Mid-20th century architecture poses unique challenges to preservationists - and for sustainability. These buildings are fragile. They are often not perceived of as historic and worth saving. In addition, they have thin or conductive walls, resulting in high energy bills and uncomfortable spaces for their occupants - and buildings that unaltered will have difficulty meeting the annual energy usage requirements of Grenelle or the Plan Climat de Paris.

La Tour Lopez was designed and constructed between 1952 and 1959 as the offices for "La Caisse Centrale des Allocations Familiales de la Rťgion Parisienne" (CAF). The architectural team was Raymond Lopez, Marcel Reby in collaboration with Simone Lopez, Michel Holley and Henri Longepierre. The building represented mid-century experimentation with architecture, technology and urban design. Lower height buildings lined the street edge, but the tower was rotated at the center of the block. Architecturally, the buildings, especially the tower, explored the open floor plan with a central core and structure that allowed the maximum flexibility in the arrangement of interior offices. The tower utilizes a steel structure in contrast to the reinforced concrete structures more typically used in France and represented an experiment in the construction efficiencies and economies of this industrialized material and a plasticized curtain wall assembly.

Historic view of La Tour Lopez
Historic view of the CAF

La Tour Lopez, April 2010
La Tour Lopez in April 2010, completed and awaiting its new occupants.

In 1997, the CAF sought to restructure their lot in the 15th arrondissement in Paris and requested a demolition permit of the tower. In their eyes, the tower had become obsolete and the workplace conditions were poor, due to the total absence of acoustic and thermal insulation, minimal to no protection against fire, and degraded facade panels.

The demolition request triggered the French Ministry of Culture to seek protection of the tower as a significant 20th century structure. It was listed on the French Inventaire Supplťmentaire des Monuments Historiques in 1998, but the CAF contested this listing in court. In 2002, the highest court, the Conseil d'Etat, ultimately de-listed the building, and the CAF again submitted a request for a demolition permit for the tower to the city of Paris.

The permission to demolish the tower was not granted, although demolition of some of the smaller buildings on the lot was granted by the SDAP. While the tower itself was not listed, any proposed work on the tower and its adjacent buildings remained subject to the review and approval of the Service Dťpartemental de l'Architecture et du Patrimoine (SDAP) of Paris, under the "titre des abords des monuments historiques" (see Endnote 1).

In 2002, OGIC and BNP Paribas Immobilier became involved. Together with the architects Dominique Hertenberger and Arte Charpentier, they designed a transformation of the lot and the tower. Their goals: for the tower to retain the functionality, transparence and modernity of its original conception, while providing the modularity, luminosity, and energy performance of a contemporary office building. The tower boasts a new double glass skin facade and has earned the HQE label (dťmarche Haute Qualitť Environmentale). The secondary buildings have been replaced with 150 housing units, including 40 social housing units. The housing buildings are broken up to provide views from the street of the central tower.

The environmental successes include the following.
- improved integration into its urban site
- reduced impact of the construction site on its neighbors and environment (acoustics, run-off, waste, truck management, etc.)
- exemplery energy performance benefiting from the city's district heating system and a new double glass facade designed to admit light but control solar gain, and allow for natural ventilation
- interior designed to provide a comfortable workplace environment, with acoustical separation from the urban site, high ceilings to allow daylight to penetrate deep into the interior, operable shades to allow control of direct sunlight adjacent to the windows, etc.
- reduced operational costs: in addition to reduced energy usage, rainwater will be recuperated to irrigate the gardens, and controls help reduce use of potable water.

Model of the proposed renovation
Architectural model of the proposed renovation

Views of the tower are afforded between the new apartment buildings along the street

The reflection of La Tour Eiffel
The Eiffel Tower is visible from La Tour Lopez. This view also shows the windows shades located within the double skin curtain wall, provided to give occupants solar control.

Typical office interior
A typical office floor benefits from a raised access floor and tall windows and high ceilings that follow the upward taper of the structural beams.

The cafeteria
The renovated cafeteria on the first floor reuses the bubble skylights of the original design.

le livre "La Tour Lopez"
More information on the project can be found in the book, "La Tour Lopez: 1952 > 2009 La Renaissance d'une Belle Endormie"
Endnote 1. The "titre des abords" is the rule that anything within a 500 meter radius or that is co-visible with a historic monument is subject to review and approval by the Minister of Culture. Initial review occurs at the dťpartemental level, by the SDAP. If the proposed work is controversial or has a substantial impact on a historic monument, then a secondary review may occur at the regional level (DRAC), and a third and final review may occur at the national level, by the Commission Supťrieur des Monuments Historiques.