often given in museum literature as 70 degrees F and 50% RH. However, it can be seen from Table 1 that these levels are outside the preferred range for many types of materials. This is most obvious for temperature, with the majority of collection objects surviving significantly longer at lower temperatures. Historically, the traditional standard of 70 degrees was chosen for human comfort, not for the preservation of the collections. Object "comfort" would dictate control systems based around relative humidity, with less attention paid to temperature, exactly the inverse of most smaller museums. For collections in temperate climates, a much better set of specifications would be 35%-55% RH with a temperature no higher than 40-45 degrees in the winter (no heat at all is acceptable), and no more than 90 degrees in the summer (76-78 degrees if air conditioning is utilized). These levels are preferred for several reasons.

First, most smaller institutions currently maintain warmer temperatures in the winter with no control of the relative humidity. If temperatures are kept around 60-70 degrees, it is common to find relative humidity readings below 10% - far too low. 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, crazing and cleavage of finishes, paint, gesso and gold leaf, cockling and foxing of paper, and other types of deterioration found on historic objects. In fact, environmental monitoring at historic institutions has shown that totally uncontrolled environments (no heat, cooling, humidification or dehumidification) maintain much better preservation conditions than environments that are heated only to human comfort levels.

Second, in order to achieve a wintertime 50% RH level at an interior temperature of 60-70 degrees interior temperature, it is necessary to add significant quantities of 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, some of this moisture migrates through the walls towards the outside. At some point inside the wall, the dew point will be reached and moisture will condense on the wall components, potentially leading to rotting of wood, peeling paint, and spalling of masonry surfaces.

Most of this damage occurs over long time periods and may not manifest itself for years. Ultimately, the building may need to be significantly repaired or replaced at some time in the future. If the building is historic, this level of deterioration cannot be tolerated. The lower the interior temperature, the less the amount of moisture that must be added to the air, reducing the risk of damage to the building. In fact, if the temperature is low enough, it may not be necessary to add any moisture to the air, completely eliminating the threat of building deterioration.

Third, recent research being conducted at the Conservation Analytical Laboratory (CAL) of the Smithsonian Institution has indicated preliminarily that collection objects can tolerate a larger range of relative humidity than previously thought. Virtually all objects are composites of various materials. Most of these individual materials respond in differing amounts to RH changes. The staff of CAL tested each of these materials to determine in a worst-case scenario how much stress could be tolerated due to RH changes before permanent deformation occurred.

Utilizing the measured levels from the least tolerant (and most sensitive) materials, the maximum range could be predicted for the object as a whole. For example, they looked at panel paintings, typically composed of a wooden support, a gesso layer and oil paint. The lower RH limit was due to the gesso and the upper limit to the wood. Based upon their research, an oil painting on panel would tolerate a RH range from 28%-66% without risk of damage.

Needless to say, these preliminary results have generated considerable controversy in the museum world. Until a larger group of materials can be tested, and theoretical laboratory predictions are borne out in real-world applications, environmental standards can not be relaxed prudently to such a wide range. However, a more modest range of 35%-55% is reasonable.

Fourth, keeping lower wintertime temperatures will greatly reduce the heating costs, saving your institution a significant amount of money. Very few other preservation procedures can make this claim!

Each collection and building are unique, and the specific solution chosen to control the environment requires careful consideration of the needs of both. This process begins with a program of monitoring the currently-existing conditions, followed by determining the appropriate settings and equipment needed to provide acceptable environmental levels.

RH and Temperature Monitoring

A comprehensive program of monitoring the temperature and relative humidity allows intelligent steps to be taken in controlling the collection's environment. Monitoring data 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. Additionally, the readings will allow determination of which spaces are more appropriate for storage, and which are causing accelerated deterioration. Otherwise, environmental control efforts are just guesses, with no way of determining if the desired effects are actually occurring.

In general, inexpensive manual monitoring systems require more human involvement than high-tech automated ones, which require more money. Therefore, if staff time is available and money is not, begin with a manual system and seek funding for a less labor-intensive system in the future. Take manual readings in each area that contains (or is projected to contain) collections in storage or exhibition. It is also helpful to take a reading outside the buildings to record the exterior conditions and to note where the readings are taken in each room.

A hygrometer/thermometer that is a good compromise between accuracy and price is made by Airguide. This digital unit also records maximum and minimum T and RH. Several suppliers sell it for prices ranging from $40-$70, but it is also available as a service to the museum community from American Conservation Consortium at our cost of approximately $20 each. Mount them on an interior wall or surface away from heat ducts or radiators, one per room, at a vertical elevation that matches the majority of the collection items, generally about 2-3 feet from the floor. Record the maximums and minimums of RH and T on a chart once a week, preferably on the same day of the week. This data can be entered into spreadsheet or other computer software to produce charts covering an entire year.

Eventually, it may be advisable to replace the manual system with recording hygrothermographs or data loggers when funding is available (cost: $500-$800 each). These units keep a much more precise and detailed record of RH and T. Onset data loggers (about $80-$95) present a low-cost alternative [Onset Computer Corporation, 470 MacArthur Blvd., Bourne, MA 02532, 508-759-9500]. While not sufficiently accurate for very precise high-tech HVAC systems, they are ideal for low-tech monitoring. The suggested unit is the Hobo H8 4-channel, which logs T, RH, light, and has a fourth channel for an external probe. Disable the 4th channel and set the recording interval for 20 minutes, and the logger will hold over a month of monitoring data. Download once a month, and charts for an entire year can be produced easily.

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. Rather, 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 buildings themselves.

Achieving Desired Levels

In order to achieve environmental control, the building envelope must be as tight as possible. The greater the amount of air exchange with the outside, the harder systems will have to work to control the environment, and the less efficient passive buffering systems will be. Keep windows and doors closed. Consider the use of storm windows and doors. Install weatherstripping. Insulate access holes for wires and pipes. Close fireplace dampers and consider capping chimneys. A tight building envelope may allow the use of a simpler environmental control solution, and is certain to save you money in utility costs.

The most basic environmental control is a tight building envelope without any mechanical systems. While no alteration of the environment is possible, long-term variations of the naturally occurring interior relative humidity are somewhat limited. It is unlikely that the RH will fall below 35%-40%, although highs will likely reach 90%. This is greatly preferable to the typical heated-only environment, which often will have a 5% to 90% RH range. The temperature range in an uncontrolled interior environment is likely to be about 10-90 degrees, varying somewhat with the geographical location. However, as discussed previously, temperature fluctuation has a relatively minor effect on museum collections. This uncontrolled temperature range of 80 degrees is roughly equivalent to a RH range of 13% based upon the "rule of thumb" guideline presented in the first part of this article.

The first step to improve an uncontrolled environment would be to add dehumidification in the late spring, summer and early fall. If properly installed, this would limit the high end of the RH range during warmer weather to about 55%. With the exception of times in the winter when the RH is too high, most of the year RH levels would be between 35% and 55%. Thus, a vastly improved environment can be created with only a dehumidifier, with very modest equipment or utility costs.

Portable dehumidifiers will work well. They are available in various sizes and range in price from $100-$300. For the typical historic house, consider installing one per floor and monitor the environmental results. If required, install additional units until they can keep the RH below 55%. Be sure to tie the dehumidifiers directly to a drain, so that emptying of containers is not required. If the dehumidifiers have difficulty in maintaining an even level of about 55% RH (sometimes holding 40%-45%, sometimes 60%-65% at the same setting), it may be because the dehumidistatic control inside is not sufficiently precise. An external wall-mounted dehumidistat (turns on when RH rises above a set point and off when it drops sufficiently) can be installed to prevent this problem. A common model is the Honeywell H46E1013.

Unfortunately, common portable dehumidifiers will not function at a temperature below 60 degrees. This situation can be remedied, however, by utilizing dehumidistatic control of a heat source during the late fall, winter and early spring to prevent RH levels from rising above 55%. If a heating system exists, a dehumidistat can be substituted for the thermostat and set for around 55%. When the RH rises above this level, the heat is turned on, forcing the RH down. If a heating system does not exist, portable heaters can be controlled by dehumidistats to accomplish the same function. Be sure to use a heater that is UL rated and does not create a fire hazard.

By utilizing dehumidistatically controlled heating in the late fall, winter and early spring, and a dehumidifier in the late spring, summer and early fall, environmental conditions can be very simply and inexpensively controlled to between 35% and 55%. The vast majority of environmentally-induced deterioration will be prevented. However, the temperature will be fully uncontrolled.

Attempting to control both relative humidity and temperature significantly complicates control systems and makes them far more expensive to install and operate. In addition, it may introduce deterioration threats to the building, as described in part 1 of this article. If the building is modern, this may not be a problem, but historic buildings cannot tolerate unnecessary risks.

For the safety of the collections, it is best to segregate human use areas, such as offices or libraries, from collection areas. If possible, these should be in different buildings. If collections and people must share the same building, segregate them by floors, ideally with the human use spaces above the collections spaces (since heat naturally rises). In this manner, the lower collections rooms can be kept cooler in the winter, with a resulting higher relative humidity. Try not to be tempted by the appeal of heating for visitor comfort. If it is cold enough outside to turn on the heat, visitors will be wearing appropriate warm clothing. Additionally, it is preferable for visitors to keep their coats on to minimize damage from carried coats banging into collection objects.

Generally, the first temperature control system to be considered by most institutions is a heating system. Unless the heat is kept very low or is controlled by a dehumidistat as described above, sufficiently high RH levels cannot be maintained without the addition of moisture into the air. The lower the temperature, the less the amount of moisture that must be added to keep the RH above 35%. Generally, if the temperature is kept at 40-45 degrees, the RH will fall below 35% only in the coldest weather.

Humidification can be provided by portable units which must be filled periodically (and religiously), or by units that are tied to a plumbing source, which is the preferred alternative. A good unit that has its own fan and is self-contained is the Aprilaire 350 (cost approximately $180-$250). Control is provided by a wall-mounted humidistat (turns on when the RH drops below a set point, turns off when it rises sufficiently). To minimize the amount of moisture added to the air, choose a winter RH standard of 35% at a temperature of no greater than 50 degrees. As the temperature is raised above this level, the risk of damage to the building increases dramatically. To hold a RH level of 35% at 70 degrees, only the newest of buildings with full vapor barriers and weather-tightening features will not be at risk of damage.

In many locations, summer temperature control is not required. If cooling is necessary, use the smallest possible air conditioning unit that will provide the required lowering of temperature. If too large a unit is used, it will not remove a sufficient amount of moisture and RH levels can easily rise above 90%. A large unit cools the air so quickly that it does not run long enough to remove much moisture, called short-cycling. This is further compounded by the fact that as the temperature is lowered, the RH inherently will rise. Depending upon your specific circumstances, it may be necessary to use a dehumidifier in consort with air conditioning.

If outside temperatures and relative humidities are very high, and the interior temperature is kept very cool, it is possible that moisture from the outside will begin to migrate through the walls and condense inside them, creating similar deterioration threats to the building as winter humidification. To prevent this, consider keeping temperature specifications higher, perhaps 78-80 degrees.

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. Similarly, air conditioners should be run continuously when they are needed, not shut off at night.

The final section of this article examines environmental control dilemmas occurring at actual historic sites, and offers solutions to these challenges.

Air Conditioning

A large 18th century historic house in the southeast installed a new air conditioning system, which was set at 70 degrees. Shortly thereafter, mildew was noticed growing on the upholstery of chairs in the collection. Examination of recording hygrothermograph charts indicated that the RH was routinely above 90%. Air conditioning was supposed to remove moisture and lower the summertime relative humidity while it cooled. The curator could not figure out what was wrong.

The answer lay with the sizing of the air conditioning unit. The HVAC contractor wanted to be sure that the unit would handle the hottest weather and chose the next larger size just to be sure. Unfortunately, this caused the unit to run only for short time periods, called short-cycling. The moisture that was extracted from the air was still condensed on the cooling coil and had not yet run into the drain. It evaporated back into the air before the cooling unit turned on again. In combination with the lower air temperature (causing RH levels to rise naturally), the system could not control the relative humidity.

The solution is to replace the unit with a smaller one, probably at least two sizes smaller. Alternatively, a dehumidifier could be used to supplement the air conditioning unit. Additionally, in a hot climate, a temperature specification of 70 degrees is too low, with a risk of condensation of moisture from the outside in the building walls. A specification of 78-80 degrees would be preferable.

The lesson illustrated by this example is the importance of utilizing a qualified consultant who understands the significance of relative humidity in historic houses to develop system specifications. Most heating contractors have previously thought only about human comfort issues. They are unfamiliar with the needs of historic structures and their collections. Additionally, it is extremely common for heating contractors to use rule-of-thumb calculations and educated guesses in designing and sizing systems. More often than not, this leads to systems that do not perform within acceptable limits and may even cause greater damage to the collections than no system at all.

Office Space

A mid-19th century building was used for both offices and exhibition space. In an attempt to segregate human use from collections, the offices were placed on the first floor, with the exhibition space on the second floor. Heat only was installed in the building, with separate zones for the first and second floors. In the winter, the second floor thermostat was turned completely off in order to keep the relative humidity higher, thus helping to preserve the collections. The offices were heated to about 65 degrees in the winter. Amazingly, the wooden objects on the second floor showed fresh cracking, warpage of flat surfaces and delamination of veneer.

Monitoring with inexpensive Airguide hygrometer/thermometers revealed that the temperature on the second floor was reaching the mid-60's, even though the heat was turned off. The natural tendency of heat to rise accounted for this. The relative humidity on the second floor was 15%-20% due to the higher temperature.

Two solutions exist for this problem. The first is to switch the two functions, moving the human use to the second floor and the exhibitions to the first floor. Thus, when the heat is kept on for human comfort on the second floor, the first floor will remain cold. Replacing the first floor thermostat with a dehumidistat to run that heating zone will assist in providing even greater wintertime relative humidity stability. The second alternative is to heavily insulate the ceiling of the first floor and place a vapor barrier on the warm (lower) side of the insulation. A minimum of 9-12 inches of fiberglass insulation is recommended. This will serve to prevent most of the heat loss to the second floor exhibition areas. As in the first solution, replacing the collections thermostat on the second floor with a dehumidistat is suggested.

Low-tech storage

A local historical society owned a single late-19th century building. The only storage space available to them was the attic, a notoriously poor location for collection objects. Attic temperatures are exceedingly high in the summer, often producing dangerously low RH levels. In addition to dramatically accelerating chemical deterioration processes, these conditions can cause softening of surfaces and damage more commonly found in heated winter environments, such as cracking of wood. During the entire year, heating of the roof by the sun, followed by cooling at night, creates severe daily fluctuations of both temperature and relative humidity. For most historic structures, venting of attics is recommended year-round to reduce the environmental extremes.

The excesses of attic environments can be brought within acceptable parameters by constructing a room-within-a-room in the attic. A separate free-standing structure is built within the attic. The walls, floor and ceiling are insulated with a minimum of 6 inches of fiberglass insulation. A vapor barrier (6 or 8 mil polyethylene sheeting) is placed on all interior surfaces. A single door of sufficient size provides access to the space. The door is well-insulated and well weather-stripped. The room is built so that it does not touch the attic walls or ceiling, allowing air circulation around it. Full ventilation of the attic is provided, enhanced if necessary by ridge and soffit vents or power ventilators. While conditions outside the room will still vary on a daily basis, inside the room, very little change will occur. Seasonally, there will be a gradual temperature shift, but the relative humidity should remain within a fairly narrow range. This is being accomplished without the use of any mechanical systems. Obviously, proper HVAC system design and installation can improve further upon the passive control methods. The room-within-a-room concept can be used for storage in other building spaces as well, with a significant improvement in their environmental conditions.

Retrofitting for a Library

An historic site wanted to move its library and archival storage into the ground level of an early 19th century building. Unfortunately, this combination of collections and human use on the same floor is not desirable for collections preservation. The building was built into a hillside, with one end of the floor at grade level, and the other end nearly completely underground. The storage areas were to be located at the far end where they were substantially beneath grade, and the library and human use areas were to be oriented toward the above-grade end. Perimeter drains were installed to eliminate the risk of water infiltration. The building had been in historic use until recently, and the site staff considered all of the fairly modern architectural interior to be of significance. Any new system installation would have to avoid an already extensive network of wiring and water piping that had supplied the currently-disconnected system of hot water radiators on the ground level and upper two floors.

Three criteria were of importance in designing a system. First, the archival collections needed to be environmentally protected. Second, the upper two floors were used for storage of the collections, primarily wooden objects, and system operation could not cause an increased threat of damage to them. And third, the building itself should not suffer deterioration.

Based upon these factors, initial temperature and relative humidity specifications were selected as 64-70 degrees and 45%-55% RH. A local plumbing and heating contractor was asked to provide an estimate for a system. He proposed forced air heat, with the human use areas supplied by a large spiral duct, smaller individual ducts to the storage areas, and appropriately-located return ducting, all of which would be highly visible once installed. One temperature zone would control the storage areas and a second zone would handle the human use areas. A humidifier in the duct system would provide moisture in the winter, while dehumidification would be provided by a central air conditioner. The overall system cost was around $11,000.

The curator was concerned that the ducts would present a visual intrusion into the historic architectural interior. She enlisted the services of a conservator and an HVAC engineer working as a team to evaluate the proposed system. The proposed main library room had been part of a year-long environmental monitoring program. Results showed a maximum high summer temperature of 74 degrees, confirming empirical suspicions that air conditioning was not necessary and would not be able to provide dehumidification. Relative humidity levels during the same time were often in excess of 90%.

A temperature of 60 degrees and 30% RH in the human use areas was chosen, since it represented approximately the same amount of absolute moisture in the air as 45 degrees and 50% RH. These design specifications would prevent undesired moisture migration between the two functional areas and allow the use of a single humidity zone.

With the specifications determined, suggestions were made on system design. It was felt that the buffered, mostly underground location of the storage rooms would not require temperature control at any time of the year. Therefore, hot water heat on a single zone would be supplied to the human use rooms only. Radiators with non-mechanical balancing valves would minimally impact the interior architectural appearance. Humidification would be provided by a self-contained humidifier, such as the Aprilaire 350 (with the humidistat located in one of the storage rooms). During the summer, the temperature would be uncontrolled, although the below-grade location of the storage rooms would probably keep it in the 50-60 degree range, with human use area temperatures generally between 65-78 degrees. A conventional dehumidifier would bring the RH down to 50% in the storage areas. This system could be installed for approximately $6000-$7000, a significant savings over the initially-proposed system. In addition, it would provide better control of environmental conditions in the collection areas and would minimize the risk of damage to the historic building.

The solutions to environmental control problems at historic properties are varied, as can be seen from the preceding examples. However, all combine certain factors. First, proper relative humidity and temperature specifications must be chosen for the preservation of the collections and the building. Second, environmental monitoring allows proper evaluation of system needs. And third, utilizing the expertise of qualified consultants will avoid costly and damaging mistakes in system design and equipment choices. Combine all of these factors with creative problem-solving, and it is possible to provide very acceptable environmental conditions for preservation of your collections on a modest budget. 

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