In order to meet the general objectives, work was principally focused on the following aspects:

  • Building envelope
  • Heat generation
  • Heat recovery through ventilation
  • Energy consumption metering
  • Implementation process in an occupied site

The building envelope

The building envelope which separates heated areas from non-heated areas was designed to be sustainable, energy-efficient and aesthetic.

Comprehensive insulation on the façades, roofing and ground floor, including replacing windows and French windows, ensured excellent building insulation and a marked increase in comfort levels.

The creation of loggias, principally to eliminate thermal bridging on balcony slabs, creates a buffer area, increasing passive solar gains.

It became essential to develop construction methods enabling work to be carried out on an occupied site, keeping negative impacts to a minimum and conserving comfort levels for residents as far as possible.

With this in mind, the renovation project utilised prefabricated elements, considerably reducing assembly times on site.


Heating and domestic hot water generation is, for the main part, provided by solar energy and ambient air exchanges.

The system comprises heat pumps combined with approximately 1700 m² of unglazed solar thermal sensors and an iced water accumulator enabling the use of latent heat linked to phase changes (water/ice).

Installation of dual-flow ventilation with high-efficiency heat exchanger in each walkway.

Introduced into living areas via a new network of the shafts integrated within the new envelope and recovery of air through the existing network from the sanitary areas.

Heat recovery is essential in buildings reaching this level of performance, with air renewal consuming the majority of heating requirements. Work was closely monitored by SIG to ensure alignment between the energy concept and the work completed.

To ensure continuous optimisation of the system, metering and consumption monitoring was put in place and will also be operated by the University of Geneva as part of research financed by the OFEN.

A screen is set up in each building indicating the consumption of the walkway and the whole area, with all stakeholders being able to benefit from remote controls for any type of screen.

Le chauffage 01Le chauffage 02

La facade du bâtiment 02

Financial aspects

Cost of work: CHF 19’900’000.-

Estimated subsidies: CHF 2’000’000.-

A decision was taken from the outset to limit rent increases to respect the principles and ethics of the cooperative.

Long-term guarantees are in place, conserving a rental level per room at approximately 50% below market average.

The effective increase is less than CHF 50 per room per month, thanks to offsetting through fixed cost reductions.

Once the building has been renovated, the results will also depend on the residents’ ability to take onboard certain behavioural changes.

To this end, there was a focus on explaining both the concept to the residents as well as the practical aspects of living in these ‘new apartments’. This involved:

  • Setting up an information centre to answer questions
  • Creating entertaining diagrams
  • Creating a “code of conduct”
  • Setting up multimedia screens in building lobbies