1. Building description

The building consists of three parts with different ages and is used as a primary school and a secondary school (Hauptschule). This combination is common in Germany. The block of buildings to be renovated was built in several sections during the thirties, the fifties and the seventies (1936, 1957, 1970). The floor area of 5260 m² includes the classrooms, halls, lobbies and staircases and a gymnasium. The total volume is about 22470 m³. A typical classroom is about 60 m² and meant for 20 to 25 pupils. There are 25 classrooms and 3 practical rooms. Each part of the construction was typical for its origin period. Thus the building represents the average school building in the western part of Germany. The work in the school building started in the summer break 1996 and was finished in the summer of 1997.

Building 1 is a solid brick construction. The second building segment is a reinforced concrete building with solid brick cladding. Building 3 is made by reinforced concrete with a multi-layer chipboard/insulation combination on the outside and on some orientations on the inside. The windows were mostly composite windows with wooden frames (double glazing) without sealing. The roofs had wooden/concrete constructions filled with peat dust (segment 1+2) or were constructed as rib floors with insulation (segment 3). The cellar ceilings were made of concrete with no insulation.

Former heating / ventilation and lighting system
The low-pressure steam boilers from 1969 of 800 kW capacity were originally fired with coal and later converted with to dual fuel with oil/gas burners. Building segment 1 was still heated by steam, building 2 and 3 had a hot water heating system fed by a heat exchanger. Piping and radiators dated from the time of construction. The circuits had weather compensation control with fixed time settings.
Ventilation was ensured by the windows and infiltration through the building envelope.
Prismatic diffusers and louvred luminaires with flourescent tubes were used for lighting. The lighting and the shading systems were manually controlled.

Problems / damages
All building segments suffered from severe heat losses, especially at the building component connections. The windows showed air leakages due to poor sealing. The energy use in the building was very high (210 kWh/m²a).
The heating system had to be replaced.
The lighting situation was to be considered to be poor due to strong glare-effects. Therefore the artificial lighting was switched on during daylight hours despite sufficient daylight supply, whilst the blinds were closed.

2. Retrofitting energy saving features

A list of possible thermal insulation and new heating systems was developed. For each different variation the energy saving potential was compared to the investment costs. By using the value annual energy savings devided by costs the variations were sorted and the most efficient ones were realised.

Building construction
The walls were insulated by composite thermal insulation (external insulation with plaster) up to 14 cm or by using styrofoam as internal insulation where the original facades have to be kept untouched. Parts of the roofs were insulated by the teachers and students with 18 cm styrofoam. The windows were replaced by low-E-coated glazings in mostly wood-aluminium frames.

Heating / ventilation and lighting systems
The boiler house with boilers and piping and the low pressure steam system were completely replaced by a condensing boiler for base load and a low temperature boiler for peak load, both with low NOx-gas burners. The peak load of the boilers was reduced by 60 %. The radiators of building 1 are oversized to provide a quick heat-up suitable for the intermittent operation of a school. The teacher has to press a button at the door to continue heating the classroom for the following hour in order to reduce heating during non-use. Otherwise the temperature drops to a base value. In building 3 the radiator valves were replaced by tamper-proof thermostats. The offices of the headmaster and his staff got a separate control circuit, the apartment of the caretaker a completely separate heating system.
No ventilation system was installed. The improvement of insulation resulted in a more air-tight façade. Ventilation by windows had to be intensified. The teachers and the housekeeper have been informed about this aspect of ventilation.
In a few test rooms daylight dependent artificial lighting control and more effective lighting elements were installed. All rooms were painted to improve the reflective surface. The control systems could not be integrated in all classrooms because of the high investment cost.

3. Energy savings

Heating
The heating energy consumption of the early nineties averaged about 210 kWh/m²a, the original value of 1977 being 382 kWh/m²a. This was mainly the result of “good housekeeping” that is 'no-cost measures' enforced by the City of Stuttgart. After retrofitting the heating energy consumption was monitored as 49 kWh/m²a. The efficiency of the heating system with the two boilers was monitored and shown to be 95 %.

Electrical Energy
The painting of the rooms decreased the electrical energy for lighting from 10,6 kWh/m²a to 8,6 kWh/m²a. The new lighting system was more effective than the old one. The electrical energy consumption was 2,6 kWh/m²a in comparison to 8,6 kWh/m²a. The daylight dependent artificial lighting control system was not profitable in comparison with the new manually controlled one since the electrical energy consumption was just slightly lower (2,2 kWh/m²a).

4. User Evaluation
At the end of the monitoring period the school employees were questioned about the new building, heating system and lighting system. This shpwed that they were fully satisfied with the building itself and the new heating system. The daylight dependent artificial lighting control was recommended by the users.
However the automatic control of the shading device was accepted by the users. The noise of the shading device while moving up and down following the sun radiation on the façade happens abruptly and is not controlled by the users. This leads to distraction and loss of concentration for the pupils and teachers. Additionally all rooms on a façade get shading the same time, even if some rooms are allready shaded by trees. The headmaster insisted that the automatic control had to be deactivated after the monitoring period.

5. Renovation Costs
The replacement of the windows cost 420 €/m², the composite thermal insulation systems cost 110 EUR/m², the internal insulation cost 320 EUR/m² for the gymnasium and 165 EUR/m² for the other walls, the roof cost 68 EUR/m² and the cellar ceiling of segment 3 cost 48 EUR/m². The whole improvement of the building envelope cost about 1.432.000 EUR or 272 EUR/m² floor area.
The HVAC-system came to 360.000 EUR of which 247.000 EUR was for the installation and 113.000 for the plant. The new lighting system resulted in 54.000 EUR comprising in 23.400 EUR for installation works and 30.600 EUR for hardware such as new lamps and the control systems.
The total refurbishment cost was 2.120.492 EUR or 403 EUR/m²a floor area.

Project Coordination: Stuttgart Municipal Office of Environmental Protection
Building Physics: Fraunhofer Institute for Building Physics (IBP), Stuttgart
Heating Technology: IKE, Institut für Kernenergie und Energiesysteme
Monitoring: IKE in collaboration with Fraunhofer Institute for Building Physics

6. Funding

National Support program German Ministry of Economy and Technology, Projektträger Jülich (PTJ)

This case study was part of IEA ECBCS Annex 36 "Retrofitting of Educational Buildings".