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PlusEnergy building renovation with prefabricated thermal façade elements: Kapfenberg

Highlighted Case August 2015
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Project summary

The case study of this PlusEnergy renovated building has been proposed as a practical and innovative approach to use as a potential solution for retrofitting the huge quantity of existing post-war residential buildings constructed during the 1950s -1980s in Austria.

The project is located at Johann-Böhmstraße 34-36 in Kapfenberg (Austria) and is a typical residential building from the 1960's. It was originally made of prefabricated sandwich concrete walls without insulation and various heating systems: central gas heating, electric and coal furnaces, electric night storage heaters, etc.  The ventilation was only accomplished by opening the windows. This situation caused very high heating and operating costs for the residents.

After energy renovation, the building can provide more energy over a year than its occupants need for their thermal comfort, including hot water and electricity. This is possible due to an overall energy saving strategy based on the thermal design of the building envelope in conjunction with the energy supply system. This, as well as the initial objective of achieving an PlusEnergy House which is fine-tuned and matched in all its construction elements.

Energy concept

Firstly, the objectives were defined, which then brought to light the high necessity for renovation:

  • 80% energy reduction.
  • 80% ratio of renewable energy sources.
  • 80% reduction of CO2 emissions.

Secondly, after analyzing the existing building elements, the overall renovation concept was determined by:

  • Energy efficiency measures: reduction of transmission, infiltration and ventilation loss in the building envelope.
  • High ratio of renewable energy sources: the ratio was increased with solar thermal and PV systems.
  • Intelligent integration of existing energy sources: Connection to an existing local district heating network and PV system integration with the electricity grid.

The main solutions for the renovation that were implemented: new façade (prefabricated active and passive elements), new windows, new roof (flat roof instead of pitched roof) and new building systems (local district heating and solar thermal panels for domestic hot water (DHW) and heating, mechanical ventilation system with heat recovery and photovoltaic panels). These interventions made it possible to achieve the highest energy standard while simultaneously providing considerable advantages to the residents, both during the construction phase (“inhabited building site”) and also during the subsequent operation (comfort combined with cost reductions).


Project partners

Energy concept and coordinator: AEE INTEC

Owner and constructor: Gem. Wohn- u. Siedlungsgenossenschaft ennstal reg.Gen.m.b.H. Liezen

Design: Nussmüller Architekten ZT GmbH

Building physics: Rosenfelder & Höfler GmbH & Co KG

Collaborators: Institute for Materials Testing and Building Materials Technology at TU Graz, Kulmer Holz-Leimbau GesmbH, Geberit Huter GmbH, other partners from the research and technology program "Building of Tomorrow" as well as support from the Province of Styria


Time schedule

Built: 1960-1961

Renovated: March 2012- Jan 2014


Surfaces areas and use

Total Ground Floor Area (GFA): 2.845 m2

Total Volume: 8.538 m2

4-storey residential building with apartments varying from 59 to 88 m2 (32 apartments in total)


Construction costs

Building cost: ca.1.500 €/ m2 (without PV system)

PV system cost: ca. 2.500 €/ kWp

Total renovation costs: 4.3 M €.  There is a 35% reduction of innovative additional costs from the Austrian Federal Ministry for Transport, Innovation and Technology (Bundesministerium für Verkehr, Innovation und Technologie, BMVIT) with support from the Styrian Department for Renewable Energy and Social Housing and the City of Kapfenberg)


Energy consumption

Primary energy demand: 94.10 kWh/m2.year (excluding ST and PV systems)

Primary energy production:  95.25 kWh/m2.year (ST and PV systems)

Surplus: +1.15 kWh/m2.year (positive annual energy balance) (including overall system)

CO2 Emissions: 12.6 kg/m2.year (includes overall system)

Certification TQB (Total Quality Building) - ÖGNB (Austrian Society for Sustainable Building): klima:aktiv-Gold with 943 points and 15,1 kWh/m2y in heating demand (


A building monitoring phase was planned for a two year period after finalization of work. Therefore, overall data from user behaviour and extensive monitoring will be available for a new ¨lessons learnt¨ assessment in 2016.


Envelope performance

Tested U-value averages:


Area m2

U-value before renovation


U-value after renovation










Windows, doors









Details of individual elements:

  • Façade: 4-storey timber-framed panels with 24 cm insulation (mineral wool).
  • Ceiling (and ground floor): the existing basement ceiling was insulated with 6 cm on the bottom side.
  • Windows: triple glazing.
  • Roof: Flat roof with 35-40 cm base slope insulation and gravel.
  • Thermal bridges: thermal bridge free construction, covering the existing building with precast elements, cutting off the old balconies.
  • Thermal inertia: existing massive ceilings and walls in solid bricks and concrete.


Energy systems

Heating system:

  • Solar Thermal
  • Local district heating (115 kW)
  • Low emission radiators

Cooling system: no cooling system.

DHW system:

  • Solar Thermal and local district heating.
  • Small storage tanks installed in each flat with connection to a central tank.
  • Ventilation: Centralized and mechanical ventilation system with heat recovery (65% efficient)
  • Energy management system: BMS with remote maintenance.


Renewable Energy production system (ca. 80% of total energy use):

  • Around 115 kW of a district heating network with 40% waste use (renewable) and 60% gas use (non-renewable).
  • 144m2 (ca. 100,8 kW) of Solar Thermal collectors.
  • 550m2 (ca. 80 kWp) of PV panels located on the roof.
  • 80m2 (ca. 12 kWp) of PV panels located on the south façade.


Improvement measures applied

  1. Existing brick façade covered by large timber-framed panels like those  used in traditional rear-ventilated constructions (passive façade elements) which can incorporate active façade elements such as photovoltaic modules, solar collectors or solar "combs".
  2. Roof replacement of a new flat roof which is highly insulated with an approximately 35-40 cm. thickness
  3. Window replacement with new triple-glazed windows having a high thermal quality and integrated into the existing prefabricated façade modules.
  4. Renewal of old heating and DHW systems and replaced with a centralized and automatically controlled local district heating system.
  5. Integration of a mechanical ventilation system with heat recovery and carefully adjusted to supply temperature.
  6. Installation and Solar Thermal and PV systems for renewable production to increase renewable contribution.

Acronym of the case


Lessons learnt

All the residents living in the building were informed prior to the renovation and around 85% were also consulted during the renovation of the building. Their expectations were generally met in respect to the retrofitting of the building. They were also satisfied with the housing association and the different companies involved. After renovation, minor complaints were received for natural lighting inside the apartments, low temperatures at the beginning and some noise as a result of renovation works in the second construction phase. The most satisfying response came from the information sessions regarding user awareness for the control and use of ventilation, heating and DHW systems.

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Start date - End date

Thursday, 1 March, 2012 to Sunday, 1 June, 2014