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THE TRANSPARENCY OF ITS SHIMMERING FACADE CHANGES WITH THE LIGHT

TECHNICAL & PRACTICE

The new Public Library in Des Moines, Iowa, was designed by David Chipperfield Architects and opened in April this year. Part of a long-term regeneration plan, it includes, in addition to the large public library, community and commercial spaces, all set within parkland.

'It's awesome. The public loves it, ' says Kay Runge, director of the Public Library in Iowa's state capital Des Moines. 'Prior to the opening, people kept asking, fiwhere are the windows? fl' When people went inside and understood the transparency of the building, they wanted to know how it worked. Runge calls it 'David's magic'.

It all started as a park - a new, six-block-long park to regenerate the western edge of downtown Des Moines and link it to the leafy suburbs further west. But the city soon realised that a park needs activities to bring it to life, and commissioned Chipperfield to design a new library-cum-community centre for the site, following on from his museum in nearby Davenport (where Runge was previously based). Asked to engage the community by presenting several designs which treated the park in different ways, Chipperfield proposed four schemes: a compact four-storey block hugging the eastern edge of the park near downtown; a linear, low building stretching along the park's northern frontage; a rambling courtyard scheme; and an organic 'fingers in the park' approach. The library board and the public converged on the fourth 'building in the park' option, seduced by the vision of reading areas overlooking the new green spaces.

The architect took panoramic views to their literal extreme, opting for a fully glazed facade rather than windows punched in walls or conventional cladding with beams and spandrels. Not quite Philip Johnson's Glass House, but close. Stacks for books were buried in the centre of the plan and the curving perimeter of the building was maximised to create informal reading areas. But how to keep out the hot summer sun, control glare and provide a bit of privacy so that library visitors could relax and read next to a big glass wall? Mirror glass was ruled out as aesthetically inappropriate and because of the dulling effect of its tint on exterior views. Sophisticated exterior shading devices were rejected because they are not widely used in America.

The architect did not, however, go with a home-grown solution. The library is clad in Okatech, a German insulating glass panel made by Okalux, with an expanded copper-mesh interleaf, chosen for its insulating capacity and its shimmering visual effect that changes with the light. In daylight occupants can see out but passers-by cannot see in. At night the illuminated library reads as a glazed box, revealing the activities within. Chipperfield had considered the product for a Japanese project, but never used it. In Des Moines, Okatech met all the design parameters: exterior visual appeal; maximising park views; and control of solar gain and glare.

Project architect Martin Ebert explains that 'the beauty of the product is that it is just a glazing unit which can be applied to a bog-standard glazing system'. Using Okalux meant, however, no more curves, so Chipperfield's organic forms became rectilinear.

Okalux developed bespoke panels for Chipperfield, as it had for Rem Koolhaas' Seattle Library, which was on site while the Des Moines library was in design. In Seattle the metal is untreated aluminium, a uniform material, different from copper with its natural variations. Okalux had used copper mesh on small projects in Germany, but never on the scale required by the Des Moines library - 3,500m 2. What's unusual about Okalux metal meshes is that they are both directional and three-dimensional, so they perform like a fixed screen of micro-louvres and can block out up to 84 per cent of solar transmission, depending on the angle of the sun. The more vertical the angle, the greater the screening, in keeping with the need to block the sun's glare. Furthermore, the copper mesh is see-through and visually attractive at close range and from afar. From inside, the transparency varies according to the weather. Sun on the facade creates a copper sheen which reduces visibility. On a cloudy day, the mesh is more transparent.

Each 4 x 14 foot panel (the building was designed in imperial dimensions) is triple-glazed, with an exterior leaf of 6mm low-iron glass; a 2mm cavity with the expanded copper mesh; a middle 6mm leaf of toughened glass with an inward-facing low E coating (N32) which reduces heat loss; a 14mm air cavity; and another 6mm leaf of toughened glass. The panels are hermetically sealed to prevent any particles or moisture damaging the copper, and each panel weighs almost 250kg.

The panels were fabricated in Germany and shipped to Iowa, where they were fixed to aluminium frames by Architectural Wall Systems using structural silicone glue. The glazing panels have a 27mm border of copper-coloured fritting, so that the silicone and spacers behind are not seen through the glazing. Pairs of panels were mounted one above the other in a subframe, then delivered to site and hung on steel brackets in the first-floor slab.

The building has an elegant base detail, which makes it appear to float about nine inches above the ground. A precast fascia panel below the glazing reads as the floating base of the building. Another visual illusion is the fact that only about 60 per cent of the cladding panels are transparent glazed screens.

About 30 per cent of the panels are backed with insulation and aluminium panels on the interior, where back-of-house uses do not require panoramic views. The remaining 10 per cent of the panels are doors. From the exterior, the panels are visually indistinguishable (except at night).

The library has a sedum green roof, which Ebert (who is German) sees as 'normal', though it is unusual in Iowa. The green roof was specified for aesthetic reasons to enhance views from overlooking buildings. Interestingly, it did not result in significant energy savings, though it promotes biodiversity.

The restrained simplicity of Chipperfield's scheme derives its visual drama (and reduced cooling load) from the shimmering glass-and-copper facade. One cannot help but query the embodied energy of the heavy cladding panels, manufactured in Germany and shipped halfway around the world to Des Moines. This globalised sourcing of materials should not be quietly dismissed.

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