One week ago, on Sunday night around 8:00pm, I received an emergency call that made my heart stop — an ADT rep told me that the smoke detectors in the Arlington Historical Museum had just been set off, and they had notified the fire department. It was the type of call I’d feared most as Chair of the Museum Building Committee. Our museum and extensive local artifact collection is housed in the historic 1891 Hume School building, by noted Washington architect B. Stanley Simmons.
When I arrived at the museum about 15 minutes later, this was the horrifying scene I encountered, sure to make any museum director’s or board of directors’ blood run cold.
Fortunately, the ladders and hoses were just fire department procedure. There was no actual fire, just smoke pumped out by a malfunctioning AC/furnace unit. After the worst of the haze and odor cleared out, we faced an urgent decision about the 17-year-old furnace. Once we ruled out the repair option and decided to replace it, the question was still: replace with what?
HVAC systems and environmental controls, particularly for relative humidity (RH), represent one of the biggest (and most expensive) conundrums for museum and historic site directors. This November 2008 post from the National Trust for Historic Preservation highlights the key challenges. Relative humidity, defined as the amount of moisture in the air expressed in percent of how much moisture the air can hold, is one of the most important factors affecting historic artifacts. Continuous subjection to high or low extremes, or fluctuation in humidity can cause certain artifacts to undergo dimensional changes, potentially cracking, warping, or deteriorating in other ways.
Museum standards for RH and temperature have evolved as more research has been conducted. Some museums used to aim for a year-round constant RH of 50%, within ±3 — almost impossible to achieve, particularly in an old, leaky building. Now the Smithsonian Institute recommends a 45% RH, ±8 at 70°F for a mixed collection, which translates to a low of 37% and a high of 53%. Others suggest 40% to 60% is just fine for some museums.
To put this into perspective, outside RH often reaches highs of 100% during humid summers in the Washington, D.C. area, and indoor air conditioning can’t sufficiently de-humidify the air. In the winter, indoor heating can send your home’s RH plummeting to 25% or lower, worsening static electricity and dry skin.
Everyone seems to agree there’s an ideal range, but the disagreement runs over how wide that range should be, and how to control it. The issue is complicated by the difficulty of detecting and measuring damage to artifacts, and isolating the cause to RH levels.
Overzealous control of RH can have a negative impact on buildings. When the dewpoint between outside and inside is reached somewhere within a building’s exterior walls, condensation and mold can develop, creating another nightmare by potentially damaging historic surfaces and rotting the structural framing.
In her master’s thesis, Becky Wood, Director of Architectural Restoration at Historic Kenmore in Fredericksburg, Virginia, examined RH standards for museums in historic buildings. She found that most types of artifacts can probably withstand a wider range of RH levels than previously thought, when subject to gradual changes in seasonal humidity levels. After all, the furniture and contents of historic homes like Thomas Jefferson’s Monticello survived for almost 200 years without the benefit of sophisticated HVAC systems.
For the Arlington Historical Museum’s replacement furnace, which handles the second floor collections storage area, we’re opting for a furnace that’s able to interface with specialized humidifier and dehumidifier add-ons, which we can purchase in the future, via fundraising or grants. And we’re looking at other ways to control RH besides just the HVAC system.
But with our artifacts in surprisingly good shape, it’s good to know a comfortable range of relative humidity may be wider than once thought. And I’ll no longer think of humidity just in terms of the havoc it wreaks on my hair.