CASE STUDY: BUILT TO LAST

Photo courtesy of SmithGroupJJR

The team constructed a large 20’x30’, full size, mock-up to model some of the most difficult conditions on this project, and to experiment with alternate cavity materials.

Photo courtesy of SmithGroupJJR

Rigorous mockups and testing can demonstrate problems such as the need for greater overlaps at curved seams as shown in this photo of a flashing prototype at the National Museum of the American Indian.

On September 21, 2004, 80,000 people gathered at the National Mall to honor the opening of the Smithsonian's National Museum of the American Indian. Marcus Wilkes, AIA, LEED AP, BD+C, Principal at SmithGroupJJR and one of the project architects for the museum, recounts with enthusiasm the complexity and collaboration that led to the completion of this important Capital Hill museum. After more than a decade, the Minnesota limestone facade maintains its integrity and testifies to the hours of careful detailing and collaboration by the owner, architect and contractor. There are high standards for the construction of a Smithsonian museum. From the beginning, the Design Team was challenged to produce a “one-hundred year” building requiring very low maintenance and a envelope capable of containing a high humidity environment.

The building evokes a natural formation “carved from wind and water” The resulting curved projections are a product of over five hundred circle centers with thousands of different radii. In section, this building is a traditional wall system built of 4” stone veneer, 2” ventilation cavity, 2” of insulation, an air/vapor barrier and a concrete masonry backup wall. However, in three dimensions, the wall system was a complex sequence of splitting/non-planar surfaces creating numerous opportunities for both moisture penetration and gaps in the air/vapor barrier.

Wilkes described the architects' initial approach to prevent water damage in the wall system involved the selection of stainless steel flashing. Miles of flashing were required and numerous, individually formed, welded, and crimped pieces of flashing would have been necessary to accommodate the curves. There were many cases where a ledge condition needed to transition to a soffit condition and back again. The wall cavity was compartmentalized, and vertical joints were strategically placed at the critical transitions and end dam locations. The collaborative design and construction team discussed various approaches to solving the basic problems of how to curve insulation at tight radii, curve shelf angles and how to provide an effective flashing system early in the construction phase. This resulted in small scale sample installations where flexible membrane flashing was explored along with curved plate and tab shelf angles, both of which were incorporated into the final design.

The team constructed a large 20'x30', full size, mock-up to model some of the most difficult conditions on the project, and to experiment with alternate cavity materials. They tested several solutions for all three problems. The insulation problem was partially solved by using spray-foam insulation. Even with fully adhered thin wall panels, curving layers of ridged insulation proved both difficult and time consuming to do well. Their original plan to use stainless steel flashing was abandoned once they understood the labor and time required when using this product on the project's curves. Their alternative was to investigate a flashing membrane. They required a flashing solution that would both match the mortar color and provide invisible assurance of moisture protection compatible with the insulation, sealants and vapor barrier. They were concerned that membranes can appear to “fish mouth” along a building facade and the number of laps required would be a challenge for quality control.

A museum expert, Wilkes explains that museums typically require strictly controlled envelope systems to protect from both moisture infiltration (exterior) as well as vapor migration (interior). Relative to other building types, they often maintain very high interior humidity conditions, upwards of 50% RH. High humidity creates a strong vapor drive from the interior during the winter, which demands a very airtight membrane. A fundamental principle for museum design is to detail a vapor impervious wall membrane on the warm side of the insulation. Special care must be given to the continuity of the vapor plane, with no breaks or gaps allowed in the entire envelope. The architect needs to select a product with a very low perm rating so that no moisture goes through the wall. For this museum, they investigated both a “peel-and-stick” self adhered membrane as well as a fluid adhered membrane. Both systems had advantages and disadvantages for this project, but the self-adhered membrane worked best with the schedule and was compatible with the chosen membrane flashing material.

The contractor, assisted by the manufacturer of the membrane flashing, constructed sample installations of typical curved conditions on the project demonstrating that the membrane flashing could be controlled to avoid fish mouthing, and that joint lapping could be properly inspected.. Other advantages of the membrane flashing, for this project included it's ability to be edge rimmed to match the rough edges of the split face stone and it could be color matched to the Kasota stone, allowing a better blend with the finish masonry than a stainless steel edge.

This project won over 20 awards for building design, construction and craftsmanship, including the Building Design and Construction Magazine's - Platinum Building Team Award, indicating a uniquely strong collaboration between Owner, Architect and Contractor.

According to Wilkes, “The team which included a consulting collaborative of Native American architects, the Smithsonian, Clark Construction and the architect SmithGroupJJR, shared a common and collaborative vision, leading to the success of this great, durable American museum.”