BTP Consultants is assisting Citynox with the rehabilitation of the iconic Nuage Towers in Nanterre

Alongside Citynox, the industrialist in charge of developing stainless steel facade blocks, BTP Consultants acts as a technical controller and ATEx (Experimental Technical Assessment) rapporteur.

A crucial mission to secure the entire innovation process and to enable Citynox to apply for the future renovation of the facades of the next four towers concerned.

An extraordinary project for an exceptional heritage

Built between 1973 and 1981, the Nuage Towers form a unique architectural ensemble, immediately recognizable by their rounded silhouettes and facades once adorned with mosaics by Fabio Rieti. Their advanced age has led Nanterre Coop' Habitat to undertake an ambitious thermal renovation program, while preserving the original spirit of Aillaud's project.

The work, carried out by the architecture firm RVA and the designer Pierre di Sciullo, relies on a metal cladding made of solid-colored stainless steel, whose varying matte finish creates light effects reminiscent of the "nebulosity" characteristic of the artwork. Residents remain in their apartments throughout the construction period, which necessitated a completely dry construction solution that was quick to install and adapted to the curved geometry of the towers.

An innovative facade solution, developed by Citynox

The renovation involves installing exterior thermal insulation made of prefabricated stainless steel blocks incorporating rock wool. Each block, approximately 2,60 m high, is custom-made to fit the specific curvature of each tower. The façade of pilot tower no. 15 represents a total of nearly 5,000 m² of stainless steel cladding.

For this tower, Citynox designed, in collaboration with RVA and Pierre di Sciullo, blocks which integrate pre-frames and, where applicable, exterior joinery reproduced in their original shapes (round, square, or so-called "sage leaf"), in accordance with the original architecture.

Lucas Reha, the installation company of the Citynox-Lucas Reha group, then carried out their implementation on site.

Today, Citynox is building on the experience gained during the initial project on Tower 15, for which several ATEx (Technical Assessment) certifications have already been obtained with favorable opinions, to offer version 2 of its EWI (Exterior Wall Insulation) blocks, incorporating various improvements. With the support of BTP Consultants, the manufacturer has submitted its application for a new ATEx in October 2025, an essential prerequisite for securing the contract to renovate the facades of the next four towers: Towers 07, 11, 19, and 95.

An atypical and proactive ATEx approach

Faced with the technical complexity encountered during the pilot tower n°15, the project owner has evolved its process for awarding the contract for the renovation of the facades of the next towers: Nanterre Coop' Habitat has decided to commission upstream, before final consultation, the production of the ATEx necessary for the implementation.

A novel approach allowing a much smoother transition to the work, while securing the entire project.

For Citynox, this ATEx case B was obtained as early as October 2025.

BTP Consultants' mission: to secure innovation and support Citynox

Since March 2025, BTP Consultants has been assisting Citynox in preparing and obtaining the ATEx certifications necessary for the implementation of their façade system. As rapporteur, BTP Consultants:

studies the technical file provided by the manufacturer
analyzes the risks associated with implementation on construction sites and in service
evaluates the means of production, control and self-control
examines the feasibility of the experimental application
identifies the risks of potential disorders
verifies compliance with regulatory requirements
formulates a conclusion submitted to the CSTB expert committee

Technical inspection during the construction phase

Beyond the ATEx process, BTP Consultants provides technical inspection services for:

verify that the work complies with the recommendations of the expert committee
to ensure the safety of installations and processes
ensure that the requirements of the ATEx file are taken into account
to support the increased precision of construction solutions derived from tower 15

This comprehensive support provides enhanced security on a construction site where every millimeter of curvature and every construction interface represents a challenge.