Setting relative humidity (RH) levels for your collection

Setting and maintaining appropriate RH levels is key to promoting the long-term care of your collection. These are our recommendations if you're setting RH levels for the first time.

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Regulating temperature and relative humidity levels really go hand in hand with keeping collections stable. We've previously discussed setting temperature levels, but what about RH? Why does it matter and how should we approach setting a level?

What is relative humidity?

RH is a measure of how closely the air is to being fully saturated with water vapor at that current temperature. Beyond 100% RH, the air cannot “hold” any more water, resulting in condensation, fog, frost, etc.

Humidity will vary depending on moisture being introduced into or taken out of your building. If it's pouring rain outside and lots of people are coming and going through the door to your collection, you can expect the absolute level of moisture to rise. This is the concept of absolute humidity - more moisture in the air.

Relative humidity is different from absolute humidity. RH levels will vary depending on the absolute humidity and what the temperature is. As temperature rises air can absorb more moisture - a hot room can absorb a larger amount of moisture. So if the temperature rises in your collection and the absolute amount of moisture stays the same, then the relative humidity will go down. This is the concept of relative humidity - it's a bit like how you can dissolve more sugar in hot coffee than in cold.

So that's a mildly technical explanation. Let's dive into the practical question - how should you set an RH level for your collection.

This isn't the first time you've heard this from us. Whatever your collection environment is right now is OK. Our goal isn't perfect. Our goals are 1) to get a sense of what's going on now and 2) to figure out how we can make it just a bit better. Perfect is the enemy.

Setting your initial RH levels

The goal of environmental monitoring is to quickly identify environmental conditions that can damage collection objects. To this end Conserv allows you to set ideal RH levels for your collection.

So what's an appropriate RH for your collection? This seems like a simple question and yet… it depends. It depends on the needs of your collection as well as the capabilities of your buildings.

tl:dr When setting you first RH level we recommend a range of 50-70% with daily fluctuations less than +/-10%. These levels will be adjusted over time based on your unique collection. For RH low fluctuation is more important than a tight range.

An ideal environment for a collection avoids RH extremes and fluctuations.

Extreme RH is anything below 50% and above 70%. Higher RH drives biological growth, speeds up chemical reactions, and causes materials to expand in size. Signs of this include mold, rust, and warping.  Low RH causes materials to contract, stiffen, flake, and crack. A range of 50-70% is a good place to start.

Extreme RH fluctuations are any movements greater than 10% over a 24 hour period. High RH fluctuations are one of the most damaging environmental conditions for your collection. As RH rises your collection objects will take on moisture and expand. As RH falls objects give up moisture and contract. Fluctuating RH causes a rapid expansion and contraction of materials which accelerates damage. A range of +/-10% is a good place to start.

Larger fluctuations in temperature generally cause objects to crack and warp, as they expand and contract. A range of +/-5°F is a good place to start.

Despite what you might hear, there's really not a standard RH for collections. Collections are the same but also unique. One smart approach is to use your current RH range as a level and then tighten it over time as you improve.

Adjusting RH levels for your collection

A “50/70” standard, with 50% RH and 70°F, has long been stated as a guide for RH/temperature levels for collection care. The AIC’s environmental guidelines further discusses this rule and its history. In reality, many cultural materials can and do reside successfully somewhere within a 40-60% RH range.

Collection Objects

When setting RH levels, consider the types of objects in your collection. High RH levels can alter the mineral content of terra cotta, causing salts to rise to the surface, removing glazes. Low RH levels, on the other hand, cause textiles to become brittle. 

Unique Spaces

Regularly monitoring your building environment can help with identifying areas best suited for storing materials. Certain areas of your building might be cooler and drier than others. Matching collection types to conditions most favorable for them would be helpful in efficiently utilizing available storage conditions. 

Building Capability & Weather

Relative humidity will also vary naturally 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.

Next step: read our article on "Setting temperature levels for your collection"

Additional Resources