Book Storage And Humidity – A Brief Q&A

by | Jul 25, 2023 | Blog, Library Collection

Mold, red rot, insect damage, and corrosion of metal elements can be some of the most concerning forms of deterioration for book collections – and they are all directly or indirectly related to relative humidity levels. As such, relative humidity levels in storage areas are a crucial aspect of maintaining the longevity and integrity of book collections.

As a collections care professional, understanding the relationship between relative humidity and book preservation will be essential for safeguarding your valuable materials. Here are some common questions related to humidity and book storage, so you can make informed decisions for the care of your collections.

Do all books require the same humidity levels?

There is no specific number for the best relative humidity levels, and most institutions do not have the funds to have separate areas for books with different component materials, so ranges can be very useful. Different materials and bindings call for specific humidity ranges to ensure their preservation.

According to the Northeast Document Conservation Center (NEDCC), optimal relative humidity (RH) levels for paper materials lie somewhere between 30% and 50% with a certain preference for the lower end to slow down deterioration reactions. (NEDCC, 2018)

The Library of Congress, for example, suggests about 35% RH for works on paper. However, this low RH may not be suitable for all books in your collection. For example, leather-bound books may require higher humidity levels to maintain their flexibility and avoid cracking or tearing (CCI, 2018).

To determine the ideal relative humidity ranges for your collection, consult guidelines from reputable organizations such as the NEDCC, the Library of Congress, or the American Institute for Conservation (AIC). Remember that books are composite objects made of various materials which must be kept in mind when deciding for environmental parameters.

Also consider that conservators have traditionally adhered to a narrow set of ideal parameters or a golden rule to preserve collections. But recent research has revealed that materials can actually be preserved under a much wider range of parameters. To discover more about the evolution of preservation standards, click here.

Do fluctuating humidity levels affect books significantly?

This depends on how fast the fluctuations happen. Rapid changes in humidity (for example within a single day or a week) can cause significant damage to your collection. Books are hygroscopic, meaning they easily absorb and release moisture from the surrounding environment. When humidity levels fluctuate rapidly, this can lead to physical changes in the materials, such as swelling, shrinkage, and warping (CCI, 2018).

To reduce the impact of fluctuating humidity levels, it is essential to maintain a stable environment. Monitoring and controlling the RH within your storage area is critical. Invest in reliable humidity monitoring equipment, such as a high-quality hygrometer or data loggers, and develop a plan for addressing fluctuations when they occur. Remember to build a good relationship with your facilities staff, so you may collaborate with them and minimize the risk of unexpected fluctuations caused by your mechanical systems or infrastructure.

How are mold and pest problems related to book storage humidity?

High humidity levels can create an ideal environment for mold growth and pest infestations. Mold thrives in humid conditions (65% and above), and if left unchecked, can cause permanent damage to your collection, including staining, weakening, and even disintegration of materials (Conserv o Gram, National Park Service, 2007). Additionally, many pests, such as silverfish and booklice, are attracted to warm, damp environments and may cause further mechanical harm to your books as they graze over the surfaces (AIC Wiki).

Maintaining proper humidity levels is one of the most effective ways to prevent these issues. If your storage spaces are in a high-humidity area, and you have little mechanical control over your buildings, try to ensure that your storage area is well-ventilated and monitor humidity levels regularly. Finding pockets of humidity in time or sudden spikes caused by external factors can be essential to preventing a mold problem or pest infestation from getting out of hand. If you suspect either, consult a conservator or pest management professional for guidance on treatment and prevention.

Are dehumidifiers and humidifiers always the solution?

While dehumidifiers and humidifiers can help you control the humidity in your storage environment, they may not always be the best or even the only solution. Depending on the size and layout of your storage area, these appliances may not provide even humidity distribution, leading to pockets of higher or lower RH levels.

Before investing in dehumidifiers or humidifiers, check with colleagues and nearby institutions to see if you can borrow some to test out.

You can also explore other cheaper and more sustainable options for maintaining a stable environment. If you have access to building engineers and facilities staff, ask them about passive methods, such as properly sealing windows and doors, insulating walls and ceilings, and using vapor barriers (Check out some examples measures taken at the Whitworth, in Manchester, UK). There are also some Open Access articles available online such as this one with useful guidelines to help you achieve passive environmental controls.

Don’t forget that you can always play with the temperature in your rooms in order to affect relative humidity levels. This is called conservation heating and you can read a bit more about it in this article by Sarah Staniforth.

If all fails and active controls are necessary, it is advisable to consult with a specialist to determine the best solution for your specific needs. If you don’t have access to one, don’t forget that the Conserv Community has several members with decades of experience in HVAC engineering and facilities management for museums!

How do I know what’s going on in my storage areas?

If humidity levels are a serious concern for your book storage areas, it is essential that you begin with a monitoring program to find out just how much you need to be concerned. At the simplest level, you may start with a handheld hygrometer and walk around your spaces. These will give you good spot readings, but will not be able to provide a complete picture of what’s going on in your spaces over time due to daily and seasonal fluctuations. To get a fuller picture of the relative humidity levels throughout the days, weeks, or months, it will be better to get automatic data loggers. If this is the first time you will be beginning an environmental monitoring program in your spaces, we recommend first making sure you start by defining your environmental monitoring goals and deciding on how you will collect your data.


For collections care professionals, there are few things more stressful than widespread mold or pest infestations that have been caused by uncontrolled environmental factors such as relative humidity.

We hope this short article on some common questions about relative humidity and book storage help you make informed decisions to improve the longevity and integrity of your collections.

If you have any questions about environmental monitoring, integrated pest management, or just want to talk about preventative conservation, please reach out to us! Don’t forget to check out our blog or join our community of collections care professionals where you can discuss hot topics, connect with other conservators or even take a course to get familiar with the Conserv platform.

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