Temperature and Relative Humidity

Most organic materials absorb and release moisture as the humidity within its surrounding environment increases and decreases. This fluctuation of moisture can accelerate the degradation of objects sensitive to humidity. Potential problems from extreme variations in temperature and relative humidity include biological, chemical, and mechanical deterioration. For care of any collection, both elements should be monitored and controlled. Of the two, temperature is the easier element to understand, measure, and control. Understanding and controlling relative humidity is a bit more complex.

Relative humidity is determined by comparing the amount of moisture in the air at a given moment to the maximum amount of moisture the air can hold at that temperature. Relative humidity is expressed as a percentage of moisture content in the air at a given time and temperature. These elements are interdependent, as relative humidity varies with changes in temperature. When the temperature increases within a specific volume of air, the relative humidity decreases. When temperature cools, relative humidity decreases. Most museums, including the National Gallery of Art, expend extraordinary effort and expense to maintain a stable environment in relation to temperature and relative humidity. Measuring these fluctuations is often the first step in determining the degree of control needed to circumvent any damage resulting from sudden changes.

Here at the National Gallery, we work closely with our HVAC engineers and use a variety of measuring devices to record data throughout the year and in special environments. With accurate measurements, we can determine the degree and rapidity of change that occurs throughout the seasons and within the collection environment. Most severe or rapid fluctuations happen during sudden changes in weather patterns, as when a warm and humid day becomes cold and dry with a swiftly moving front. Armed with the data and details, a collection manager can implement preventive measures. The principal objective is to buffer or prevent any rapid changes that may occur and create stress within objects.

Museums work within narrow parameters in relation to temperature and relative humidity because of the broad range of materials often found in their collections. Sometimes a single object is created from multiple and diverse materials, all of which may react differently to an environmental shift. A sudden change can damage an artwork as that object, or any of its constituent materials, adjusts rapidly to the new relative humidity. These fluctuations add to or create new and unnecessary stress within hygroscopic (moisture sensitive) materials such as paper, wood, ivory, and textiles.

Fortunately, there are simple procedures that anyone can follow to diminish the potential damage caused by rapid fluctuations in temperature and relative humidity. Surrounding an object with buffering materials, for example, may slow, if not completely prevent, sudden changes in relative humidity. Tools and materials that assist us with this task include silica gel, microclimate packages, and even the simple addition of acceptable framing materials with graphics, such as additional mat board.

For objects coming to the Gallery on loan from a much different environment, we are often required to create and maintain a unique environment outside of our normal collection set points. This typically occurs with objects borrowed for special exhibitions. Working with our design department, we construct custom exhibition display cases, in which we can contain, measure, monitor, and adjust an atypical environment appropriate for the object. This usually entails conditioning silica gel to the required relative humidity, then using the gel to maintain the exhibition case within a specific relative humidity range.


American Institute for Conservation of Historic and Artistic Works

Smithsonian Museum Conservation Institute

Canadian Conservation Institute

Published Resources

Ashok, Roy, and Perry Smith, eds. Preventive Conservation Practice, Theory, and Research. London: International Institute for Conservation of Historic and Artistic Works, 1994.

McGriffin Jr., Robert F. Furniture Care and Conservation. Nashville: American Association for State and Local History, 1983.