Off on a totally new track

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. Pfaffengrunder Plateau is a residential complex that forms part of the overall project – and passive house-certified Schöck Isokorb XT is one of the many components that are playing an indispensable, efficient and economically viable role in achieving the ambitious goals.

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 then be home to some 5,000 people and a total of 7,000 jobs will be created there. Construction commenced in 2008 and so far, around one-third of one of Germany's largest construction projects has been completed. The total cost is estimated to be around two billion euro, with higher construction costs per square metre of residential area than are being seen elsewhere in this city with its rich university traditions: 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 to become the owner of one of the passive houses or apartments.

Extremely comfortable and superbly healthy living conditions

The fact that these manufacturing costs are above the normal average is partly due to the superb quality of the building and other materials, and partly to the intelligent passive house construction process incorporating triple glazing, a controlled ventilation system with heat recovery, and an extremely well insulated building envelope. In return, residents gain an extremely comfortable and superbly healthy home that costs very little to heat and thus ensures fast payback of the additional investment cost over the years. In keeping with the passive house concept, 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 about twelve euro per square metre, or around one-third less if they benefit from subsidies. This urban design envisages a colourful mixture of young and old, single people and families, and a cross-section of all income classes to fill the area measuring 116 hectares with life.

Smart construction solutions

One of the numerous construction projects in this quarter is Pfaffengrunder Plateau in quarter W4. This centrally located residential complex is made up of ten houses in total which form a block perimeter with occasional passages through to the inside, and was designed by Gramlich Architekten, a Stuttgart-based architects' firm. The three- to six-floor buildings – some with penthouse floor – are home to 77 leased and 39 owner-occupied apartments, all of which 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 area with balcony makes them appear larger than their actual size of 56 to 127 m2. The façades are clad with a composite heat insulation system to satisfy the stringent requirements for thermal insulation of the envelope. This ETIC system has slanted edges where it joins the window reveals – a clever construction and design trick that reduces shade while optimising solar gain.

Energy efficiency standard-compliant

Equally smart is the method for connecting the supported, and in some cases free cantilevered, balconies using Schöck Isokorb 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, while at the same time affordably and efficiently eliminating the conventional problem of thermal bridges associated with such connection details. A total of 432 of these intelligent building components are concealed in the building envelope structure and reliably prevent thermal loss through the linear thermal bridge beneath the balcony door or base of the wall. Different variants of the Isokorb XT were used, depending on whether the balconies – which measure as much as two metres in depth – are free cantilevered or supported on reinforced concrete pillars: The QPXT variant for supported balconies with peak loads only in specific spots, and the KXT variant for free cantilevered balconies. Installation of the building components is an exercise in efficient timing; they are put in place while laying the reinforcement and covering the floor slab with concrete.

Structurally safe and optimally insulated

Compliance with the energy efficiency standard for passive houses would undisputedly not have been possible without the passive house-certified Schöck Isokorb XT, given the magnitude of the influence exerted by thermal bridges from the balcony slabs on the energy efficiency of the building, and the considerable risk of mould forming in the interior in the absence of the thermal break.

In multi-storey apartments in particular, the installation of staircases without creating acoustic bridges is just as important as thermal insulation in terms of comfort – in the Pfaffengrunder Plateau residential complex, more than 200 Schöck Tronsole type AZ elements guarantee reliable insulation of the impact sound between the in-situ landing and the staircase walls.

In addition, nearly 350 Schöck SLD type dowels transmit the relevant shear forces across the expansion joints without the need for the usual inner slab beams or corbels. This enhances both the structural safety and the design scope 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 flagship project that is Bahnstadt. Schöck building components have played a key role – the importance of their contribution might not be visible, but is nevertheless indispensable.