Since opening in 1968, the Neue Nationalgalerie on Berlin's Kulturforum has been seen as an architectural icon of modernism. It is the only building that Mies van der Rohe realized in Germany after emigrating in 1938. With its glass facade and deeply cantilevered steel roof resting on just eight columns, this modern art museum is a masterpiece of transparency and minimalism. Like a modern temple, the glass pavilion stands atop a stone plinth housing the museum's lower level.
Ludwig Mies van der Rohe had previously proposed very similar designs for the Bacardi Administration Building in Santiago de Cuba (1957) and the Georg Schäfer Museum in Schweinfurt (1960), neither of which was built (see illustration below). With the Berlin Senate's 1962 commission for a new museum building, Mies was able to realize the idea of a flowing space that seems to dissolve the boundaries between outside and inside. His column-free exhibition hall is an adaptable universal space structured by a mere few elements such as cloakroom fixtures, utility cores and stairs. Noteworthy to no small degree is the characteristic floor-to-ceiling glass facade with oversized panes of up to 3.46 × 5.37 metres - the maximum size that was feasable at the time of construction.
Complex task of refurbishment
After nearly 50 years of use as a museum it was time for a comprehensive overhaul and technical modernization. The built fabric of the two-storey building was dilapidated and had numerous defects, including damage to the concrete and the plinth facings, a leaky roof and cracks in the terrace paving. The defects of the glass facade were particularly glaring: For one thing, condensation on the windows impaired the view; for another, there were repeated instances of glass breakage over time. The building services were also to be revamped to meet the latest standards so that lighting, indoor climate and fire safety meet the requirements of a contemporary art museum. The costs are expected to total about 100 million euros.
After a thorough survey of the existing building and preservation aspects, the Federal Office for Building and Regional Planning held a two-stage VOF procedure in 2012 and commissioned David Chipperfield Architects Berlin to carry out the refurbishment. The architects followed the principle of preserving as much of the original building as possible and making as few visible changes as possible. "Upon completion of the construction work, the only thing to be seen will be a major work of late modernism that has been renovated with great care", says Martin Reichert, one of two partners heading the project at David Chipperfield Architects. The architectural historian Fritz Neumeyer and American architect Dirk Lohan aided the firm as consultants. The latter, a grandson of Mies van der Rohe who worked in his grandfather's office, had been construction supervisor for the Neue Nationalgalerie.
After the museum was cleared out in 2015, the extensive renovation work began. Around 35,000 original building components and fittings like interior wall cladding, light fixtures, facade panels and terrace slabs were restored in special workshops and then reinstalled after the refurbishment work on the structural carcass and roof. There are just two small spatial changes: To improve barrier-free accessibility, the architects have installed an exterior ramp and an additional lift for visitors. And they converted two former art storerooms on the lower level into a museum shop and a cloakroom. There is now a new storeroom with ancillary spaces on the lower level below the entry terrace.
Cause for concern: the facade
Refurbishment of the characteristic steel and glass facade that crucially defines the exhibition hall's appearance proved to be an especially complex task. Over the years, glass panes had repeatedly broken and needed to be replaced, so when the refurbishment work began, only a few original panes of glass remained. After a detailed analysis with measurements over several months, the architects detected several causes for the glass breakage: First, the panes were undersized with a thickness of only 12.5 mm; second, severe corrosion occurred at the glazing beads; and third, the facade was unable to adequately withstand the substantial roof deformations caused by temperature expansion and snow and wind loads.
The first two flaws are remedied by using glass that is twice as thick and by controlled ventilation and drainage of the rebate space. To deal with the problem of the construction's lack of capacity to absorb movement, the architects changed the juncture between roof and facade: Instead of a continuous steel flat, short steels fins are now used above the mullion posts (see figure on p. 66). In addition, three facade mullions per side are modified in such a way that they are able to absorb movements in the structure - with, at the same time, rigidly configured facade corners (see figure on p. 66).
Technical renewal versus monument protection
The way the facade was built posed a particular challenge for the architects: The mullion-and-transom structure consists ofsolid steel profiles that are not thermally separated and single glazing with a Ug value of 5.8 W/m²K, which leads to condensation on the glass at low outdoor temperatures. The detail was no longer state of the art at the time it was built, but Mies accepted the disadvantages so as to realize his ideal of the most minimalistic facade possible.
Planning the rehabilitation measures raised the fundamental question of how to deal with the outdated constructive design. The architects first developed different variants for the facade, in which they considered improvements such as replacing the single glazing with insulated glazing and using thermally separated profiles (see figure at left). In the end, however, they concluded that these measures would impair the appearance of the listed historic building too severely: For one thing, insulating glass would have had a different tint, diminished transparency and twice the reflections; for another, the original, sharp-edged solid steel profiles are among the essential characteristics of the facade of the Neue Nationalgalerie's facade.
Hence in consultation with the client and the monument protection authority, the planners decided on a minimal solution with only minor interventions: The original monolithic panes of 12.5 mm thick cast glass are replaced by laminated safety glass twice as thick, ensuring high bending tensile strength and structural stability even in the event of potential damage. Thus it was possible to preserve the original facade profiles; coatings and corrosion protection were renewed in conformity with the original condition. Solely the glazing beads had to be modified due to the thicker glass panes, but this is nearly indiscernible. The existing facade's disadvantages from the excessively high U value cannot be entirely rectified but can be significantly diminished by optimizing the historical a facade ventilation system: air outlets in the floor create a warm air curtain behind the facades and minimize the formation of condensation.
Pitfalls of the glass format
The glass itself posed an additional diffuculty for the facade refurbishment. For his design of the Neue Nationalgalerie, Mies had chosen an oversized format of 5.37 × 3.46 metres - the widest dimension available for the cast glass sheets commonly used back then - for the upper panes. Shortly afterwards, however, the industrial float glass process with its maximum production width of 3.21 metres prevailed. Thus each glass pane that broke over time needed to be replaced by two panes connected by a silicone joint - which severely impaired the facade's minimalist aesthetic.
The refurbishment called for glass panes of the original size, which, under building law, required special individual approval. The search for a manufacturer capable of producing this oversized format proved difficult, however. It took extensive research throughout the world and the help of a European glass dealer to find a Chinese manufacturer - the only company able to do the job - and likewise just one glass processing company capable of laminating such large panes. Now, panes of laminated safety glass with a total thickness of 24 mm are used, comprising partially tempered glass bonded with a stiff plastic film interlayer. This format did not allow for a sun or heat protection coating. In terms of colour, the white glass used approximates the original panes.
Opening in sight
In the summer of 2021, the Neue Nationalgalerie will reopen after being closed for more than six years . At that point, it will again be possible to access this important building - Mies van der Rohe's last and culminating work - and to experience it in all its beauty. The refurbishment will not be very noticeable to visitors, because although the damage will have been repaired and the building will shine in its former splendour, the countless interventions and the great efforts undertaken will ultimately remain indiscernible.
