Environmental Guidelines for Museums - Temperature and Relative Humidity (RH)by David Grattan and Stefan MichalskiCanadian Conservation Institute Website

IntroductionThis general introduction to the current approach to controlling ambient RH and temperature in museums is intended for all museum professionals. It is based on the “Museums, Galleries, Archives and Libraries” chapter in the American Society of Heating, Refrigeration, and Air Conditioning Engineers Inc. (ASHRAE) Handbook, a rather technical document intended primarily for engineers designing, maintaining, or operating HVAC systems in buildings that house heritage collections. The approach taken in the chapter represents a departure from earlier more traditional thinking about museum environments, which called for stringent control of RH and temperature. In the current approach, RH fluctuation is linked to measurable damage in artifacts. Certain types of artifacts are much more sensitive to RH fluctuation than others, and it is neither economical nor environmentally acceptable to have very tightly controlled conditions if they are not necessary.The Role of Temperature and RHTemperature and RH are directly related: when a volume of warm air is cooled, its RH goes up when a volume of cool air is warmed, its RH goes down

Because of this interdependence, temperature and RH are usually considered together. The damage caused by incorrect temperature and incorrect RH falls into three broad categories: biological chemical mechanical

General Set PointsThe target value of temperature or RH that a mechanical system is designed to maintain over time is known as the “set point”. However, even the best mechanical systems will produce values that fluctuate above and below the given set point.The term "set point" can be used in two ways: to refer to the setting of the thermostat or humidistat over a short period of time (hours, days) to refer to the average annual setting of the thermostat and humidistat (because the set points may

be adjusted over the year for various reasons, such as energy saving)

Note that the "set point" is often defined by museums as 50% RH with the temperature between 15 and 25°C, although it can also be based on the historic averages. In practice, it may be defined by factors such as the needs of the collection, the performance of the building plus the HVAC system, and the climatic variation in temperature. On the other hand, class of control is defined by the degree of fluctuation in temperature and RH.   And it is fluctuation rather than set point that we now strive to control because fluctuation provides the main threat to most artifacts and Class of Control defines the allowable degree of fluctuation. There are five Classes of Control: AA, A, B, C, and D. Within control levels AA, A, and B, seasonal adjustments are noted separately from permissible short-term fluctuations. For control levels C and D, the wide fluctuations specified could result from either purposeful seasonal adjustments, or from short-term fluctuations, or from both (which is usually the case). What Best Describes Your Collection?1

Collection Type: General museums, art galleries, libraries, and archives (all reading and retrieval rooms, rooms for storage of chemically stable collections, especially if mechanically medium to high vulnerability)

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RH and temperature set points: historical annual average for permanent collections or 50% RH with the temperature between 15 and 25°C.Maximum fluctuations and gradients in controlled spaces Class of controlShort-term* fluctuations and space gradients

Seasonal adjustments in system set point

±5% RH±2°C

RH no change.Up 5°C and down 5°C.

AAPrecision control, minimal seasonal changes to temperature only.

±5% RH±2°C

Up 10% RH and down 10% RH.Up 5°C and down 10°C.

AGood control, some gradients or seasonal changes.

±10% RH±2°C

RH no change.Up 5°C and down 10°C.

AGood control, seasonal change to temperature only.

±10% RH±5°C

Up 10% RH and down 10% RH.Up 10°C (but not above 30°C) and down as low as necessary to maintain RH control.

BControl, some gradients plus winter temperature setback.

Within range 25–75% RH year-round.Rarely over 30°C, usually below 25°C.

CPrevent all high risk extremes.

Reliably below 75% RH. DPrevent damp.

*Short-term fluctuations are any fluctuations less than the seasonal adjustment; however, some fluctuations are too short to affect some less-sensitive artifacts and those that are enclosed.A Note of Caution Environmental conditions affect objects in many ways. Some objects are vulnerable in conditions that may not affect other objects at all. Some attempts to improve conditions for an object might actually affect it adversely. For example, moving an object from a poor environment to a theoretically better one might cause severe mechanical damage. There is simply no one-size-fits-all pattern for good environmental control strategies, nor is there likely to be, no matter how good HVAC engineering becomes. Understanding how environmental factors affect collections helps conservators make consistently good choices.Collection Type: Archives and libraries (storage of chemically unstable collections)Set point Maximum fluctuations and gradients in controlled spacesCold storage: -20°C 40% RH 

±10% RH±2°C  

Cool storage:10°C 30–50% RH

Even if achieved only during winter setback, cool storage is a net advantage to collections as long as damp is not incurred.

Collection Type: Special metalsSet point Maximum fluctuations and gradients in controlled spacesDry rooms:0–30% RH RH not to exceed some critical value, typically 30% RH.

1. The next three tables are adapted from Table 3 "Temperature and Relative Humidity Specifications for Collections", that appears in the chapter of the ASHRAE Handbook, on p. 21.13.

Damage Caused by Incorrect Temperature and RHThe damage caused by incorrect temperature and RH falls into three broad categories: biological chemical mechanical Biological DamageThe biological threat directly related to RH is mould growth. Mould is always undesirable in a collection. It causes irreversible and often devastating damage.

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Mould spores are naturally present in the air around us, and it is impossible to eliminate them. However, it is only when the mould spores have sufficient nutrients, time, and moisture that they grow into destructive mould. As suitable nutrients are readily available in almost any environment, the route to controlling mould is to control moisture. Without moisture, mould spores cannot grow.The safe RH boundary usually cited to prevent mould growth is 65%. Below the safe RH boundary, mould will not grow at any temperature. In contrast, mould is very likely to grow at high RH. For example, at 85% RH, mould will probably appear in less than a week. Establishing what happens in borderline conditions (in RH between 60% and 70% with different temperatures) is a bit more challenging. Graphs that provide more precise answers to these questions can be found in the ASHRAE chapter mentioned previously and in Technical Bulletin 23 (Guidelines for Humidity and Temperature for Canadian Archives).It is relatively easy to maintain ambient RH and temperature levels that will preclude mould growth. The danger, which is often hidden, lies in microclimates that can inadvertently be created within a building.Many rooms have temperature gradients, and colder spots have higher RH than warmer ones. For example, during cold weather, exterior walls or floors may become considerably colder than the rest of the room, creating pockets of high humidity or even condensation. The tools designed to condition the air can create their own problems: near the output of humidifiers, RH will be too high; and near dehumidifers, there is the risk of water leaks. Even attempts to protect objects by enclosing them in a protective wrapping can backfire. Consider what happens if a package containing an object is stored where the temperature is uneven (for example, one end of the package is touching a cold wall). The cold end will have much higher RH than the warmer end, leading to mould growth. In summary, protective wrapping is beneficial in many ways, humidifiers and dehumidifiers can play important roles, and most storage rooms are better for collections than no storage room at all. Each of these improvements to collections, however, needs to be applied carefully so as not to create new sources of damp and mould.Chemical DamageChemical damage is caused by chemical reactions taking place within a material. The key reactions are hydrolysis and oxidation, which account for most natural aging processes.“Hydrolysis” is a reaction between a substance and water that results in the chemical breakdown of the original substance and the formation of one or more new substances. "Oxidation" is a reaction between a substance and oxygen, often resulting in physical breakdown.Temperature and RH both affect chemical processes: heat speeds up any chemical reaction (the rule of thumb is that the rate of reaction approximately

doubles for every 5°C increase in temperature) RH is significant because some reactions require moisture before they can take place

Acid hydrolysis is a primary concern for museums and, especially, archives. It affects cellulose-based materials, including photo negatives and cine film, magnetic media, and paper. In paper, acid hydrolysis splits the long cellulose chains into shorter ones, making the paper less flexible, more brittle, and more susceptible to damage. This breakdown process will continue as long as acid is present and, because the process itself produces acid, the degradation will actually accelerate as it progresses.The lifetime of paper is highly variable. It depends on the make-up of the paper, with fibre type, fibre length, degree of beating of the pulp, chemicals used in manufacture, basis weight, and thickness all affecting how long the paper can be expected to last. Although none of these factors can be changed, the temperature and RH conditions of the environment where paper is stored can influence its relative permanence.Isoperms or "record lifetime multipliers" provide a graphical representation of the relationship between the lifetime of materials that deteriorate rapidly from acid hydrolysis, and the temperature and RH at which they are stored. Isoperms can be used to plot the relative lifetime changes associated with changes in temperature or RH. Isoperm graphs and further explanations on their use can be found in the ASHRAE chapter and in Technical Bulletin 23 (Guidelines for Humidity and Temperature for Canadian Archives).Mechanical DamageExtremes of temperature or RH make many objects vulnerable to mechanical damage. For example, materials that become stiff or brittle when cold are more likely to break at low temperatures. Luckily, extremes are usually easy to avoid, except in rare cases such as sudden failure of an environmental control system.

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It is fluctuations in temperature or RH that cause the most mechanical damage. Unfortunately, these are less easily controlled and not as well understood.The explanation for the dangers of fluctuations lies in the basic mechanical properties of materials: inorganic and organic materials (e.g. metal, stone, paint, and wood) respond to temperature

changes by expanding when hot and contracting when cold organic materials, which are hygroscopic, respond to changes in RH by shrinking when RH drops

and swelling when RH climbs

These responses do not cause damage in and of themselves. However, damage can occur when objects combine materials that respond in different ways, placing one material under restraint from another. For example, a wooden panel that is not restrained in any way can expand and contract in response to changing conditions without harm. However, if the wooden panel is part of a chest of drawers, or an agricultural machine, or an architectural element, and it is attached to other elements that restrict its movement, then damage is likely when the wood attempts to move during RH changes. A painting presents a more complex example. A typical painting is composed of many layers that expand and contract in differing ways. The internal stresses thus created can cause cupping and craquelure over time.The damaging nature of very wide fluctuations in RH and temperature has been known for a long time. The early specifications for acceptable fluctuations in museums assumed that even very small fluctuations would produce some form of damage. Specifications such as 50±3% RH and 21±1°C emerged. Since the early 1990s, a consensus has grown among scientists and conservators with wide experience of collections that objects have a range within which they tolerate fluctuations without damage. The ASHRAE chapter currently provides the best review of the issues and the relevant literature.

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Vulnerable ObjectsHighly or very highly vulnerable objects are defined by Stefan Michalski in “Quantified Risk Reduction in the Humidity Dilemma” (APT Bulletin, Vol. 27, No. 3, 1996, pp. 25–29). These definitions of object “vulnerability” are based specifically on mechanical damage. Please note that this classification has been applied mainly to wooden objects - so far. This information is intended as an indication of likely damage to certain materials and is included to exemplify the problems which may be encountered under different climatic regimes. It is not intended to be an exhaustive list of all possible vulnerabilities. Classifying the Mechanical Vulnerability of Objects to Fluctuations in Temperature and RHObjects can vary greatly in their vulnerabilities to fluctuations in temperature and RH. Stefan Michalski has defined four categories of vulnerability: very high, high, medium, and low (“Quantified Risk Reduction in the Humidity Dilemma”, APT Bulletin, Vol. 27, No. 3, 1996, pp. 25–29; “Climate Control Priorities and Solutions for Collections in Historic Buildings”, Historic Preservation Forum, Vol. 12, No. 4, Summer 1998, pp.8–14). By way of example, a wooden object with aged glue, varnish, lacquer, gesso, or oil paint bridging a joint in which the grains of the wood components are at right angles to each other would be considered “very high” vulnerability. However, a single piece of wood with a paint or varnish layer would be considered a “medium” vulnerability object provided the wood had no cross-bars or restraints to its movement. Numerous examples of classified wooden artifacts can be found in the Vulnerabilities of Wooden Artifacts table.The ASHRAE chapter uses these vulnerability classifications to describe the risk to collections from the various classes of control. Vulnerabilities of Wooden ArtifactsThis table is adapted from the work of Stefan Michalski (“Quantified Risk Reduction in the Humidity Dilemma”, APT Bulletin, Vol. 27, No 3, 1996, pp. 25–29.

Artifact ExamplesVery High Vulnerability  ±5% RH: gradual fatigue fracture ±10% RH: fracture possible each cycle±20% RH: fracture definite first cycle

This class of wooden artifact breaks the rules of cautious woodworking, or else the fracture of these coatings has never been considered disfiguring, e.g. painted doors.

Aged glue, lacquer, varnish, gesso, or oil paint that bridges joints where wood grains meet at right angles (lap joints, mortise and tenon joints, etc.; also, any knots in wood components).

Aged glue, lacquer, varnish, gesso, or oil paint that bridges a crack formed by a check, knot, side-by-side butt joint, or mitre joint.

Inlays of metal, horn, ivory, or shell (but not wood; marquetry is considered below as medium vulnerability, since it is much tougher and more resilient). The longer the inlay runs across the grain, the more vulnerable the piece.

High Vulnerability  ±5% RH: zero fatigue fracture±10% RH: gradual fatigue fracture, or plastic deformation±20% RH: fracture possible each cycle±40% RH: fracture definite first cycleMuch of this type of fracture has already occurred in old artifacts. Only artifacts from less fluctuating or higher annual average conditions, or those recently re-attached by inflexible treatments, will fall in this category.High Vulnerability (continued)

Veneer over corner joints where wood grains meet at right angles (lap joints, mortise and tenon joints, etc.; also, any knots in wood base components).

Lacquer, oil paints, or gilding over single knot-free wood components, or over joints that use feathered inserts, fabric, tissue, etc. that are still sound. When new and fairly flexible, these layers may drop to only medium vulnerability.

Fretwork, applied ornaments (especially if the wood grain follows the notch), and assemblies with metal bands, bolts, nails, or screws that

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Artifact Examplesrestrain the wood unevenly.

Checked timber or sculpture with a hard new fill.

Cracked panel or panel painting in a rebate or cradle that jams.

Large pieces of recently seasoned wood such as folk art, also all knots and uneven grain in wood must be considered prestressed in this way.

New plywood, newly steam-formed wood held by other components.

Panels near intermittently damp walls, floorboards over damp crawlspace, and glued veneer or joints where the glue bond has softened and readhered at the expanded component position.

Medium Vulnerability±10% RH: zero fatigue fracture±20% RH: gradual fatigue fracture, or plastic deformation±40% RH: fracture possible each cycle

Most wood joinery, veneers, and marquetry over single, clear pieces of wood at crossed grain; any prestressed pieces from above that have stress-relaxed for more than a decade.

Any wood with little or no coating, subjected to an RH fluctuation shorter than its response time. This leads to warping or surface checking, e.g. backs of picture frames; on exposed end grain it results in end-checks, e.g. tenons, dovetails, feet of furniture, overhangs in carved totem poles.

Low Vulnerability±40% RH: possible accumulation of fatigue fracture or plastic deformation if the freedom to move, or the coatings, or the slowness of the fluctuation are less than perfect

Aside from possible cracks in any varnish, and given either a slow enough change in humidity or good moisture barrier coatings, then:

o already loose joinery

o floating panels

o loose tabletops

o tongue-and-groove or lapped planking nailed or bolted at one point only, e.g. wainscotting

o boxes on farm machinery (unless jammed due to painting or warping)

o hollowed-out totem poles

o hollowed-out sculpture

o most single-component tool handles

o veneer on wood with parallel grain

o joined wood components with parallel grain.

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Classes of ControlAA (precision control, minimal seasonal changes to temperature only)Permissible short-term fluctuations of ±5% RH and ±2°C, with a seasonal set point change in temperature of up 5°C and down 5°C. No seasonal change in RH set point is allowed. The images below show the worst case scenario with maximal permissible fluctuation.

 

 Collection risks and benefitsNo risk of mechanical damage to most artifacts and paintings and there is minimal risk even to objects of very high vulnerability. Some metals and minerals may degrade if 50% exceeds the value that is critical for a particular element. Chemically unstable objects will become unusable within decades.CommentsThis is the traditional, very narrow specification for collection climate control, with a modest amount of seasonal temperature adjustment. However, it must not be understood as the “perfect” specification. It does not address the large problem of chemically unstable materials in 20th-century collections, and it may be unnecessary for much of the collection. Also, even systems that keep fluctuations within this

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range 97% of the time will usually fall outside it a few days a year. (Currently, the ASHRAE specifications are ambiguous about the strict design definition of these classes of control.) In spite of these limitations, this is a plausible target for an institution with the mandate and the resources to provide the best, as long as the resources allocated to the system and its operation do not diminish resources needed to address much larger risks to the collection.A (good control, some gradients or seasonal changes, but not both)This degree of control has two subcategories: short-term fluctuations of ±5% RH and ±2°C, with a seasonal temperature change of up 5°C and

down 10°C, and a seasonal humidity change of up 10% RH and down 10% RH short-term fluctuations of ±10% RH and ±2°C, with a seasonal temperature change of up 5°C and

down 10°C

The images below show the worst case scenario with maximal permissible fluctuation.

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 Collection risks and benefitsSmall risk of mechanical damage to high vulnerability artifacts; no mechanical risk to most artifacts, paintings, photographs, and books. Chemically unstable objects will become unusable within decades.CommentsThe letter “A” was assigned to this specification because it was felt to be the most cost-effective degree of control for most collections, given the ability and mandate to provide a climate-controlled building. Such systems will probably operate within AA levels most of the time. To stay reliably within A conditions year-round is a resource-intensive task in most climates. The larger seasonal adjustments in set points are a recognition that even major museums must face energy and sustainability constraints, and that these suggested temperature changes are not a significant risk.  

B (control, some gradients plus winter temperature setback) This degree of control allows short-term fluctuations of ±10% RH and ±5°C, with a seasonal temperature change of up to 10oC. Note that the temperature can not be allowed to rise above 30oC but can fall as low as necessary to maintain RH control.The image below shows the worst case scenario with maximal permissible fluctuation.

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Collection risks and benefits Moderate risk of mechanical damage to high vulnerability artifacts; Little risk of damage to objects of medium vulnerability. Tiny risk to most paintings, most photographs, some artifacts, and some books; and no risk to many artifacts and most books. Chemically unstable objects will become unusable within decades and sooner if collections are routinely maintained at 30°C. Cold winter conditions may retard the rate of deterioration considerably. CommentsHigh vulnerability artifacts in most historic collections have already experienced large fluctuations, and have formed the cracks that are possible. Further fluctuations of the same size have a very low risk of further damage. This is known as the “proofed fluctuation”. Thus the risk to high vulnerability objects stated above must be recognized as applying only to objects that have come from better conditions, or have been repaired. B class of control is suggested as a reasonable target for collections in historic buildings that can tolerate the intervention of some mechanical systems but must not be damaged by the humidity conditions.

C (prevent all high risk extremes)Within 25–75% RH year-round. Temperature rarely over 30°C, usually below 25°C. Collection risks and benefitsHigh risk of mechanical damage to high vulnerability artifacts; moderate risk to most paintings, most photographs, some artifacts, and some books; and tiny risk to many artifacts and most books. Chemically unstable objects will become unusable within decades. Cold winter conditions may retard the rate of deterioration considerably. CommentsHigh vulnerability artifacts in most historic collections have already experienced large fluctuations, and have formed the cracks that are possible. Further fluctuations of the same size have a very low risk of further damage. This is known as the “proofed fluctuation”. Thus the statement of risk to high vulnerability objects stated above must be recognized as applying only to objects that have come from better conditions, or have been repaired. C class of control recognizes that most of the damage potential of uncontrolled climate can be prevented simply by avoiding extremes in humidity. Between 25% RH and 75% RH, hygroscopic response is only one-half to one-third that seen below 25% RH or above 75% RH. Mould and rapid corrosion are avoided. C class of control is suggested as a target for open display collections in historic buildings that are as important as the collection.If parts of the collection are still at risk from these conditions, then provide climate control for these objects through the use of cases, cabinets, packaging (in storage), and special rooms or zones. 

D (prevent dampness)

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This degree of control dictates only that humidity is “reliably below 75% RH”.Collection risks and benefitsHigh risk of sudden or cumulative mechanical damage to most artifacts and paintings (including objects of low vulnerability) due to low humidity fracture; however, high humidity delamination and deformations, especially in veneers, paintings, paper, and photographs, will be avoided. Mould growth and rapid corrosion will be avoided. Chemically unstable objects will become unusable within decades. Cold winter conditions may retard the rate of deterioration considerably. CommentsAvoiding mould and rapid corrosion due to damp is probably the single most important aspect of climate control for the smaller (and larger) museum. Although such a target is not normally considered an engineering design condition, it is when applied to basic historic buildings with poor quality building envelopes. It is also a legitimate target for any collection in a simple building in a humid climate, such as in the Maritimes. Well engineered systems rely more on careful design than the sheer power or ultimate capacity of the HVAC system. If parts of the collection are still at risk from these conditions, then provide climate control for these objects through the use of cases, cabinets, packaging (in storage), and special rooms or zones.

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Special CollectionsCool StorageSet point 10°C; 30–50% RH(even if achieved only during winter setback, cool storage is a net advantage to collections as long as damp is not incurred)Collection risks and benefitsChemically unstable objects will remain usable for a century or more. Books and papers tend to have a low mechanical vulnerability to fluctuations. CommentsNo specific fluctuation specifications are offered in the ASHRAE table. One can apply the classes of control AA, A, and B, if desired. A much more detailed discussion of climate control specifications for such material is available in Technical Bulletin 23 (Guidelines for Humidity and Temperature for Canadian Archives).  Cold StorageSet point -20°C; 40% RH Maximum fluctuations

±2°C; ±10% RH Collection risks and benefits Chemically unstable objects will remain usable for millennia. RH fluctuations under one month do not affect most properly packaged objects at these temperatures.  (Time out of storage becomes the lifetime determinant). CommentsCold storage is used in archives and libraries, as well as for 20th-century collections of plastics and rubbers. This level of control allows chemically unstable objects such a photographic film to remain usable for millennia. Properly packaged objects are insensitive to monthly fluctuations in RH. The lifetime of objects in cold storage is determined by the time spent out of storage. A much more detailed discussion of climate control specifications for such material is available in Technical Bulletin 23 (Guidelines for Humidity and Temperature for Canadian Archives).Dry Rooms Set point0–30% RH FluctuationsThe RH should not exceed a critical value. This value is often 30% RH.CommentsThere is no specific note of risks and benefits in the current ASHRAE table; however, the benefit can be presumed to be the prevention of corrosion, especially on metal objects with contaminants such as chlorides. Fluctuations are not an issue per se — it is simply a question of not exceeding a particular RH.

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