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The small Bavarian city of Regensburg, like other urban areas in southern Germany, engaged in a postwar building boom, which continued into the 1960s, to house the city’s growing industrial workforce. StadtBau Regensburg (SR), a city-owned development company, built over 4,000 social housing units during this period. Those now aged structures require overhauls, so, to keep the developments viable, SR has embarked on several substantial renovations to bring them up to contemporary building-performance and occupant-comfort standards.

One such project is a 14-story tower with 58 units, dating from 1967, which has undergone its own modernization and the addition of a conjoined tower, led by Munich-based architect studiomolter. Most conspicuously, the project, completed early last year, entailed cladding it in 8,300 square feet of photovoltaic (PV) panels and recycled corrugated-aluminum sheets.

Champagne-colored photovoltaics and aluminum sheets now enclose the building, and windows have been enlarged. Photo © Roland Halbe, click to enlarge.

The existing 64,600-square-foot tower block was built of poured-in-place and precast concrete and enveloped in gray fiber-reinforced concrete (FRC) panels. Within, the mechanical systems and residential appliances were powered by outdated greenhouse-gas-emitting systems. The initial redevelopment plan, from 2018, was to build an adjoining 34,900-square-foot building comprising three residential units per floor, with an egress stairwell on its eastern flank—new fire-safety regulations rendered the sole existing stairwell, within the old building core, insufficient. The existing apartments were to be renovated, in line with those of the new, and both would be wrapped in a more thermally robust skin of new FRC panels and mineral wool insulation. For the duration of the project, beginning in 2018, the tenants were gradually and temporarily relocated to nearby apartments also owned by SR.

Germany’s increasingly stringent material circularity rules had made it difficult for the developer to dispose of similar FRC cladding. The product is often unsuitable for recycling, due to its heterogeneous composition; steel or synthetic fibers can entangle machinery and adhere to crushed material. So they decided to explore other options for the building envelope, ones that could be easily dismantled and recycled, along with other sustainability upgrades, through a design competition.

In 2020, studiomolter was awarded the commission, and shortly thereafter began its investigation process, mainly to evaluate the structural capacity of the tower’s concrete masonry units, located behind the exterior cladding. With those assessments in hand, the design team moved forward with detailing the new ventilated and thermally broken envelope system, which consists of nearly 8 inches of mineral wool insulation, an aluminum substructure, and thin sheets of corrugated-aluminum and PV panels in lieu of FRC. Together, the building envelope components are expected to last some 50 years, and the aluminum panels, owing to their ease of disassembly, can be removed and recycled for a range of uses. The design team was also required to add a concrete barrier at the exterior face of each floor plate to mitigate the fire risk associated with the envelope’s air gap and the PV panels’ electrical wiring.

Initially, PV panels were to be placed on the east, south, and west elevations, as well as the roof, for a system capable of producing well over 100 kWh during peak hours—enough to provide power to numerous buildings within the neighborhood. But such a high output would have led to the developer’s official classification by the German government as an energy supplier, which comes with a host of additional regulations and tax levies. So, instead, plans for the east elevation were scrapped to ensure a maximum peak production of (just) 98kWh.

“We also explored installing solar panels throughout the neighborhood, which is made up predominantly of four-story flat-roof residential buildings, but this proved too ambitious for the project,” says Philipp Molter, founder of studiomolter. “We are still very proud of providing an affordable and self-sufficient energy source to social housing residents.”

The shift in course for the cladding led to protracting the beginning of demolition until spring 2021. As the existing building’s interiors were stripped and the cladding carted to a landfill, the adjoining tower was erected at a pace of one floor per week. The new high-performance windows, for which openings had been enlarged to allow more daylight penetration within the old building, were installed in the summer of 2021. With the building watertight, interior upgrades could then commence, which included installation of radiant-floor heating systems. The mineral wool insulation was in place by fall 2022, and the rest of the envelope system was completed the following year. Along the way, owing to the 2022 Russian invasion of Ukraine and the subsequent energy crisis, the developer decided to swap out the building’s gas-fired cogeneration system and went fully electric for both its heating and appliances.

The multiple interventions—the total project cost was just shy of $22 million—has yielded impressive results. The estimated energy use intensity (EUI) of the original building was approximately 18 kWh per square foot annually. Today, the entire structure, with its additional 40 units, has an EUI of just 3, setting it up for another, greener half-century ahead.

Click graphic to enlarge

Click graphic to enlarge