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Case Study:
Central Library

Des Moines, Iowa

Ecological Literacy: A copper-infused glass skin blurs this library’s boundaries.

By Nadav Malin

When you’re inside the building, it feels like you’re sitting in the park,” says Paul Mankins, who was the Partner in Charge for Herbert Lewis Kruse Blunck Architecture (HLKB) on the new Des Moines Public Library. Mankins, who is now a partner at Substance Architecture, was referring to the building’s glass skin, which he considers the most remarkable element of the deceptively simple-looking library. It’s an ethereal skin designed to blur the boundary between indoors and out. Achieving that effect while providing a respectable thermal envelope forced the designers to collaborate with the glazing manufacturer to incorporate shading—in the form of a fine expanded copper mesh—into the triple-glazed 4-by-14-foot panels, and then to install those panels without exterior mullions. Mankins admits that his team hadn’t fully anticipated the visual effect of the glazing: “At sunset you see the building go from opaque copper to transparency. It dematerializes.”

Central Library
Photo courtesy David Chipperfield Architects and Des Moines Public Library/Farshid Assassi
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Des Moines, Iowa (confluence of Raccoon and Des Moines rivers)
GROSS SQUARE FOOTAGE: 145,500 ft2 (13,520 m2)
COST: $30 million
COMPLETED: December 2005
ANNUAL CARBON FOOTPRINT: (predicted): 40 lbs. CO2/ft2 (197 kg CO2/m2)
PROGRAM: Reading areas, stacks, offices

Sky Conditions   Temp./Dew Points   Heating/Cooling

OWNER: Des Moines Public Library
ARCHITECT: David Chipperfield Architects
ASSOCIATE ARCHITECT: Herbert Lewis Kruse Blunck Architecture FAÇADE
CONSULTANT: W.J. Higgins & Associates
ENGINEERS: Arup (mechanical, electrical, and plumbing), KJWW Engineering Consultants (mechanical, electrical, and plumbing), Jane Wernick Associates and Shuck-Britson (structural)

METAL/GLASS CURTAINWALL: Custom made by Architectural Wall Systems
GLASS: Facade – Okatech by Okalux Glazing (insulating glass panel with triple-glazed copper mesh)
ROOFING: Carlisle Waterproofing Systems
GREEN ROOF: Roofscapes
FLOORING: Tate Access Floors
CARPET: Lees Commercial Carpet

Having prevailed in a competitive selection process, London-based David Chipperfield Architects offered four distinct concepts from which the city could choose. Chipperfield’s presentation to the selection committee was carried on local public access television (and repeated several times on the content-hungry station), and the public weighed in. The library selected the scheme that had also proven most popular with the public—a curvilinear two-story form with tentacles stretching into a surrounding park. The slender fingers of that design “had to be bulked up to contain the collection,” says Mankins, but the general layout remained intact.

“The unusual shape allows for a lot of privacy for our customers,” says acting director Dorothy Kelly. Compared with the one large reading room of the previous library, the new space offers visitors a much better chance of finding a quiet corner, even though visitation has increased threefold since the building opened in April of 2006. “One of the things that our customers, as well as our staff, particularly like is that there is so much light coming in,” says Kelly.

After Chipperfield and its consultant, Arup, created the concepts, they collaborated with associate architect HLKB, engineer of record KJWW and the other consultants, to develop and document the design. Realizing Chipperfield’s minimalist aesthetic required meticulous attention to details. “A lot of time was spent on alignments [of planes and surfaces], to make sure that those made sense,” says Jeff Wagner of HLKB.

The green roof was not in the original design, but was added at the request of a neighboring office overlooking the building. That amenity will lend aesthetic value to others over time, according to Mankins, who notes: “The library is in the middle of what is going to be a densely developed part of Des Moines.” Beyond the aesthetics, however, the green roof provides a valuable stormwater management function by modulating runoff. “This part of Des Moines has combined sewer and stormwater overflow,” notes Mankins—when the stormwater system’s capacity is exceeded, raw sewage is released into the river.

The choice to include a green roof affected the facades as well, according to Brett Mendenhall, who was a project architect for HLKB, and is now with OPN Architects. Because of the open interior plan with few full-height walls, there were limited locations for roof drains. Fewer drains would require longer and higher slopes on the roof. This meant the parapets would have needed to be raised significantly to accommodate a conventional sloped roof drainage system. “We knew that there were manufacturers who would guarantee the waterproofing on a dead flat roof,” says Mendenhall, so the designers chose to eliminate the pitch entirely. The green roof was bid as an alternate, however, so when they made the decision to use a flat roof they were taking a risk. If the green roof were not implemented, “we would have had a big flat bathtub,” notes Mendenhall. Fortunately, the green roof survived, thanks to a favorable bidding environment, and to the fact that lowering the parapet height reduced the size of the facades, saving enough money on the glazing panels to offset half the cost of the green roof.

Extending the minimalist aesthetic to the interior meant using a raised floor system with underfloor air distribution to limit clutter at the ceiling level, or what Scott Bowman of KJWW calls “shifting money from ceiling to floor.” The system delivers air at constant volume in the central core, while over 200 fan-coil units at the perimeter provide heating or cooling as needed, an approach that was suggested by Arup’s U.K. team. Demand-based controls for the ventilation and an enthalpy wheel, which exchanges heat and moisture between incoming and outgoing air streams, were instrumental in reining in energy costs. Together with other measures, they helped win the project a $42,650 efficiency incentive payment from the local utility, MidAmerican Energy.

The Weitz Company was on board early on as construction manager, providing cost estimates during design, and helping to manage a complex process involving 22 separate bid packages. “Had they bid it as a single $25 million project, they only would have had a couple of bidders,” says Mankins. Instead, many smaller contractors competed for the work, which resulted in a $1 milllion cost savings and allowed the library to maintain more control.

The exposed concrete ceilings and columns contribute significantly to the building’s thermal mass, which helps to stabilize indoor temperatures. “It’s hard to move [temperatures in] that building,” notes Bowman. Having learned how the building responds thermally, the facility managers now take advantage of its benefits: “No energy is put in when it is unoccupied. The thermal mass really carries it through,” says Bowman. The exposed concrete also generated a lot of extra moisture as it cured. “When we first turned the system on, we were dehumidifying for over a year—that has now settled down a bit,” Bowman reports.
Bowman’s firm was also responsible for commissioning, which, given the project’s size and complexity, went smoothly. “We found only 50 items that required correction,” he said. Most of them were minor and Bowman felt that planning to commission improved the outcomes.

Based on nine months of utility bills, it appears that the library’s electricity and gas usage savings for its first year will exceed predictions by about 23 percent. Given the library’s popularity and the extended hours that it has been used, this rate suggests that the energy simulation was reasonably accurate. With further attention to managing energy use, the building may well improve in efficiency over time, taking it from a reasonably efficient library to one that is exemplary.

While the community continues to actively debate the project’s merits as a city landmark, the library staff has no reservations. “We love it, and the public loves it,” says Kelly. “We’ve got tremendous circulation—it’s being used beyond our wildest dreams.”

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This article appeared in the April 2007 print issue of GreenSource Magazine

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