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Construction Refurbishment Other

Case Study: nZEC - Energy refurbishment of 137 dwellings in Liebenauer Hauptstrasse, Graz, Austria

Name of organisation: GIWOG - Gemeinnützige Industrie-Wohnungs-AG

Stage of development: completed

Year of finalization: 2014

Type of project: refurbishment

Area: urban

Scale: group of buildings

Type of building: Apartment in a block of six to ten stories

Number of units/dwellings: 137

Tenure: social rental

Street: Liebenauer Hauptstrasse 302 - 306

Postcode: 8060

City: Graz

Region/ County: Steiermark

Country: Austria

Last Update: 17.08.2015

Funding Certification Thermal Insulation Windows and Shading Heating and Hot Water Ventilation

Short Description

The project is one of the latest projects of retrofitting a multi-family block into passive house standard interest. Specific feature of the technical solution is the façade: a pre-fabricated panel system of a ventilated solar façade with integrated photovoltaic elements instead of thermal insulation compound system with EPS . That enables e very short period of construction (appr. 10 months) while residents are disturbed to a minimum. The photovoltaic system function to (pre-)warm the water. Thermal retrofitting was not only aspect: the roof has been renewed as well as elevators. Loggias were renovated as well as doors.

Key Elements


Measured heating demand before renovation:135 kWh/m2

Calculated heating demand after renovation: 8 kWh/m2


Mix of own funds, bank loan and public subsidies + Extra grant for innovative renovation from federal ministry (There is an annuity grant extended by the provincial promotion scheme which covers 40% of repayments for a loan for 25 years. An additional grant for innovative modernisation was extended by the Federal Ministry).


Thermal Insulation

The insulation of the basement and roof was improved. A special solar façade was installed which not only functions as insulation but also makes passive use of sun light to temper the façade (cellulose comb behind glass panel).

Due to a special formation of elements overheating during the summer period is prevented (GAP-system)  

Loggias were c.losed with windows which may be opened completely during summer period.

Windows and Shading

Passive house windows with outside sun protection installed.


Each dwelling equipped with automatic ventilation unit including heat recovery.

Heating and Hot Water

The existing system – central gas heating for space heating – was not altered. It is planned to monitor the functioning of the system for some time and then eventually to replace it. The existing electric boilers for hot water production also were kept in place but are connected to photovoltaic façade systems.

Main Results

The project is one of the latest of multi-family block retrofitted to passive house standard in Austria.  Specific features (pre-fabricated solar façade) were used to obtain this standard. The integrated photovoltaic elements support the hot water production. The loggias were closed as an additional element of insulation; since the window system allows complete opening during the summer period their functionality was kept.

All together costs of construction were high. Calculations show that costs for a “normal” thermic insulation (bringing down the heating demand to 60kWh/m2a instead of 8kWh/m2a) would have cost 200,--€/m2 less. The effects of the extra investments are additional savings per m2 and month of appr. € 0,57/m2 and month – which could not compensate for financing the investments without subsidy.

Lessons learned

Since the renovation was finished only recently it is too early to draw conclusion especially as regards the functionality of the system and usability for tenants. Nevertheless: good experiences have been made with the façade system in former projects.

Additional Information

Ms Eva Bauer
Gbv - The Austrian Federation of Limited Profit Housing Associations