Museum Space Humidity Control

By Joey Balducci, PE, CxA | Associate, Discipline Manager, Mechanical Engineering, Orange County

Special considerations in mechanical systems are necessary for museums to protect valuable art, artifacts, and other media types in storage or on display from damage caused by changes in temperature and humidity. Every space is different, but the goal is always the same: maintain stable indoor conditions that help preserve collections over time. In this article, we dive into understanding the risks and considerations when selecting an approach for humidity control. Involving the mechanical engineer early in the design process allows these risks to be evaluated before floor plans, equipment selection, or control strategies unintentionally limit the ability to maintain a controlled environment.

Economizer Risks and Alternatives

Many buildings use air economizers, which bring in cold outdoor air to provide “free cooling” to a facility in the morning or during cool weather times. This provides energy savings since the compressors and chillers are not running during those times. It is common and often required by code to add economizers to air handling equipment. While this approach can be effective for other buildings, it can create challenges in museum spaces.

Outdoor air conditions are subject to high fluctuations in temperature and humidity. The introduction of large quantities of outdoor air into the building poses a threat to maintaining the tight temperature and humidity tolerances required in exhibit and archival spaces. As a result, mechanical equipment often needs to be increased in size and capacity to control the large humidification and dehumidification loads. Any energy savings gained from using outdoor air may be offset by the increased demands placed on the humidity control equipment.

For museums without economizers, energy savings can be recuperated by running smaller humidifiers during dry days, as well as reducing cooling coil size during humid days. Allowing the units to recirculate most of the air and limiting outdoor air to only the amount required for occupant safety, achieves tighter control of the indoor air conditions. Some facilities can even utilize dedicated outdoor air handling units that pretreat incoming outdoor air before distribution to the air handling equipment serving the museum spaces. The mechanical engineer understands that discussing these options early with the owner and design team helps identify a solution that balances preservation needs, energy use, and code requirements.

Local Climate Considerations

Depending on where the museum is located, the winter conditions (cold/dry) or the summer conditions (hot/dry) may govern when selecting the humidification equipment. In Southern California, for example, system size is based on the Santa Ana wind conditions which usually take place in late summer. During these conditions, the local weather can reach 105°F with a relative humidity of 5% (10 gr/lb) so cooling and humidification would be required. In winter conditions, the opposite may be true. When the most demanding seasonal conditions are identified early, mechanical engineers can guide equipment selection and control design strategies based on the most critical preservation conditions rather than average weather data.

Advanced Dehumidification Solutions

The ASHRAE handbook provides guidelines for safe operating conditions in museums depending on the building age and space type. An acceptable climate standard is typically 70°F and 50% relative humidity, with minimal fluctuation. Some collections, however, require more specialized conditions. Identifying these requirements early allows the mechanical engineer to discuss humidity control options with the design team that consider both costs and long-term operational impacts.

When a space requires a very dry condition, standard cooling equipment may not be sufficient. The mechanical systems can be enhanced with specialized dehumidification equipment designed to remove moisture more effectively. These systems can be larger and more costly, so they are often just used to treat outdoor air before being fed to a standard air handler unit.

For optimal performance of the mechanical systems, humidity sensors should be placed at the system return where they best reflect actual room conditions. Locating sensors near return air paths or within the spaces themselves helps provide the best sampling for mechanical systems to respond accurately to changes in humidity.

Architectural Design Considerations

Mechanical systems alone cannot maintain stable conditions if the building itself allows infiltration of uncontrolled air into the exhibit, gallery, and collection spaces. Early coordination between the architectural and mechanical team is critical, as building layout, circulation patterns, and design details can directly influence humidity control strategies.

When designing museums, layouts should minimize exposure to outdoor air by locating exhibit spaces at interior zones away from exterior doors. If exterior access is required, vestibules can create a buffer between the outdoor air and sensitive gallery spaces. Crowd control protocols, such as limiting interior and exterior vestibule doors from opening at the same time, can further reduce the impact of outdoor air on the performance of the mechanical systems. Additionally, full height walls and door sweepers are two architectural details that can help reduce unwanted air transfer from adjacent interior spaces.

Working Together for Long-Term Preservation

Maintaining the right humidity in museum spaces is important for protecting art and artifacts. By choosing the right equipment, considering the local climate, and designing spaces to limit outside air, museums can create safe environments for their collections. Bringing mechanical engineers into the conversation early, encouraging evaluation of multiple system approaches, and working together with experts helps museums keep their treasures in good condition for years to come.

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