With Hortus, you realised the second building for the same client in Allschwil. How did this happen?
Alexander Franz: Basel is a globally significant location for life sciences and the pharma industry. Roughly ten years ago the Life Science Park was created at the urban fringe. An investor from St. Gallen recognised the potential of the site for creating a new hub and commissioned us to design a laboratory building. The future users were unknown at the time. The labs required flexible organisation, yet Basel is located in an earthquake zone, and thus, the only option was a concrete construction. We then received our next commission: a radically sustainable office building. Again, a program didn’t exist. However, the office function permitted using timber for the structure. The client only defined sustainability goals, such as a maximum carbon footprint and rapid amortisation of embodied energy.
What did this mean for the planning process?
The new building needed to produce a significant amount of power and reduce embodied energy. It is a free standing volume, and thus, the facade and the roof were extensively covered with photovoltaic modules. Simulations showed that energy can be produced on the shaded side. Only in the interior courtyard, sunlight is insufficient. We achieve an energy surplus of roughly 40 % annually. The building requires no additional power and embodied energy was no functional program, we didn’t design any interior partition walls. Thus, the ceilings needed to receive the mass. The result is a hybrid ceiling consisting of a timber beam structure and loam. The architects, the engineers, and the client met remotely in regular intervals. Martin Rauch contributed his experience with loam construction. We developed a full scale mockup of a ceiling module. It was certified for one hour fire resistance according to Swiss regulations. Companies are currently conducting 90 minute tests in order to meet standards valid in other countries.
Did you use commonly available modules?
We used standard modules from a Swiss company. They are black and frameless and are currently the most efficient ones on the market. The dark PV strips define the facade. With Hortus, the modules became an integral part of our architecture for the first time. Only the visible rafters signify the timber frame to observers.
Was the height of the facade strips driven by design?
Glass has a large carbon footprint. Room-height glazing wasn’t sensible. Daylight simulations resulted in an ideal 50 % share of fenestration based on the optimal ratio of daylight intake to power generation. As insulation for the timber parapets, we initially wanted to use straw bale. However, the material is difficult to procure in large quantities and the insufficient U value would have increased wall thickness. Eventually, we selected cellulose fibre sourced from waste paper. The adequate insulation values allowed us to use double glazing. Due to the lightweight glass, we opted for room-height triple glazing only for the ground floor. We also selected centre-pivot windows that enable efficient air circulation.
What concept is the green interior courtyard based on?
The courtyard was developed in cooperation with landscape architect Piet Oudolf. Climbing plants support summertime heat protection. The public shrub garden benefits the building and the entire area. By raising the volume on stilts, a stack effect sets in. In summer, cool air circulates underneath the structure and warm air is vented upwards. The courtyard remains cool even on hot days. We reduced the share of concrete to a minimum. There is no basement, only point foundations beneath the columns, reaching two metres deep below the ground. The courtyard facade corresponds to the exterior envelope and features black wood siding instead of PV modules. In order to maintain the colouration, we tested Japanese wood carbonization. Eventually we opted for a type of slurry paint that is durable and can be refinished simply. Our material studies encompassed all building parts.
Did the client pay for these studies?
The client financed a seven months long research phase. The aim was to develop a new building part that allowed reducing the carbon footprint. Together with the engineering firm ZPF we analysed the greatest potentials for savings. 40 % of emissions are hidden in the ceilings, another 10 % in the columns and walls. We needed thermal mass for summertime heat storage. Typically, solid interior walls serve this purpose. A timber structure can hardly meet such demands. Since there was no functional program, we didn’t design any interior partition walls. Thus, the ceilings needed to receive the mass. The result is a hybrid ceiling consisting of a timber beam structure and loam. The architects, the engineers, and the client met remotely in regular intervals. Martin Rauch contributed his experience with loam construction. We developed a full scale mockup of a ceiling module. It was certified for one hour fire resistance according to Swiss regulations. Companies are currently conducting 90 minute tests in order to meet standards valid in other countries.
The arched elements consist of rammed earth. Did you also consider using earthen block or pouring liquid loam?
Our elements comprise timber beams and threelayer panels on top, connected by screws. They are reversed and loam is introduced between beams without reinforcement. Ramming creates the vaulted shape, which supports optimal load transmission. Setting earthen block is too expensive and requires decoupling due to fire protection purposes, which the timber structure would need to meet alone. In our modules, loam is an integrated element. Liquid loam contains hazardous additives that increase its liquid and elastic properties. Our rammed earth ceiling is free of pollutants. The material serves many different purposes: It provides thermal mass, enabling heat storage, moisture control at a constant 40 to 60 % range, and filtration of volatile gases. It also has beneficial acoustic properties. The Swiss minimum width for single office spaces and maximum structural grid dimension for non-glued timber led to elements sized 2.8 by 5.6 m. The prefabricated timber modules fit on a truck trailer. The material, sourced from excavated soil to 76 % and amended with regionally available marl, was applied in-situ. The composite ceiling produces only 10 % of carbon emissions of a concrete slab with 28 cm thickness and identical loadbearing capacity. Considering the 40 % share of the ceiling slabs these savings are substantial.
Which species of wood did you use?
The types of wood are related to loadbearing purposes. On the upper floors, spruce is sufficient for the columns along the facade. On the lower floors, we used beech. Both were sourced from regional forests. The earthquake proofing is achieved by diagonal beech bracing. The secondary beams of the modules consist of spruce, primary beams are made of beech. We omitted laminated veneer lumber and instead opted for blockboard with large block cross sections to minimise the share of glue. We achieved an 80 % share of solid timber. We use plug connections, the secondary beams are set into recesses, and ceiling slabs rest on top of the columns. For this purpose we designed bearings measuring 8 cm. The overall dimension is 32 × 32 cm.
Which energy values did you achieve?
In order to meet the Swiss 2050 climate goals, a new building must not produce more than 9 kg of CO2 per m2 annually. We score at 5.3 kg, not counting the PV system. During operation, no emissions are caused, since we omit using fossil fuels. Heating and cooling are supplied by geothermal systems. Facade convectors with a low flow temperature of 26 °C enable distribution. Floors contain fill consisting of building waste, since sand isn’t sustainable. Instead of PE foil, we use oil paper. For soundproofing, the initial choice was sheep’s wool. However, wood fibre performs even better. The sustainability approach led to an exceptional planning process. We didn’t begin with the urban design scale and conclude with details, but reversed pen einbauen. Wir entschieden uns für unbehandelten Schwarzstahl, der problemlos recycelbar ist. this course. When we finished the studies, we presented full scale construction details. We were able to enter the approval and tendering stage with a completely elaborated project.
Which functions were specified?
The ground floor was supposed to be publicly accessible. The building is intended to offer room for 600 new jobs. The figure for the entire site is 10 000. Thus, we need gastronomic functions. The building features a bar and a cantina. The cantina operators are not required to pay rent. Instead, costs are covered by office tenants. They can use the ground floor functions for free. We specified the partition walls in the tenant units as wood stud walls with wood fibre insulation and loam render. We did have to make concessions in the openly designed stairwells. We couldn’t built timber stairs for the emergency routes. Thus, we opted for unfinished black steel that can be recycled without difficulty.
How were the office floors structured?
Per floor, up to eight tenants can move in. All office spaces are technically fully equipped. We developed lamps consisting of glass and metal. They feature components that can be exchanged as required. The exceptional part are the sensors: The lamps communicate with each other in order to produce homogeneous lighting levels. They also measure humidity, temperature, CO2 content of the air, and volatile gases. Data are processed centrally. Mechanical ventilation is automatically activated when the CO2 content is high. To us, the building is an inventoried resource for spare parts. This is why a clear systemic separation between technical services and loadbearing structure was important. Service lines are exposed and building parts can be reversibly sorted by type. In an ideal case, the facility is maintained as long as possible. That would be the most sustainable solution.
The related project documentation can be found here.
