Temperature and Relative Humidity for Exhibits

• Know the environment. Monitor an exhibit space for one year to obtain baseline information about the temperature and relative humidity. Review these environmental data for each exhibit to determine if existing conditions meet the conservation criteria.
• Control the environment within the entire exhibit space. In general, keep temperature between 60 and 70°F (15.5 and 21°C) and relative humidity between 40 and 60%, eliminating rapid cycling of temperature and relative humidity. Requirements for special objects and certain geographical areas may vary.
• Locate sensitive objects in the most stable locations. Do not place moisture-sensitive collections in the path of direct sunlight, near heating or air-conducting ducts, against external walls, or in damp locations such as basements. Avoid putting cases and framed works along exterior walls.
• Provide additional control for sensitive objects. When appropriate use sealed cases to slow air exchange and thus stabilize environments inside cases. Consider creating a microclimate by incorporating silica gel or other climate control products within cases that contain moisture-sensitive materials.

General Guidelines

Conservation criteria for temperature and relative humidity are often necessary because of the range and severity of physical, chemical, and biological damage caused by exposure to excessive or rapid cycles in temperature and humidity. Organic materials are sensitive to temperature and relative humidity levels. Inorganic materials, on the other hand, are usually unaffected by moderate cycles.

Temperature affects the rate of most chemical deterioration processes, which double for every increase of 18°F (10°C). Some materials, such as plastics, photographic emulsions and rubber-based materials, become soft or tacky at higher temperatures. Other materials become more brittle and are easily damaged by handling or physical stress at low temperatures. In general, temperatures in the exhibit area should be kept below 80°F (26.6°C), even in hot climates, and above 32°F (0°C). Dramatic and rapid changes in temperature must be checked to prevent thermal shock to glass, enamels, and some other inorganic materials.

An acceptable range of relative humidity for many parts of the country is generally 40-60% RH, with a winter low of 35% in colder, northern climates. In areas that experience dryer ambient climates, such as the Southwest, a lower humidity range (30-40%) is appropriate. In tropical climates, museum collections have been safely displayed in atmospheres approaching 75% RH if adequate ventilation is provided. In addition, objects tend to adjust to (reach equilibrium with) the environmental conditions from which they originate and to those which they are continually exposed.

Whatever the ambient environment of a region, the primary goal is to maintain as stable a relative humidity as possible. Therefore, the rate of daily RH change and fluctuation should not be excessive: ideally 3% but no more than a total of 5% per day. Seasonal change in relative humidity should also occur with very gradual transition. Moreover, it is important to note that some materials are very humidity sensitive (for example, ivory teeth and some minerals) and must be maintained under stricter environmental ranges.

Monitoring Conditions

Some understanding of the general climatic patterns in the museum's geographic region can be gained from annual summaries available from the National Oceanic and Atmospheric Administration. A real understanding of the exhibit environment, however, can only be gained by monitoring the conditions in the exhibit space for one year to establish a baseline. If such monitoring is not feasible, projecting or estimating the climatic conditions in the exhibit space may be possible. Evaluation of the climatic records:

• Allow the environmental conditions of an exhibit space to be considered when choosing objects for display
• Enable staff to modify existing heating and air-conditioning systems or choose equipment that allows a more optimal range of temperature and RH
• Facilitate the location of more sensitive collections in areas with the most stable conditions
• Point out the need for additional measures, such as providing a microclimatically controlled environment
• Guide the technical requirements for exhibit case seals

Macro Solutions

The concept of a box within a box is a useful one when considering environmental control. The building itself is the outer box, while the exhibit room creates a second zone. The exhibit case further defines the environment around an object. Controlling the environment at these different levels, or at different levels within each "box" is a cost-effective strategy that is appropriate for certain exhibit situations.

The initial selection of an exhibit location should consider the recommended environmental criteria for the exhibit. Making the exhibit area as self-contained as possible helps to stabilize the environment. To encourage a more constant environment, block unnecessary windows and doors with insulating material; enclose the exhibit area with well-fitting doors; and provide unrestricted airflow for the heating and air-conditioning systems. Ideally, an exhibition space should have these features:

• No door or windows opening directly outdoors
• Few exterior walls
• Weather seals and insulated walls, floor, and ceiling
• Vapor barriers in the walls, floor, and ceiling

The options for environmental control begin with the overall exhibit space. Climate control throughout the area provides protection for all the objects on display. Macro control is appropriate when most or all of the objects are sensitive to temperature or humidity. In other situations, the susceptibility of the objects, the available human and financial resources, and the schedule will influence the degree of control that is practical.

Improving conditions within an entire area does not always mean adding expensive equipment. Low-cost options include:

• Setting the temperature at a level appropriate for the geographic region (in the Northeast, Northwest, and upper Midwest maintaining a lower temperature in winter will help prevent excessively low relative humidity)
• Maintaining temperature set points at constant levels on a 24-hour basis, not lowering temperatures at night
• Controlling the heating and air-conditioning (HVAC) system with a humidistat rather than a thermostat
• Using portable dehumidifiers during problem periods, such as the last weeks of summer when visitation is traditionally high

Design Solutions

Object location is important when designing the exhibit layout:

• Place humidity-sensitive objects away from sources of humidity fluctuation, including exterior door- ways, windows, air vents, artificial and natural light and heat sources, and exposed pipes
• Do not place objects in damp basements
• To prevent damage from desiccation and dimensional change, avoid overheating objects resulting from displaying them too close to a lighting source or allowing heat from lights or transformers to build up inside an exhibit case.

The choice and placement of light fixtures can cause thermal change and therefore influence relative humidity and the moisture content of objects on display. Inappropriate exhibition lighting heats up the objects. More than 90% of the energy from an incandescent lamp is heat, and every watt of light adds 4.15 BTUs (British thermal units) to the heat load of the building. Some exhibitions incur air-conditioning expenses year-round as a result of the heat produced by poorly designed lighting systems.

When the overall climate conditions in the exhibit do not meet the conservation criteria for humidity-sensitive objects, a tightly sealed exhibit case can offer the required protection. By decreasing the rate of air exchange, sealed cases buffer the objects from the worst ambient humidity swings. The use of silica gel can stabilize the humidity level within a sealed case in exhibit spaces with little or no humidity control.

To prevent the transfer of exterior temperatures from affecting the internal temperature and humidity of exhibit cases, cases should not touch a building's outside walls. Similarly, framed works should not be hung on external walls. When the location of a case or frame cannot be changed, consider the use of insulating construction materials and thermal pane glass.

Guideline: Modifying Exhibit Climate
Effective measures [design, controls and policies] must maintain object-safe climatic conditions that fulfill the Conservation Requirements
OR: The exhibit design must provide exhibit objects the required climatic conditions

Inappropriate relative humidity (RH) and temperature are both damaging to objects. Relative humidity is also directly affected by temperature because, as temperatures rise, RH generally goes down (the warmer the air, the more water it can contain); as temperatures fall, RH goes up. Consequently, fluctuations in temperature will also cause fluctuations in RH. Because of this relationship, RH control should be conducted in tandem with temperature control.
Inappropriate RH can create a host of problems for organic materials, causing them to disintegrate and discolor, to buckle and tent, to shrink or swell. Some minerals will hydrate or dehydrate when exposed to inappropriate RH, and metals containing salts can corrode. Damp conditions (over 75% RH) cause molds, which stain and weaken organic and inorganic materials. Fluctuations in RH can also be damaging. Collection objects exposed to excessive or rapid cycles in humidity can suffer a range of physical, chemical, and biological damage. Composite artifacts (made from two or more different materials) are most at risk from RH fluctuations because the materials from which they are made will react to RH changes at different rates and to different degrees and will pull or push at each other as they shrink or swell.
Temperature affects the rate of most chemical deterioration processes, which doubles for every increase of 18F (10C). Organic materials, especially those that are chemically unstable (e.g., acidic paper, color photographs, nitrate and acetate films) will disintegrate and discolor if temperatures are too high. In fact, most materials decompose gradually at room temperature, although the time scale for complete destruction can be in millennia. Some materials, such as plastics, photographic emulsions and rubber-based materials, become soft or tacky at higher temperatures. While other materials, including paint and other polymers, become more brittle and are easily damaged by handling or physical stress at low temperatures.
Temperature fluctuations can also cause a range of physical, chemical and biological damage in collection objects. Such fluctuation can cause fractures and delamination in brittle, solid materials, especially if they are layered. Dramatic and rapid changes in temperature can cause condensation and thermal shock to inorganic materials such as glass and enamels.
Temperature and RH can be brought to acceptable levels through the use of controls and hardware. Exhibit design and museum procedures can be used both to eliminate sources of damaging temperature and RH within the museum environment and also to ensure that the controls function as intended.

Best Practice: Technological controls that maintain a stable relative humidity are included in exhibit design when necessary

What conditions should relative humidity controls aim to create?

To ensure that relative humidity (RH) does not damage objects it must be controlled in two ways: not only should RH be kept within a certain range, it should also be prevented from fluctuating too rapidly within that range.

The data from environmental monitoring will provide information on the range of RH and its fluctuations in the exhibit environment. Ideally, the information will have been collected throughout the year to reflect seasonal variations. (For more information on environmental monitoring, see Standard *.) Controls should create the following conditions:

• Controls should maintain a stable RH: It is important in all museum environments to achieve as stable a RH as possible whatever the ambient environment of a region. Collection objects exposed to excessive or rapid cycles in temperature and humidity can suffer a range of physical, chemical, and biological damage.
  • Daily RH change and fluctuation: The rate of daily RH change and fluctuation should not be excessive—ideally 3% but no more than a total of 5% per day.
  • Seasonal RH change: Seasonal change in RH should also occur with very gradual transition.
• Controls should generally aim for a range between 40-60% RH
  • For many parts of the country an acceptable range of RH is generally 40-60% RH [for exceptions see below].
  • Maintain a moderate RH (preferably below 60% RH and never above 75%RH) to discourage growth of mold and mildew.

• In certain areas it is appropriate to aim for a range outside of 40-60% RH
  • In colder, northern climates a winter low of 35% is allowable.
  • In areas that experience dryer ambient climates, such as the Southwest, a lower humidity range (30-40%) is appropriate.
  • In tropical climates, museum collections have been safely displayed in atmospheres approaching 75% RH if adequate ventilation is provided.

• Controls should always meet the conditions set by the conservation requirements
  • Always follow the Conservation Criteria to create more stringent conditions as necessary. It is important to note that some materials are very humidity-sensitive (for example, ivory teeth and some minerals) and must be maintained under stricter environmental ranges. Metals generally benefit from RH levels that are as low as possible. Organic artifacts require more moderate RH levels to prevent desiccation or embrittlement. Objects also tend to adjust to (reach equilibrium with) the environmental conditions from which they originate and to those to which they are continually exposed.

Technical Information: Setting Relative Humidity (RH) Parameters

What are the main controls available for managing relative humidity?

Macro scale: RH can be controlled on the macro scale (the exhibit building or gallery) by using an HVAC system or radiators with window mounted a/c units. Climate control throughout the area provides protection for all the objects on display. Such macro control is appropriate when most or all of the objects are sensitive to temperature or humidity. Appropriate exhibit design can enhance the functioning of RH controls by allowing good air circulation. Insulating the exhibit building or exhibit space will also improve climate control. It should be noted that controlling the RH at the building or room level can be expensive and may cause structural damage if not done properly.

Micro scale: On the micro scale (an exhibit enclosure or display case), RH can be controlled by using a well-sealed case that will allow for a stable environment in which humidity can be controlled by humidity absorbers (silica gel, etc.).

Best Practice: Technological controls that maintain a moderate temperature are included in exhibit design when necessary

What conditions should temperature controls aim to create?

As with relative humidity, both an inappropriate range of temperature as well as rapid temperature fluctuation can be damaging to objects. And fluctuation in temperature can cause damaging fluctuation in relative humidity. The data from environmental monitoring will provide information on daily and seasonal temperatures in the exhibit environment. Controls should achieve the following:

• Controls should maintain a relatively stable temperature: The ideal range of temperature fluctuation is no more than 10F [correct?].
• Enough heat should be provided to keep the relative humidity below 60% to prevent mould and discourage infestations and corrosion.
• At an absolute minimum, temperatures in the exhibit area should be kept below 80F (26.6C), even in hot climates, and above 32F (0C). In general, temperatures should be kept to between 60F and 70F (15.5°C and 21C), with the optimum range for museum objects often given as 68F to 72F (20 °C and 22C), eliminating rapid cycling of temperature and relative humidity and the damage they cause. Cooler temperatures are beneficial to most artifacts. Any temperature above that required for human comfort can be viewed as too high.
• Controls should create the specific conditions recommended by the Conservation Requirements. Higher temperatures can cause faster aging and dessication of organic materials. Temperatures that are too low can cause embrittlement of other types of artifacts, particularly those with resins and varnishes.

What are the main types of control for temperature?

• Controls to bring the temperature down:
  • The HVAC is the main tool for maintaining a moderate temperature in the exhibit space. It works most effectively when the exhibit space is properly sealed.
  • Ensure the HVAC system has multiple zones, is of adequate size for load conditions, and provides good distribution of air.
  • If you don’t have a building-wide HVAC, consider installing an air conditioner in the exhibit room.
  • Lighting: Certain light systems provide a large percentage of their energy as heat rather than visible light. In fact, more than 90% of the energy from an incandescent lamp is heat, and every watt of light adds 4.15 BTUs (British Thermal Units) to the heat load of the building. The amount of infrared radiation produced by lighting is therefore an important consideration in efforts to control temperature and relative humidity in an exhibit.
  • For information on lighting options, see Standard *

• Controls to bring the temperature up:
  • Almost all buildings will have some form of heating and this will be the main tool to ensure that the exhibit space does not get too cold for the objects. An important consideration is ensuring that the heat is not set too high for object safety.

[Other options for controlling temperature? What are the best types of heat source from a conservation viewpoint since some will dehumidify the air more drastically than others?]

Best Practice: Exhibit design strategies are used to protect objects from inappropriate climatic conditions

What are low-cost options for controlling relative humidity and temperature?

Improving conditions within an entire area does not always mean adding expensive equipment. Low-cost options include:

• Setting the temperature at a level appropriate for the geographic region (in the northeast, northwest, and upper Midwest, maintaining a lower temperature in winter will help prevent excessively low relative humidity).
• Maintaining temperature set points at constant levels on a 24-hour basis and not lowering temperatures at night.
• Controlling the heating and air-conditioning (HVAC) system with a humidistat rather than a thermostat.
• Using portable humidifiers during problem periods such as the last weeks of summer when both humidity and visitation is traditionally high.
• Using portable heaters or air conditioners to correct local problem areas.

How can design protect objects from inappropriate temperature and relative humidity?

By allowing for good air circulation and insulating the exhibit space, exhibit design and construction can create a more constant environment in which temperature and relative humidity (RH) are more easily regulated. Conversely, inappropriate design choices can expose objects to harmful climatic conditions. The following design strategies can enhance climate control efforts:

Exhibit design can include structural modifications that will help to seal and insulate the exhibit location and stabilize the environment. Such modifications include:

• Incorporating a vestibule at the exhibit entrance to control airflow.
• Enclosing the exhibit area with well-fitting doors.
• Blocking unnecessary windows and doors with insulating material.
• Caulking or gasketing doors and windows to limit air infiltration through cracks and gaps in the building
• Blocking the transfer of humidity by installing an effective vapor barrier where missing (e.g. walls, ceiling and floor)
• Provide unrestricted airflow for the heating and air-conditioning systems
• If it is not possible to eliminate all windows, minimize their impact by using double or triple glazing on windows to help stabilize the room climate in winter climates, or filtered glass to eliminate excessive heat ingress.

A properly designed exhibit case can provide effective protection from RH on a micro scale:

• A well-sealed exhibit case can protect humidity-sensitive objects from constant fluctuation of RH. Because the rate of air exchange is decreased, the environment inside the case is stabilized, protecting objects from the worst ambient humidity swings.
• The inclusion of silica gel and other humidity control products can stabilize the humidity level inside a sealed case in exhibit spaces that have little or no humidity control.

[For more information on case design, see Standard *.]
The layout of exhibit furniture should not interfere with proper functioning of climate controls:

• Ensure that furnishings do not block proper air circulation

Tall furnishings such as panels, shelving, and tall cases, should not create obstacles to the circulation of air.

• Ensure cases are situated in locations that will not compromise climate controls:

Cases should not touch a building’s outside walls: exterior temperatures could affect the internal temperature and humidity of the case. Similarly, framed works should not be hung directly on external walls. (When the location of a case or frame cannot be changed, consider the use of insulating construction materials and thermal pane glass.)

Cases should not be placed near heat sources such as radiators, heaters or in front of windows

Exhibit layout should allocate the most climatically stable locations in the exhibit area to sensitive objects:

• Place sensitive objects in the most stable locations--those locations in the exhibit area that are least prone to humidity and temperature fluctuations.
• Do not place temperature or moisture sensitive collections near sources of temperature or humidity fluctuation:

Avoid placing objects in the path of direct sunlight, against external walls, near doorways, windows, air vents, exposed pipes, or heat sources such as radiators. To prevent damage from desiccation and dimensional change, do not overheat objects by placing them too close to a lighting source. Objects should be placed at least 36 inches from incandescent or tungsten halogen light because of heat buildup and illumination levels.

Avoid using materials for construction, props or room furnishings that could increase RH.

• If organic material is used for props, ensure that it is dry.
• Do not use water to create weather and water effects; instead utilize synthetic materials.
• Indoor plants can interfere with humidity levels.

(All of the above can create hazards besides affecting humidity levels. For more information on safe exhibit materials and furnishings see Standard *.)

How can design protect objects from inappropriate temperature and relative humidity?

By allowing for good air circulation and insulating the exhibit space, exhibit design and construction can create a more constant environment in which temperature and relative humidity (RH) are more easily regulated. Conversely, inappropriate design choices can expose objects to harmful climatic conditions. The following design strategies can enhance climate control efforts:

• Exhibit design can include structural modifications that will help to seal and insulate the exhibit location and stabilize the environment. Such modifications include:
  • Incorporating a vestibule at the exhibit entrance to control airflow.
  • Enclosing the exhibit area with well-fitting doors.
  • Blocking unnecessary windows and doors with insulating material.
  • Caulking or gasketing doors and windows to limit air infiltration through cracks and gaps in the building
  • Blocking the transfer of humidity by installing an effective vapor barrier where missing (e.g. walls, ceiling and floor)
  • Provide unrestricted airflow for the heating and air-conditioning systems
  • If it is not possible to eliminate all windows, minimize their impact by using double or triple glazing on windows to help stabilize the room climate in winter climates, or filtered glass to eliminate excessive heat ingress.

• A properly designed exhibit case can provide effective protection from RH on a micro scale:
  • A well-sealed exhibit case can protect humidity-sensitive objects from constant fluctuation of RH. Because the rate of air exchange is decreased, the environment inside the case is stabilized, protecting objects from the worst ambient humidity swings.
  • The inclusion of silica gel and other humidity control products can stabilize the humidity level inside a sealed case in exhibit spaces that have little or no humidity control.
  • [For more information on case design, see Standard *.]

• The layout of exhibit furniture should not interfere with proper functioning of climate controls:
  • Ensure that furnishings do not block proper air circulation: Tall furnishings such as panels, shelving, and tall cases, should not create obstacles to the circulation of air.
  • Ensure cases are situated in locations that will not compromise climate controls: Cases should not touch a building’s outside walls: exterior temperatures could affect the internal temperature and humidity of the case. Similarly, framed works should not be hung directly on external walls. (When the location of a case or frame cannot be changed, consider the use of insulating construction materials and thermal pane glass.) Cases should not be placed near heat sources such as radiators, heaters or in front of windows

• Exhibit layout should allocate the most climatically stable locations in the exhibit area to sensitive objects:
  • Place sensitive objects in the most stable locations--those locations in the exhibit area that are least prone to humidity and temperature fluctuations.
  • Do not place temperature or moisture sensitive collections near sources of temperature or humidity fluctuation: Avoid placing objects in the path of direct sunlight, against external walls, near doorways, windows, air vents, exposed pipes, or heat sources such as radiators. To prevent damage from desiccation and dimensional change, do not overheat objects by placing them too close to a lighting source. Objects should be placed at least 36 inches from incandescent or tungsten halogen light because of heat buildup and illumination levels.

• Avoid using materials for construction, props or room furnishings that could increase RH.
  • If organic material is used for props, ensure that it is dry.
  • Do not use water to create weather and water effects; instead utilize synthetic materials.
  • Indoor plants can interfere with humidity levels.

(All of the above can create hazards besides affecting humidity levels. For more information on safe exhibit materials and furnishings see Standard *.)

Best Practice: Museum policies protect objects from inappropriate temperature and relative humidity

What policies can assist in protecting objects from inappropriate temperature and relative humidity?

[Placeholder suggestions; need amplification]
Windows and doors are kept shut to ensure climate controls work effectively

Timely repairs are performed on broken windows and on dried putty and caulking around windows and doors to eliminate cracks and leaks.

Maintenance of temperature and relative humidity controls:

• Training is provided to ensure staff know how to monitor and maintain controls so that they work effectively
• Budget includes funds to maintain controls and replenish humidity control products as needed (silica gel, etc.)
• Sufficient staff are dedicated to these tasks.