Set Relative Humidity (RH) Levels

Setting and maintaining appropriate relative humidity (RH) levels is key to promoting the long-term care of your collection.

RH measures how close the air is to being fully saturated with water vapor at a given temperature. When RH reaches 100%, the air cannot hold any more moisture, leading to condensation, fog, or frost.

Humidity levels can fluctuate based on the moisture introduced to or removed from your building. For instance, heavy rain outside or increased foot traffic can raise absolute humidity, the total moisture content in the air.

RH differs from absolute humidity. As temperature increases, air can absorb more moisture; hence, if the temperature rises while absolute humidity remains constant, RH decreases. This is similar to how more sugar can dissolve in hot coffee than in cold.

Current Collection Environment

Your current collection environment is likely acceptable. Focus on two main goals:

Assessing Current Conditions: Understand your existing RH levels.
Identifying Improvement Opportunities: Determine how to enhance the environment for your collection's stability.

Striving for perfection is not necessary; instead, aim for a balanced approach that ensures the well-being of your items.

Appropriate RH Levels for Your Collection

Determining the right RH for your collection may seem straightforward, but it depends on various factors, including the specific needs of your items and the capabilities of your facility.

Recommended Starting Range: Begin within a range of 50-70% RH, aiming for daily fluctuations of less than ±10%. This is a basic, general recommendation which does not necessarily take into account sustainability practices or local climate.
Adjustment Over Time: Over time, adjust the RH levels based on the unique requirements of your collection and local environmental conditions.
Key Consideration: While a specific RH range is important, minimizing fluctuations is also crucial. An ideal environment maintains consistent RH levels, avoiding extremes and sudden changes to protect your collection effectively.

Extreme RH

Extreme RH levels can be detrimental to your collection:

High RH (above 70%):
- Promotes biological growth (e.g., mold)
- Accelerates chemical reactions
- Causes materials to expand
- Signs: Mold, rust, warping.
Low RH (below 50%):
- Leads to contraction and stiffening of materials
- Signs: Flaking, cracking, shrinking.

Optimal Range:

Starting with a range of 50-70% RH helps mitigate these risks and maintain the integrity of your collection. If this is not a reasonable range for your particular building or location, adjustments should be made over time based on specific needs and conditions.
Consider having different ranges for different times of the year.

Extreme RH Fluctuations

Extreme fluctuations are any changes that are greater than 10% within a 24-hour period. High fluctuations lead to rapid expansion and contraction of materials, which accelerates damage to collection objects.

Optimal RH Fluctuation Range: Aim for a ±10% fluctuation to minimize risk.
Temperature Fluctuations: Larger temperature changes cause objects to crack and warp as they expand and contract.
Optimal Temperature Fluctuation Range: Aim for a ±5°F variation.

Set Your RH Levels

There is no universal standard for RH in collections, but a practical approach is to start with your current RH range and gradually tighten it as conditions improve. If your collections are used to certain fluctuations, you may find tightening is not necessary. Evaluate on a case-by-case basis.

Adjust RH levels for your collection

The “50/70” standard—which refers to 50% RH and 70°F—is widely cited as a guideline for optimal RH and temperature levels in collection care. The AIC’s environmental guidelines further discuss this rule and its history. In reality, many cultural materials can and do reside successfully somewhere within a 40-60% RH range. Read more on the history of environmental monitoring standards in this short blog post.

Collection Objects

When determining appropriate RH levels for your collection, consider the specific types of objects that you have.

High RH Levels: Elevated moisture can lead to salts rising to the surface in ceramics, which can damage glazes and affect the integrity of the piece.
Low RH Levels: Insufficient moisture can cause textiles to become brittle, making them more susceptible to tearing and deterioration over time.

Unique Spaces

Regularly monitoring your building's environment is essential for optimizing storage conditions for your collection.

Identify Favorable Areas: Different areas of your building may have varying temperature and humidity levels. By monitoring these conditions, you can pinpoint cooler and drier spaces that are more suitable for storing sensitive materials.
Match Collection Types to Conditions: After you identify the best storage areas, match your collection types to the conditions most favorable for them. This tailored approach helps preserve the integrity of your items and maximizes the efficiency of your storage solutions.
Efficient Resource Utilization: By strategically using available storage conditions, you can enhance the overall care and longevity of your collection while minimizing potential damage.
Leverage Microclimates: It is possible to keep sensitive objects in airtight enclosures with stable environments while you allow the general space a more flexible range of conditions.

Building Capability and Weather

RH also naturally varies depending on geographic location as well as season. RH tends to be lower in the winter and higher in the summer. Making gradual RH adjustments in response to these changes can help maintain a stable environment.