Preventive conservation is about reducing risk, not chasing one number

Temperature and relative humidity management are part of preventive conservation: reducing the risks that can damage collections. The Canadian Conservation Institute (CCI) distinguishes four relative-humidity risks: damp conditions above 75%; humidity above or below an object-specific critical value; any relative humidity above 0%, which can promote metal corrosion; and fluctuations. Very low humidity can also harm some materials, so consider it within the relevant object’s critical limits and material properties. A mixed collection of wood, paper, textiles, metals, and glass may have no single perfectly correct target; objects differ in sensitivity. A familiar rule-of-thumb number should therefore not be copied into every gallery or storage room. CCI’s relative-humidity guidance

Damp conditions can encourage mould or corrosion. Changes in humidity can stress wood or objects made from joined materials. Very dry conditions can also be harmful to some materials. The type and scale of risk depend on the object and its condition.

Connect a reading to its location and the objects nearby

Place monitoring instruments with their locations documented, then examine how long a reading persists and where it occurs rather than reacting to one spike. Conditions near an exterior wall, window, roof, or floor can differ from those in a central aisle. Record the time and place alongside changes to display lighting or HVAC, and observations such as leaks or condensation. CCI recommends starting with object-specific guidance such as ClimaSpec when selecting climate guidelines; a single collection can contain several groups with different sensitivities to fluctuation. CCI climate-guidelines overview

For a fictional example, imagine a small gallery with paper documents and metal objects where humidity rises each afternoon. Before buying a humidifier or dehumidifier, check the monitor’s location and calibration, condensation near windows or exterior walls, and the HVAC schedule. Map cockling or mould on paper and corrosion on metal by shelf. That can reveal a local problem hidden by the room’s average reading. If objects have remained stable and show no signs of damage, a conservator should also consider whether an abrupt change in target conditions is warranted.

A practical inspection sequence

  • 1. Group objects by material and condition; flag those known to be sensitive to fluctuations or condensation.
  • 2. Record instrument, location, and time. Compare trends with leaks, outdoor air, lighting, and HVAC changes.
  • 3. Look beyond room averages to differences inside display cases and near exterior walls.
  • 4. If risk or damage is present, consult guidance for the object type and agree on a target range with a conservation professional.
  • 5. After an adjustment, keep measuring at the same locations and record both improvement and unintended effects.

The useful question is not simply “What is the correct temperature and humidity?” It is “Which risk can we reduce first for these objects in this building, and did the change help?” A number should be interpreted with the materials, risks, and measurement location in view, rather than used as a context-free prescription.

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