DW DIE WOHNUNGSWIRTSCHAFT

B&O’s multi-storey timber construction technology is ready for serial production, marking our contribution to ecological sustainability.

B&O Wohnungswirtschaft since 2006 has been developing a residential park on the grounds of a former American military base. The centerpiece is the B&O project Building with Wood up to the High-Rise Height Limit. As part of the Energy-Efficient City subsidy program of the Federal Ministry for Economics and Technology, and in line with the general principles of the Zero Energy City, one of the most significant experiments in residential and urban development is emerging here. The first four-storey building, constructed with prefabricated solid wooden elements and using no concrete whatsoever, has already been completed.

        B&O, a general contractor to the housing industry (see the detailed report on preceding pages) is seeing through a pilot project as part of Germany’s Energy-Efficient City subsidy program on the 70-hectare grounds. It is erecting a four-storey building on the park grounds with a new system that uses prefabricated solid wood elements for multi-storied buildings.

        The use of wood for the construction of multi-storey buildings is not at all common; wood is not generally thought to be the best construction material in urban areas. B&O wants to use this pilot project to show that wood can easily replace concrete, steel and brick in the construction of multi-storey buildings in urban settings – especially because sustainability, resource-saving and healthy materials are becoming increasingly important. Work on the first four-storey wooden building started early in May, and more will follow. The height between floors is 3.2 meters and the clearance height in apartments is 2.8 meters. The building is being erected on the basement foundation of a demolished barracks building.

        Since tenants today no longer expect to live in identical apartments on each floor of a building, the building is designed using varying sizes and standards for each apartment. Variations in the floor plan of each apartment are meant to accommodate individual wishes. This is also visible in the building’s exterior design. From the outset, balconies were spaced so that occupants would not disturb each other. Built like loggias, with sidewalls and a roof, they prevent others from looking in. This offers a maximum of privacy, even in outdoor spaces.

Ideal construction material

        The entire building, including the elevator shaft and the balconies, is made of wood. It is self-supporting and does not need the support of a concrete stairway. The open steel staircase is the only exception. The wall surface behind the stairs is not wood either; plaster was used instead. These are fire protection measures. Wood is a natural construction material, renewable and sustainable. It also has many other advantages. Wood as a construction and cladding material enjoys wide acceptance among potential tenants, and B&O expects demand for the apartments will be high. Because wood and plaster have a positive effect on the regulation of humidity, the use of these materials ensures a comfortable and healthy room climate. Moreover, the touchable quality of wood surfaces can soften the rigid appearance of large buildings. The wood cladding is treated with a natural finish to retain its original color and appearance. The finish used on the wood cladding of the elevator shaft will need maintenance only once per every 10 years.

        As using wood for multi-storey residential housing is new in many ways, the planning team worked in cooperation with the ift Soundproofing Center, the Rosenheim University of Applied Sciences, and the Technical University of Munich. A total of 250 cubic meters of wood were used in the construction of the model house, which in the long term will reduce CO₂ emissions by about 250 tons.

Construction time and quality 

        With the cost of labor going up continually, intelligent building methods are becoming increasingly important. Brief construction times are a key advantage when it comes to densification projects or the replacement of old building stock. Four stories were erected in four days during the pilot project. This kind of timing is especially important in dense urban areas, as it keeps the disturbances caused by large construction machinery to a minimum. A rational sequence of operations does however require the extensive prefabrication of large building elements in the production hall of the Huber & Sohn wood construction company in Bachmehring. Prefabrication also has the advantage of increasing precision and lowering production tolerances, as well as speeding up construction times and reducing costs.

        Prefabricated façade elements open up many possibilities, not only allowing the sequence of operations to be more rational, but also making it easy to install new technology systems in the production hall rather than later at the construction site in the conventional way. ‘Active building envelope’ technology is an example. Ventilation systems can be integrated and the building shell can be equipped with solar technology in the production hall with a high level of quality.

        Load-bearing elements reinforced to be self-supporting can be made up to 12 meters in length. They consist of vertical square timber studs that are nestled with no spacing into a bottom girt, and held in place by a top girt (top horizontal member in a post frame or timber frame wall). They can also be positioned farther apart if the load is not very high. The walls have a high vertical load capacity and several stories can rest upon them. Fiber-reinforced plasterboard is used for fire protection. This turns the walls into static slabs, meaning that they are also diagonally reinforced, further stabilizing the building. These walls are manufactured for both interior and exterior use. Exterior walls are lined with a 16 to 24-centimeter thick layer of insulation, offering optimal thermal insulation and protection from heat. The final appearance of the building’s exterior is then a matter of choice.

        The use of curtain walls, especially wood planking, complies closely with the manufacture of complete wall elements with a finished outer shell and built-in windows. Blinds are also pre-installed if needed. Ceilings are made of planks glued together in order to form stacks of up to 60 centimeters in width. These are then assembled at the production hall into ceiling elements measuring 1.8 meters in width. Once they are installed at the construction site, they are connected to form ceiling slabs which supports the reinforcement of the building.

        Unlike the walls, the ceilings are not covered with plasterboard. The wood remains visible from below, and ceilings are merely finished with a light-colored glaze. Roof elements are 2.4 meters wide and consist of five laminated timber beams. The space between the beams, which are about 30 centimeters in height, is completely insulated. A layer of plywood is laid out on top. Here as well, the elements are assembled to create a static slab. The bottom side comes already lined with a vapor barrier membrane, tightly sealed after the elements are installed. It is a variable vapor barrier, which in summer enables drying out of moisture which may have leaked through, thus providing maximum safety for the roof construction. After electrical installation is completed on site, the bottom side is lined with grooved panels. Here too, the soffit is finished with a light-colored glaze. The insulation panels are installed on top, following the slope of the roof. Finally, a prefabricated UV-resistant EPDM single-piece layer seals off the roof. The elevator shaft is also constructed of plywood boards and is delivered to the construction site in one piece. The same goes for the balconies, mounted onto the façade from the outside, and made of laminated veneer lumber that is pressure treated.

Innovative ventilation technology with heat recovery

        Every apartment is equipped with a decentralized heat recovery ventilation system, which effectively lowers the concentration of dust and pollen hazardous to health. It also reduces the amount of heating needed in the apartments. Fresh air is drawn in through the façade. Ventilation ducts in suspended ceilings in the bathrooms and hallways draw this air into individual rooms. Outgoing air is collected from bathrooms and kitchens, and once heat has been recovered from it, is blown out over the roof via a duct behind the elevator. A combined incoming and outgoing ventilation duct is used in apartments that face south. The fans themselves are located in the bathrooms in the laundry area next to the washing machine and dryer. The laundry area can be separated from the rest of the bathroom by a cabinet or a simple curtain. The clearance height of the bathrooms and hallways is 2.4 meters.

        Since the model house is still experimental, different types of heating and control systems are being used. The building has an annual primary energy demand of 22 kilowatt/hours per square meter and the total energy requirement is 33 kilowatt/hours per square meter; it is connected to the residential park’s district heating network. Triple-glazed windows would significantly improve these values. Construction costs for this small, tower-like building, which can be seen as a prototype, are roughly EUR 2,000 per square meter. This could be reduced to EUR 1,500 per square meter if standard residential building geometry were used.

Retrofitting with wood

        The physical properties of wood as a natural material and the precise prefabrication of façade elements ensure good thermal insulation and a particularly high degree of energy-saving potential. The use of prefabricated elements also allows exterior building retrofits to be carried out very quickly and with a minimum amount of disturbance to occupants and nearby neighbors. A little more than a year ago, exterior building elements were used to renovate a building at the B&O residential park in cooperation with the Huber & Sohn wood construction company. Pioneering timber construction activities in Bad Aibling clearly demonstrate that wooden structures need no longer be associated with log cabins or exotic architecture. Instead it is a building material that can be used for a large variety of applications and has enormous potential in the construction of multi-storey buildings and the retrofitting of existing buildings.

Author: Arthur Schankula

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