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Controlling Relative Humidity

pc-erin downloading data
Historic New England staff member downloading environmental conditions data while on site.

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; and
  4. Install mechanical systems using simple controls and equipment in a phased manner and evaluate the environmental data after each phase.

This is an active project and our experiments continue through funding from the Institute of Museum and Library Services (IMLS). Four of our historic properties are undergoing modifications to simplify the 1990s systems. The IMLS project will culminate in a roundtable discussion with other stewards of historic house museums in the spring of 2011. 

Read an article on the analysis of the 1990s mechanical systems.

Learn about our 2011 roundtable discussion on simplified HVAC system for historic house museums.

Data Collection and Monitoring

Monitoring the environments in our historic house museum properties (PDF) 

Collecting environmental data (PDF) 

Heating, Ventilation, and Other Means of Environmental Control

Coming soon: a white paper on developing an HVAC project

Controlling Relative Humidity