The German city of Heidelberg is witnessing the emergence of a trend-setting model for intergenerational living and working. "Bahnstadt" is a project to convert the former freight depot into a new residential quarter built to passive house standards. A complex that forms part of the overall project is Pfaffengrunder Plateau and one of the many components playing an indispensable, efficient and economically viable role in achieving the ambitious goals, is the passive house certified Schöck Isokorb type XT.
When it is completed in 2022, "Bahnstadt" – located in the former freight yard, just behind Heidelberg's Central Station – will be the world's largest passive house residential complex in terms of area. The new quarter will be home to some 5,000 people and a total of 7,000 jobs will be created there. Construction commenced in 2008 on what is one of Germany’s largest construction projects, and to date, around a third has been completed.
The total cost is estimated to be around two billion euros, with higher construction costs per square metre for a residential area than has been seen anywhere in this city, with its rich university traditions. It is expensive to become the owner of one of the passive houses or apartments. The first apartments to be completed have so far cost about 3,200 euro/m2 and some prime areas are expected to command prices of up to 4,200 euro/m2.
Extremely comfortable. A superb and healthy living environment.
The fact that these manufacturing costs are above the normal average is partly due to the superb quality of the construction materials throughout; and also because of the intelligent passive house construction process that incorporates triple glazing, a controlled ventilation system with heat recovery, and an extremely well insulated building shell. In return, residents benefit from an extremely comfortable and superbly healthy living environment that costs very little to heat and helps ensure a fast payback of the initially high investment cost. Totally in keeping with the concept of passive houses, the buildings primarily heat themselves, using the internally recovered heat. Any minor residual heat that is needed is provided from the public grid.
Anyone looking to rent will pay around twelve euros per square metre, or, if they benefit from subsidies, about one-third less. The urban design process anticipates a cross section of all income classes, comprising a colourful mixture of young and old, singles and families, all of which should make the 116 hectare area a very vibrant place to live.
Smart construction solutions
Designed by Stuttgart-based architects' firm Gramlich, one of the centrally located residential complexes in quarter W4 is Pfaffengrunder Plateau. It is made up of ten houses which form a block perimeter with occasional passages through to the inside. The buildings vary from three to six stories – some with penthouses – and are home to 77 leased and 39 owner-occupied apartments. All have a spacious balcony or roof terrace. The floor-to-ceiling windows flood the two-to-five room apartments with light, while the open-plan living/dining areas appear larger than their actual size of 56 to 127m2, because of the balcony feature.
The façades are clad with a composite heat insulation system that satisfies the stringent requirements for thermal insulation of the shell. This is an ETIC system which has slanted edges where it joins the window reveals – a clever design detail that improves shade while optimising solar gain.
Energy efficiency and standard-compliant
Equally clever is the method for connecting the supported, and in some cases, free cantilevered balconies using the Schöck Isokorb type XT, which is passive house certified. As a load bearing thermal insulation element, it transmits the bending moments and shear forces occurring on a single axis in the balcony slabs, to the reinforced concrete inner slab. At the same time efficiently and economically eliminating the conventional problems associated with thermal bridges at such connectivity points. In total, 432 of these highly effective components are concealed in the shell structure and reliably prevent thermal outflow through the linear thermal bridge beneath the balcony door or base of the wall. Different variants of the Isokorb XT are used, depending on whether the balconies – which measure as much as two metres in depth – are cantilevered or supported on reinforced concrete pillars. The QPXT version is for supported balconies with occasional peak loads; and the KXT variant is for free cantilevered balconies. Installation of the components is an exercise in critical timing, as they are positioned while the reinforcement is laid and the ceiling slab then covered with concrete.
Statically reliable. Optimally insulated.
Without the passive house certified Schöck Isokorb XT, compliance with the energy efficiency standard for passive houses would undoubtedly not have been possible. The negative influence exerted by thermal bridges on cantilevered reinforced concrete slabs and resultant loss in energy efficiency is enormous and there is also the additional risk of mould forming on the interior surfaces without the thermal break.
When building multi-storey apartments, the importance of impact sound protection and the avoidance of acoustic bridges in staircases, is just as important as thermal insulation. Consequently, In the Pfaffengrunder Plateau residential complex, more than 200 Schöck Tronsole type AZ elements are installed to guarantee reliable insulation against impact sound between the in-situ landings and staircase walls.
A further Schöck product incorporated into the complex is the Schöck dowel type SLD. Nearly 350 of these dowels transmit the relevant shear forces to expansion joints without the need for the usual inner slab joists or corbels, enhancing both the static reliability and the design flexibility in equal measure.
As part of the Bahnstadt Heidelberg project, the passive house residential complex has scooped the "Passive House Award 2014", which emphasises just how widespread international interest is in the ‘lighthouse project' that is Bahnstadt. Schöck components have played a key role – and while they may not be visible, their contribution is indispensable nonetheless.