Modular Structure for Creating an Environmentally Controlled Space
A prefabricated, modular structure similar to a walk-in refrigerator can be used to create special high-quality museum collection storage space. These structures are becoming popular for storage of museum collections, either within an existing building or as a self-contained, stand-alone exterior building. Super-insulated structures can contribute to the preservation of museum collections by enclosing objects in a beneficial microenvironment. This environment is free of ambient light, dust, insects, and rodents and, most importantly, it provides stable relative humidity (RH) and temperature conditions. Stabilized, high-quality storage conditions reduce physical and chemical stress on objects and thereby slow down the rate of deterioration.
Donald R. Cumberland, Jr.
Curatorial Services Division
National Park Service
Harpers Ferry, WV 25425 USA
Tel (304) 535-6072
Preservation Assistance Div.
National Park Service
These modular structures are highly effective in maintaining an appropriate museum environment because of superior insulation and virtually airtight construction.
The storage space is formed by assembling foam-core (generally high-density polyurethane), metal clad (aluminum or galvanized steel) panels (Fig. 1). The panels from most manufacturers have an insulation “R” value above 30. The panels are gasketed along the edges and, when fastened together, form virtually air-tight joints. The panels are lightweight and are quickly and easily assembled with ordinary hand tools. Panels can be carried through the door of a room and assembled inside. Weatherproof roof panels are available to allow the building to be placed outside on a concrete slab.
Figure 1. Exploded view of prefabricated structure.
The cost of these structures is economical when compared to a conventionally constructed building having the same insulating and sealing characteristics. An added benefit is the ease of expansion or design change. It can be expanded or decreased in size or even moved and reassembled on another site with equal ease. Room sizes are limited only by the length of ceiling panels that span the space; currently, the maximum width achievable without center columns is 17 feet.
Recent in-house tests conducted by the National Park Service (NPS), USA, demonstrated the ability of super-insulated structures to buffer and stabilize RH and temperature conditions. In one situation, a calibrated hygrothermograph was placed in an unconditioned super insulated structure and an identical hygrothermograph was placed in the room housing the structure. The room was on an upper floor of an historic masonry building in which there were no mechanical climate control systems.
Records from the two hygrothermographs revealed that over the course of the two-year testing period and without any energy input for heating, cooling or humidity control, the greatest diurnal (day-night, 24-hour) RH and temperature changes recorded on the interior of the insulated structure were 6% and 6oF respectively. In comparison, maximum diurnal fluctuations of 25% RH and 47oF temperatures were recorded in the historic room itself.
Figure 2. Example of environmentally controlled structure in an historic fort.
Another benefit is derived when these structures are used within an historic building (Fig. 2). There has been concern in the museum and historic preservation fields that maintaining the “ideal” RH and temperature conditions recommended for museum collections may be harmful to the fabric of historic buildings housing those collections. For instance, during the winter season there is a point at which warmer, more humid air, being maintained to preserve museum objects inside the structure, contacts building fabric cooled by cold air outside the structure. When the dewpoint of the humid interior air is reached, condensation can occur on interior surfaces of windows, walls, ceilings, and floors. Depending on the humidity/temperature relationship, water also can condense inside the historic wall. If condensed moisture soaks into the fabric of the structure, decay can threaten structural features.
Additional problems occur if moisture is involved in a freeze/thaw cycle. The cycle of ice-crystal formation and thaw will weaken the fabric of masonry or stone structural features and cause surfaces to spall or flake. The insulating and vapor barrier characteristics of the super-insulated structure, on the other hand, prevent moisture from condensing and damaging the historic structure surrounding it.
It should be noted that the system is no more fireproof than a framed structure and would require similar fire protection systems to safeguard the collection.
Cumberland, D. R. Jr. 1985. Museum collection storage in an historic building using a prefabricated structure. Museum Collection Storage Number 1, Preservation Technical Notes. National Park Service, Harpers Ferry, WV.