Best practices from Historic New England experts

There are a multitude of environmental factors that might affect the collections within a historic house museum. The most prevalent at Historic New England include light and moisture. Mold can germinate when exposed to high temperatures and high relative humidity (RH) causing damage both to the building and to the objects.

Low RH, indicating a dry environment, can cause damage through material contraction causing wood joints to separate, paint to crack, and veneers to release. Ultraviolet light (UV) causes damage to collections. It is important to note that all light is, in fact, a source of degradation. The essence of a historic house museum and a goal of our historic property interpretation is for the public to experience, in a real and personal way, the way the lives and stories of the individuals and families who made New England what it is today. Despite the fears of a potentially unstable environment for the objects, it is important to think about the objects in the following terms:

• It is important for us to leave the objects in situ as the family owned and used them. In fact removing the objects to a “museum environment” would be pointless for a large number of the objects because their real value is in context next to the other items used by the family.
• These objects have been exposed over their lifetime in the structure to conditions that have always been unstable when compared to a traditional museum environment.
• The majority of objects have a low risk of damage due to the conditions at the houses.

For these reasons Historic New England has been experimenting with different methods to mitigate the risk of damage and long-term deterioration of its buildings and objects from environmental issues.

Controlling Relative Humidity

Controlling the relative humidity (RH) in a historic house museum is very difficult yet important for the long-range health of the structure and the objects within. The most common approach is to install mechanical systems designed to regulate and modify the overall RH in the museum. Many experts today, however, are advocating for the use of traditional and more passive methods. In the 1990s we experimented with eight different mechanical systems in eight different properties to determine our ability to regulate the RH in the properties to 45% RH with a cushion of +/- 5%. In 2008 we began a systematic analysis of these systems to determine the effectiveness of the 1990s experiment. As a result of this analysis we have refined our approach to the control of RH in historic house museum properties. Historic New England’s current four-step approach to environmental systems:

1. Collect data about the temperature and RH in the museums before, during, and after any work is implemented
2. Understand that the majority of the objects within the structure can survive within parameters of 30-60% RH
3. Perform corrective repairs to elements that are allowing water to infiltrate the building and use the environmental data to measure the results
4. Install mechanical systems using simple controls and equipment in a phased manner and evaluate the environmental data after each phase

These experiments were funded in part by a grant from the Institute of Museum and Library Services (IMLS). Four of our historic properties underwent modifications to simplify the 1990s systems. Read a white paper on monitoring the environments in Historic New England museum properties. Read a white paper on collecting environmental data. Read our white papers on ultraviolet protection.