Incorrect Relative Humidity (RH)

What causes relative humidity damage?

humidity cartoon

humidity logoWe’ve all suffered through muggy, steamy summer days. Humidity, or the amount of water vapor in the air, can be measured in several ways. Relative humidity (RH) is a measure of the amount of water vapor air holds at a given temperature.

Regardless of temperature, too high, too low, or fluctuating levels of RH can damage artifacts in our houses and museums.

High RH causes mold, mildew, and fungal growth, as well as swelling, rusting, and corroding.

Low RH causes cracking, shrinking, drying, desiccation, and brittleness.

Fluctuating RH causes swelling and shrinking; over time, the lifting and peeling of surfaces and structural weakness.

How do you protect objects from RH damage?


Store humidity sensitive objects at a stable RH. Most objects, other than metals, can be comfortably kept at 40%-60% RH. This is the range that is most comfortable to live in, as well. Metals should be stored at a low RH, to prevent corrosion.

Use humidity monitors to keep track of RH. Use humidifiers or dehumidifiers to control RH. Store sensitive metals with drying agents such as silica gel to keep RH low.

Photo: Humidity monitors

What can you do at home?

  • Keep the relative humidity in your home as stable as possible. Seasonal fluctuation is unavoidable, but you want to control the most extreme shifts.
  • Add humidifiers during the winter and dehumidifiers (air-conditioning helps) during the summer-especially in those damp spots like basements and crawlspaces.
  • Never store your most sensitive possessions in an attic or a basement because those areas will experience the most extreme RH changes! An interior closet is the safest bet.

Photos: This leather covered wood trunk has extensive RH damage. See (below) the peeling leather, lifting wood splinters and corroded nails.


Vulnerable Objects:

  • Metals
  • Musical instruments
  • Wooden Objects
  • Painted objects
  • Mixed media artwork
  • Hide or leather

For more information, email

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