On 12 August 1942, Adelaide Minogue was photographed inspecting a humidity recorder in the stacks of the United States National Archives. The machine traced the changing conditions around the records continuously. A reading taken at one convenient moment would tell the archivists nothing about the previous night’s damp air, an unusually hot afternoon or the consequences of equipment failure. That continuous record mattered because collections experience dampness, heat and equipment failures over days and seasons.
The problem is especially awkward for a small museum working in an old building. Imagine a volunteer-run historic house with nineteenth-century windows, a damp cellar, photographs and furniture sharing the same rooms, and heating installed primarily for visitors. A requirement to hold every space at 70°F (21°C) and 50% relative humidity might demand costly new equipment before anyone has established which objects need that control. Staff may be committed to preservation, yet the building and budget can make that specification impractical.
A costly mistake is to assume that one climate target or one machine can protect every kind of collection. Preventive conservation starts with the objects: their materials, past conditions, signs of damage and the way their building behaves through the seasons. Any proposed measure has to reduce deterioration without bringing another hazard into the room. A few collections require exceptionally precise conditions. Elsewhere, fixing water entry, monitoring properly and providing local protection for vulnerable objects may achieve more than a major mechanical installation.
Where did the 70°F and 50% RH rule come from?
Through much of the twentieth century, a temperature close to 20–21°C and approximately 50% relative humidity became familiar requirements in museum loans and engineering specifications. Shared numbers made agreements and system design simpler, and they can still be appropriate for particular collections. An object can deteriorate within that range or remain stable outside it, depending on its materials and history. The 2014 IIC and ICOM-CC environmental declaration urged institutions to consider their local climate, actual preservation risks and lower-energy approaches instead of assuming that full air conditioning was always the answer.
Conservators did not abandon concern about humidity changes when that guidance appeared. The declaration brought together professional positions, including the Bizot interim guidance’s 40–60% RH band for many moisture-absorbing, or hygroscopic, materials, alongside stricter provisions for sensitive objects. The 2019 ASHRAE chapter on museums, galleries, archives and libraries describes several classes of environmental control with different implications for risk and engineering. Choosing among them requires examining the collection and building. Choosing the cheapest control class without studying sensitive objects would be poor practice.
Old buildings make that judgement particularly important. Solid masonry, timber-framed windows and uninsulated roof spaces can be cold even when the rooms are heated. Maintaining high indoor humidity during winter may cause moisture to condense inside walls, around glazing or in roof structures. Humidification could then increase operating costs and leave the museum paying to repair damaged historic fabric. ASHRAE treats the historic fabric itself as part of what must be protected and advises investigating passive solutions before invasive mechanical work.
Relative humidity is a risk, not a score
Relative humidity, abbreviated RH, measures how close the air is to saturation at its present temperature. Cooling air raises its RH if its water-vapour content stays the same; warming the same air lowers RH. Consequently, a room may register a substantial humidity swing without any comparable change in the amount of moisture present. A humidity percentage alone cannot explain what has happened. Staff need to know the air’s moisture content, how cold nearby surfaces become and how the collection responds to those conditions.
The Canadian Conservation Institute (CCI) divides incorrect relative humidity into four kinds of risk: dampness, crossing a critical threshold particular to an object, very dry conditions and fluctuations. Each requires a different assessment. Readings above 50% alone cannot tell staff which risk is present. Sustained dampness may support mould, while salts and pollutants can worsen corrosion. Wood, paper, canvas, adhesives and paint layers may swell, distort or crack as their moisture content changes. The relevant levels and lengths of exposure differ between these processes.
CCI identifies about 75% RH as a broad boundary for serious dampness, while recognising that sustained exposure can encourage mould below that level, particularly on vulnerable materials. Soiled objects and surfaces rich in proteins may support growth where clean inorganic materials would not. Keeping a photographic collection at 70% for months is therefore not made safe simply by staying “below 75%”. A brief rise to 61% also provides little evidence that a previously stable object has been damaged. Temperature, exposure time, contamination, enclosure and the object’s condition all affect the outcome.
Consider two objects that react differently to the same seasonal humidity cycle. An old wooden cabinet may have moved with the seasons for decades, while a recently restored panel painting with rigid repairs may accumulate stress. Conservators sometimes call an object “proofed” by its past humidity experience. That is a way of assessing likely risk, not a promise of immunity. Earlier cracks, fatigue or new treatments can change how the next cycle affects it. Condition checks must accompany the RH records.
Measure the rooms before buying equipment
Small museums need reliable environmental records before investing in new climate-control equipment. A temperature and RH data logger can reveal conditions overnight and through weekends, holidays and changing seasons. The Northeast Document Conservation Center’s monitoring guidance calls for a written monitoring plan, named responsibilities, recorded instrument locations and notes linking readings to weather and events. When practical, it recommends observing a full year before major building-system changes. Active mould, a serious leak or rapidly corroding metal demands action sooner; an urgent threat must not be left untreated while staff wait for annual data.
Put a logger at a representative height near collections in each distinct room or store, then add instruments where conditions may differ: in a case, behind furniture on an external wall, underground or near an unreliable window. A logger next to a hot-air outlet tells you more about the heater than the gallery. Keep a record of placement, serial numbers, sampling intervals and maintenance. Readings every five or fifteen minutes may help diagnose fast changes, though collections do not universally require that frequency. Battery capacity, storage and the particular question being investigated should guide the settings.
A low-cost sensor is useful only when its accuracy is understood. Compare it with a reliable reference, follow the maker’s calibration procedure and keep the results. Saturated salt methods can provide reference conditions, but only with suitable salts, stable temperature, a properly sealed arrangement and enough equilibration time. A kitchen-table check cannot certify a sensor’s accuracy. What looks like a three-percentage-point shift in room RH might be sensor drift. The National Park Service handbook chapter on collection environments likewise treats calibration and interpretation as inseparable from collecting readings.
Once monitoring begins, compare daily and seasonal peaks, the time spent in damp or dry conditions, events such as door openings and differences between the gallery and its cases. Monthly averages may hide repeated humid nights. On the other hand, a short RH spike may end before a thick wooden object absorbs enough moisture to change significantly. The records must be interpreted in relation to materials and their response times. Staff should use those records to decide when repairs, environmental controls or better enclosures are necessary.
Dew point explains why a cool room can become a damp room
Dew point gives another way to understand moisture problems. It is the temperature to which air would have to cool, at its existing water-vapour content, before becoming saturated. A surface at or below that temperature can collect condensation. The centre of a room may appear safe on its RH logger while a cupboard against cold masonry develops mould. Local surfaces can remain dangerously damp without visible droplets. The NPS guidance explains these connections between dew point, RH and condensation in historic buildings.
Take a hypothetical gallery at 22°C and 60% RH. Its calculated dew point is about 14°C, so a window reveal or outside wall reaching that temperature may collect water locally. These figures illustrate the physics; they are not measurements from a museum. Surface temperatures can be investigated with a suitable contact sensor or infrared thermometer, although emissivity and how the instrument is used affect its reading. Recurring moisture problems call for assessment by a building specialist or conservator, especially if the building fabric has historic value.
A psychrometric chart shows why temperature and moisture cannot be managed separately. On it, dry-bulb temperature, humidity ratio, RH and dew point describe the condition of moist air. Cooling without removing water makes relative humidity rise. When air is cooled below its dew point, water condenses, as it can on a cooling coil. A small air conditioner may dehumidify effectively while running, whereas an oversized unit that stops after short cycles may remove too little water. Lowering the thermostat will not necessarily remove excess moisture.
A dehumidifier does not repair a leaking building
A portable dehumidifier may be sensible for a small enclosed store with a known moisture problem. It may also help after water damage, provided specialists address wet collections, contaminated material and electrical safety. It cannot cure rain entering through a roof, failed drainage, defective rainwater goods or rising damp. CCI’s account of sources of incorrect RH places attention on water entry and the building envelope before equipment that merely removes moisture already inside. Until a leak is repaired, the machine may run continually without solving the underlying problem.
Portable units bring their own hazards. A full tank stops water collection, a blocked drain can overflow among valuable objects, and neglected filters reduce efficiency. The humidistat on the machine may also miss a damp corner some distance away. Refrigerant dehumidifiers can perform poorly in low temperatures, where other equipment may be needed. If one is installed, arrange reliable drainage, independent monitoring, scheduled maintenance and a response to power failure. The reservoir and drain must be positioned so an overflow cannot reach the collection.
Humidification requires just as much caution. Adding moisture in winter may relieve a dangerous dry spell for moisture-sensitive materials, yet it also raises dew point. Cold windows, roofs and wall cavities can then become sites of condensation. The mist from unsuitable equipment can deposit water or mineral residues on objects nearby. Safe humidification depends on clean-water procedures, properly selected equipment, knowledge of the building and evidence that the collection requires it. A small humidifier can cause expensive damage if moisture accumulates within the building.
Cases, boxes and frames can create better climates than galleries
The air within a well-sealed display case is easier to manage than all the rooms of a historic property. A case slows exchange with the gallery; its contents and suitable moisture-absorbing materials can buffer change. In CCI Technical Bulletin 33 on silica gel, Jean Tétreault and Paul Bégin explain how silica-gel products work and how to calculate a required quantity from their buffering capacity. The amount required differs from case to case. Leakage, internal volume, outdoor climate, desired RH, gel performance and the interval between maintenance visits all affect the calculation.
A leaky case may use up its buffering capacity quickly, while a sufficiently airtight one can need much less attention. Silica gel has to be conditioned for the intended RH and positioned to exchange moisture with the enclosed air without contacting or dirtying the objects. A logger inside the case checks whether conditions are being maintained. Even a well-designed gel arrangement cannot tell staff what RH a chloride-contaminated metal or unstable glass object can safely tolerate. The object’s safe environmental range must be established before selecting the gel.
Boxes made from suitable acid-free materials, protective sleeves, cabinet doors, framed backboards and vapour-resistant packaging can also slow environmental changes and exclude dust or pollutants. Enclosure alone does not guarantee protection. A damp item sealed inside may remain wet longer, while packaging touching a cold wall could create condensation within it. Each enclosure needs to suit its contents, be properly conditioned and receive periodic checks. Inspect enclosed objects regularly, especially if they were previously damp.
A lender may require conditions narrower than a museum can maintain in the surrounding gallery. In some cases a purpose-built, buffered frame or display case provides safer and less expensive local control. ASHRAE acknowledges that well-designed enclosures can maintain precise conditions within buildings whose room climates vary more widely. A buffered enclosure may also slow climate changes during a mechanical failure. Such a case does not address every threat: light, heat, pests, pollutants and physical security still require attention.
Different materials do not share one safe humidity target
Moisture-sensitive materials include wood, veneer, ivory, parchment, gesso and painted panels. They absorb and release water as RH changes, sometimes expanding or shrinking at different rates within one object. Those movements strain joints and paint layers. Severe dryness can cause cracking or shrinkage; persistent dampness can weaken some adhesives and favour mould. Construction, past conservation treatments and the speed of change shape the response. A conservator should examine a panel with lifting paint before its allowable humidity range is widened.
Flexibility does not make a canvas painting immune to humidity. Changes in the canvas, ground, paint, varnish and stretcher can interact, leaving the canvas slack or overly tense or causing cleavage between layers. The CCI guidance on caring for paintings explains the interactions among these materials and the risks they face at different RH levels. Vulnerable works may need specific monitoring and tighter controls for display or loan, even where other objects in the same museum can tolerate broader conditions.
Paper and bound books can deform when they gain or lose moisture; their chemical ageing also accelerates with heat. Persistent dampness contributes to mould, blocked pages and cockling. Photographic collections have further problems. Gelatin layers may soften and stick when humid, while cellulose acetate and nitrate film and many colour processes remain chemically unstable at ordinary gallery temperatures. The NEDCC’s care of photographs recommends cooler, drier conditions and specialised cold storage for particularly at-risk formats. A dedicated cold store may protect vulnerable photographs better than a compromise climate imposed on the entire museum.
Archaeological metalwork may need a much drier environment than paper or furniture. Excavated iron and chloride-contaminated copper alloys can continue corroding at RH levels that would not harm books. For certain objects, appropriate levels can lie around or below 30% RH, although contamination and corrosion products determine the threshold. Lead is vulnerable to organic acids released by unsuitable wooden cases, while unstable glass and some minerals have their own humidity sensitivities. CCI’s material-specific risk tables show why furniture specifications cannot simply be transferred to a case of excavated metals.
Seasonal drift is permitted only when it is planned
Seasonal set points allow a museum to operate at deliberately different temperatures or RH levels in winter and summer, within planned limits. They do not authorise uncontrolled surges. The 2019 ASHRAE guidance distinguishes an orderly seasonal adjustment from a brief fluctuation, with limits and rates set by the chosen control class. Its A1 example permits seasonal RH movement of up to ten percentage points while restricting how quickly conditions change. The figure is a limit within one control class, not a universal RH allowance for every museum.
In a historic house, somewhat lower winter RH may reduce condensation on cold fabric, while a controlled summer range can help prevent prolonged dampness. A library holding unstable photographic film might instead need dedicated cold and dry storage. Institutions with a well-documented record of successful operation around a particular annual average may also have good reasons to retain it. Documented building and collection conditions must determine the seasonal range; energy costs alone cannot set it.
The 2014 professional declaration draws a distinction between the climate of permanent collections and conditions specified for international loans. Signed loan requirements remain binding unless the parties agree to change them. A curator cannot widen a permitted RH band merely because recent conservation research supports more flexibility in other situations. Borrower and lender can negotiate a documented solution, perhaps using a sealed case or microclimate during transport. Without evidence from particular institutions, no one can safely promise that “most lenders accept” broader targets.
The building envelope comes before a larger HVAC plant
The first building survey should look for sources of water entry. Blocked drains, broken gutters, poor ground levels and leaks around doors or windows can bring in quantities that a dehumidifier cannot economically counter. Delivery doors left open can also undermine a tightly controlled store. Roof and drainage repairs, regular inspection and appropriate draught reduction deserve attention before a major HVAC replacement. In listed or otherwise protected buildings, changes must respect historic fabric, ventilation and conservation rules. Sealing an old building indiscriminately may trap moisture within it.
Many practical improvements are modest: shift storage away from a cold external wall, raise objects above a flood-prone floor, improve appropriate shelving or mend a faulty door closer. The National Park Service collection-environment guidance describes how these measures work alongside cabinets, boxes and more conventional environmental systems. These measures limit the humidity, dust and temperature changes that reach the objects. Protecting the collection does not require every public room to operate like a specialised laboratory.
Routine maintenance also affects the collection environment. Dust can retain moisture and pollutants and add to biological risk. Mechanical filters, coils, drains and humidifiers need inspection on a defined schedule. Good storage hygiene, controlled handling and integrated pest management reduce dangers a thermostat cannot measure. If active mould or potentially hazardous contamination is discovered, routine vacuuming is not a safe treatment. Specialist advice and appropriate containment may be necessary before anyone handles the affected area.
Lighting and visitors alter the environmental load
Lamps contribute heat and may increase cooling loads. Switching inefficient fittings to appropriate LEDs can cut electrical use and local heating, but does not eliminate light damage. The risk depends on the light’s intensity, spectrum and duration and on the sensitivity of the particular textile, photograph or other object. CCI’s study of light, ultraviolet and infrared explains that visible light can fade materials even when ultraviolet radiation has been excluded. Staff still need to limit exposure times and keep direct sunlight off vulnerable objects.
Visitors also change the climate of a building. People release heat and moisture, open doors allow outside air to enter, and an exhibition opening or school visit may produce short-lived peaks. Staff should record a brief occupancy spike and check how quickly conditions recover before treating it as evidence that the HVAC plant is inadequate. Persistent overnight dampness or water condensing on cold surfaces is more concerning. Staff should record occupancy, events and equipment operation alongside the logger data to distinguish temporary loads from underlying faults.
Changing light schedules, shading suitable windows, managing doors and selecting efficient lamps can reduce the load used to size future HVAC equipment. Yet window films, exterior shutters and insulation may alter historic glazing, wall moisture and protected architectural fabric. They need to be considered in relation to the individual building. Even modest alterations can affect moisture movement through an old building.
When a larger climate-control system is justified
Significant mechanical investment is justified when collections face unacceptable risks that passive measures cannot reliably control, or when exhibition and loan agreements require tighter conditions. Unstable film in storage, particularly vulnerable panel paintings or recurring dampness may demand dedicated plant. The decision should explain which form of damage the equipment is meant to prevent and how consistently it must operate. A “museum-grade” label does not establish that a system protects the objects in that particular building.
An engineer needs a brief describing the sensitive materials, measured room conditions, outdoor climate, historic fabric, occupancy, water sources, different zones, maintenance resources and what happens when equipment fails. Temperature reduction, or sensible cooling, differs from latent control that removes moisture. The design also has to work during shoulder seasons, when little cooling is needed even though incoming air is humid. Drainage, condensate disposal, understandable controls and local servicing matter as much as headline capacity. If staff cannot maintain an oversized or complex system, its failures may leave the collections in worse conditions.
The Getty Conservation Institute’s Managing Collection Environments initiative brings research on objects together with studies of buildings and institutional decisions. Getty’s published climate-action account records trials with broader temperature and RH settings, including a 2025 storage pilot at the Getty Research Institute. These are specific programmes with monitored outcomes, not a promise of equal energy savings for every building. The trials show how institutions can test changes through environmental and object monitoring before adopting them more widely.
What the psychrometric chart can and cannot tell you
Engineers use a psychrometric chart to describe how moist air behaves during heating, cooling and dehumidification. It makes clear why cooling without taking out water raises RH, why adding moisture raises dew point, and why effective dehumidification may require cooling air below its dew point and then reheating it. The chart helps estimate loads and compare systems. It cannot decide whether a deteriorating lacquer surface or cracked panel can survive a certain humidity cycle. A conservator must assess that risk using the object’s materials and condition.
To investigate a problem, compare indoor temperature and RH with outdoor measurements, calculate dew point and measure surfaces at suspect locations. A high overnight RH reading without much dew-point change may mainly reflect cooling air. A rising dew point points to additional moisture. Condensation can develop behind furniture despite reasonable readings at the room sensor. Staff can then investigate drainage, ventilation, room use and equipment operation. None is a conclusive diagnosis without observations to support it.
A conservation specification should describe performance, not slogans
A policy can say “50% RH at all times,” yet still leave staff without instructions for managing the collection. A useful policy identifies collection spaces and vulnerable objects, describes acceptable seasonal conditions, establishes warning levels and specifies the action and personnel responsible when readings exceed them. It should also say how long an excursion may continue before staff intervene, and who can authorise changes. The final numbers need to be developed with a conservator, and where appropriate the building engineer and lender, from the actual condition of the collection.
Equipment failure belongs in the same policy. During a summer power cut, will sealed cases buffer their contents, and can vulnerable photographs be moved safely? Who receives an alarm from a damp basement, and who can stop water after a pipe bursts? A system that maintains a very tight average under normal conditions may still pose a severe risk when it breaks down. ASHRAE notes that long-term reliability can matter more to conservation than nominal precision. The climate specification also needs an emergency response plan.
Before approving major expenditure, trustees should examine recorded risks and likely savings. They can examine how long rooms exceed a relevant risk threshold, whether vulnerable objects show deterioration, what the plant costs to operate and what maintenance costs before and after an intervention. A promised energy-saving percentage is unjustified without metered data or an energy model and a baseline. Wider settings may reduce energy consumption, but the outcome depends on local weather, equipment configuration, operating hours, electricity prices and which humidification or dehumidification demands have truly been reduced.
A realistic first three months for a small museum
During the first month, identify materials most sensitive to moisture, heat, dryness and light. Inspect roofs, drains, external walls, basements and stores for immediate problems. Place calibrated loggers in representative rooms and priority cases, or check the instruments already installed. Keep a record of weather and building operation. Active deterioration should trigger prompt conservation advice; there is no reason to postpone treatment until enough readings have accumulated for a seasonal graph.
In month two, examine the early readings and revisit suspected cold or wet locations. Where suitable, try reversible, modest changes such as shifting a cabinet from an external wall, repairing a gutter, improving a door seal without damaging old fabric or enclosing photographs appropriately. Record exactly when each measure was introduced and monitor afterwards. Weather changes can produce apparent improvement on their own. A short trial can test a repair, but it cannot establish how the building will perform in both summer and winter.
By the third month, the museum can begin prioritising longer-term work. Determine which spaces need specialist advice, which objects belong in separate storage, which cases might use a carefully calculated microclimate and which machinery needs repair. Assemble an initial risk register and a programme for continued monitoring. Discuss proposed RH ranges with a conservator before altering loan conditions or paying for new HVAC. Ninety days of evidence can support early decisions, although major capital changes usually benefit from a complete seasonal record.
The difference between a plausible example and a documented saving
Picture a hypothetical county museum with a brick gallery and a photograph store. One external wall becomes damp each summer. A logger in the room records moderate RH, while another behind a cabinet shows prolonged high readings. Inspection reveals a blocked rainwater downpipe and a poorly ventilated cold corner. Repairing the drainage and moving the cabinet may reduce the immediate danger, and a sealed, monitored case might shelter a particularly sensitive object. This is an illustrative diagnosis, not an actual published case study; it provides no measured evidence of electricity savings.
Demonstrating a genuine energy saving after changing set points requires utility bills or meter readings, along with comparable weather, occupancy and records of other changes made at the same time. Claims about mould being permanently eliminated or a specified quantity of silica gel holding a particular RH need equally relevant measurements. A small museum should judge its environmental strategy by whether readings, object condition and maintenance show that deterioration risks are understood and controlled. Meeting a thermostat target matters only if the collection is being protected.
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