A STRAW BALE HOUSE IN THE VILLAGE CENTER
Pfaffenhofen is a small village near Heilbronn. It is characterized by an idyllic setting with a church, and half- timbered houses dating from the 16th and 17th centuries in the center of the village, with vineyards rising picturesquely in the background. The village center is now being redensified with a residential building in a cubature typical for the region, and that is extraordinary in several respects. For one thing, the way it is built reflects the client’s mindset and area of work. As far as possible the aim was to realize a building that was ideally made from natural and renewable raw materials which can be re-introduced into the natural cycle.
Everything was based on the idea of using bales of straw combined with clay plaster as a thermal envelope for floors, ceiling, roof and walls – a practice in use since the late 19th century and that is now making a comeback for a number of reasons: Since straw is renewable and recyclable it is consequently easier on both resources and climate than conventional insulating materials. It is also readily available and can be sourced locally. However, not only is the material itself low-tech, its handling is also simple: The bales of straw are pressed into a wooden framework to a thickness of 36.5 cm with any excess simply being trimmed off using hedge cutters.
The goal for Haus Hoinka was to realize all six facades – in other words the roof and the floor slab as well –
using this straw bale construction method. In order to dispense with elaborate sealing while at the same time still permanently protecting the straw bales in the floor slab from water the house was raised by an entire floor. The compact house rests on a concrete cross and four supports. When the wooden shutters are closed it creates the impression of an elevated wooden monolith which forms a stark contrast to the open garden level. In the process, Haus Hoinka subtly adopts both the grain and roof shape of its setting and with its staggered structure comprising a stone base and cantilevered wooden building engages in a direct dialog with the half-timbered houses in the center of the village.
A HOUSE THAT CAN BE TRANSFORMED
The simple basic form conceals a complex nested semi-detached house in which the two residential units are each connected to the garden level via a single flight of stairs. This means that the entrance doors to the apartments are each located on the ground floor. On all floors the residential units are organized by point reflection to one another so that all residents can benefit from the views in all four directions: This means that views from every apartment extend east to the church square, west to the garden, north to the vineyards and south over the roofs of the village into the distance. On the first floor the house is divided lengthwise, on the
second crosswise. This division is also legible in the interior: For example, the structure of spruce wood and the loam in the apartment facing the center of the village were painted white, while they were left untreated in the one facing the garden. But this division is also subtly visible in the silver fir facade: The widths of the planks in the board and batten cladding vary slightly in the two halves of the house thus giving a hint of the interior.
On the ground floor the elevation required for structural reasons is achieved by means of a concrete cross and four corner supports. This creates four open spaces that can be used in a variety of ways by the occupants
– conceivable options include a charging station for an e-car, a workshop or an outdoor kitchen. These uses might change in the course of the year: In the summer, for example, the rooms work as outdoor living rooms expanding the indoor living space. The client decided to realize a granny flat in one of these four spaces. Further developments such as a winter garden, a workshop or a guest room are possible options for the future. Through their choices the residents will shape the house over the course of the years. The Cite Verticale in Casablanca inspired the architects to adopt an approach that leaves the use of a building in part to its occupants. And indeed, Atelier Shen regards further building by the residents of this development as highly enriching.
ROOMS WITHOUT FEATURES
On each of the floors, eight almost square rooms measuring roughly 4 x 4 meters are arranged. Lengthwise to the house on the first floor and crosswise to the house as an enfilade on the second floor. Since all rooms are virtually identical, they can be used alternatively as kitchens, bedrooms, living or dining rooms. This means they can change their function over the building’s entire lifespan without overly many extensive structural alterations being necessary. However, some rooms in Haus Hoinka such as the bathrooms are clearly defined on account of their installations. This uniformity in the rooms is also reflected in the facade: All rooms on the upper floors have identically shaped windows. Only the balcony doors interrupt the uniformity of the elevantion – whereby they reference the passage openings of the enfilade. In the attic, wide ribbon windows were installed, and once again they are identical in form serving to underline the equal value attached to all the rooms.
This equality and uniformity makes it possible to create different apartment configurations in the house. The
two residential units of the semi-detached house can each be divided floor by floor. This means the two large maisonette apartments can be divided into four smaller units. In this scenario the internal staircase becomes a stairwell that provides access to two residential units. Then the doors opening onto the staircase become the new entrances to the apartments. In this way the house can respond flexibly to changing family circumstances. For example, when the children move out the parents can live on one floor and sublet the other as a separate unit.
This means that the living space per person can be reduced after the children have moved out and the house provides immense flexibility of use without the need for major alterations.
THE HOUSE AS A POWER PLANT
At Haus Hoinka, emphasis was placed on the use of simple and ecological materials. In addition to the use of straw, clay and wood, care was taken to ensure that all other materials used also had a good ecological balance and could be recycled and separated appropriately. As far as possible any glued connections that would be difficult to reverse were avoided. All materials including their origin are recorded in a database for sustainable building products developed by the client.
For the most part, regenerative sources of energy are used. Electricity is generated by solar modules that are fully integrated into the roof as the complete water-bearing layer. The solar modules form the smallest component in the house and correspond with the grid of the skylights as well as with the grid of the house as a whole. The PV- elements produce a total of 30,000 kWh electricity per year, which exceeds the projected required consumption by around 6,000 kWh. This amount takes into account the total electricity and heating requirements for the house. A daytime electricity storage system with a capacity of 10 kWh ensures that the electricity generated during the day remains available in the evening hours and at night. Should this electricity not be sufficient, energy is drawn from the power grid; conversely surpluses are fed into the grid.
Heating is provided by a heat pump (which can be switched to cooling), that serves a ceiling panel heating system. Thanks to the radiant heat, this system offers comparable comfort to underfloor heating and can respond markedly faster to a change in heating requirements. Overall, the house achieves the KfW 40 Plus Efficiency House and the Efficiency House Plus Standard as it is able to demonstrate both a negative annual primary energy requirement and a negative annual total energy requirement. Moreover the house will be monitored over the course of one year when occupied so as to compare the actual amounts of energy consumed with the original calculations.
Taking all of these aspects into account the building has a particularly good eco balance: Compared to a new conventional semi-detached house of the same size that is made of bricks or tiles and with classic insulation 95% of CO2 has been saved. Around 100 tons of CO2 are stored in the 140 cubic meters of wood used for the house. Especially now given the ever greater shortage of resources, a renewable insulating material like straw makes
a good alternative given that it is not only climate-friendly but also readily available regionally and at low cost. It holds considerable promise for the future of building.
As regards building type, Haus Hoinka, demonstrates how residential construction in rural areas can be rethought. A higher density for the village center is achieved through a sustainable building construction and using simple materials that can be separately recycled. Moreover, the building offers an extremely flexible floor plan that allows for various constellations of people living together. Not only will Haus Hoinka make future changes possible it will probably also be enriched by them.