What did the competition stipulate?
The clients requested a retirement home with 25 dwelling units. The existing school building needed to be demolished. It was dilapidated and didn’t meet code requirements. The clients suggested reusing the demolition waste. At the same time, they also suggested creating a timber structure in order to accelerate the construction phase through prefabrication. Our concept intentionally explored a different direction. There is no tradition of timber construction on Mallorca. Similarly as in the entire Mediterranean region, due to the availability of materials and for purposes of thermal inertia, masonry finds use. Solid construction is the basis of passive comfort in architectural terms. Here, it is the more sensible solution and the more sustainable one. We intended to recycle the demolition waste and use it as aggregate to prefabricate concrete block. We opted for prefabricated solid timber elements for the ceiling slabs. Based on this approach, we won the competition.
Are solid timber ceiling slabs typical of Spain?
For 15 years, solid timber has found increasing use in construction. Nearly 20 years ago, we had selected cross laminated timber for a building. It was likely the first time this construction type was applied in Spain. Then, the wood came from Austria. Now, manufacturers exist in the forest-covered north of Spain. Previously, it was risky to build this way, since the workforce wasn’t trained for it. Today, it is a tried-and-trusted and economically feasible construction method.
Were you able to implement the initial strategy?
We applied for a construction permit. In the permit process and as demolition began, we further developed the design for implementation. During demolition, we saw that the rendered walls concealed marès stone block. The soft limestone is typical of Mallorca. It can be easily cut, performs well thermally, and features a beautiful surface. We created the first sample blocks measuring 40 × 80 cm on the property of the demolition contractor as we did not have enough space on the construction site. Initially, we tried to remove the concrete surface manually to expose the stone. This proved to be too laborious. Instead, we cut the block with a circular saw. The stone became visible after cutting each side. This approach convinced us. In follow, we created large format slabs measuring 4 × 4 m that were cut into blocks.
What is the specific composure of the block?
There were changes to the type of concrete. Originally, we decided against a cementitious mix. Its production is carbon-intensive. This is why we opted for pure lime concrete. We experimented with different aggregate types. For instance, we added both large marès stones and gravel. This resulted in a natural stone share of 35–40 %. However, this solution was expensive, since lime concrete isn’t an industrial product. It needs to be mixed on-site. In order to make the process more economically feasible, we decided to use cementitious concrete after all. It can be produced in mixing plants and delivered in 6 m3 units by truck. On the construction site, it was mixed with lime, stone, and gravel. Cementitious concrete is more durable and cures more rapidly. After four days, a block weighing 800 kg can be lifted in place with a crane. With lime concrete, we would have needed to wait one month to do this. The cement also allowed us to reduce the wall thickness. Eventually we continued the tradition of masonry construction – by use of prefabricated cyclopean concrete elements. The costs are comparable. The time required for developing the elements and addressing questions of licensing was offset by the optimised construction processes.
Are the solid ceilings set on top of the walls without anchoring them?
The thickness of loadbearing walls is lowered incrementally from floor to floor by 10 cm, since loads decrease from bottom to top. This allowed us to reduce materials usage. This results in four different wall thicknesses ranging from 32 to 64 cm. The block otherwise features identical dimensions, 41 cm tall and 132 cm long. Five centimetre wide bearings on each side are, however, insufficient. This is why we created a 5 cm deep wall recess for setting the 15 cm thick slabs. The recess is limited to this area and the wall returns to its full thickness above, thereby stabilising the slab against flexural stress. Altogether we poured about 3300 slabs for the walls. One-tenth of these were re-cut in quarry sites. They have circular saws with three parallel blades that cut slabs into four strips. This resulted in 13 cm thick elements, which corresponds to the dimensions of traditional marès stone masonry. These thin wall elements stiffen the loadbearing crosswalls in perpendicular direction.
Can you describe the thermal conditions in the building?
The house faces the sun in a disadvantageous way. Its southern side is shaded by tall neighbouring volumes. In summer, the building requires no cooling. In winter, it needs to be heated. The unclad walls display high thermal inertia. They store heat and keep the building cool, which has a positive effect on the indoor climate. We would have liked to integrate underfloor heating. It was omitted due to budget concerns. However, it would have been more efficient and more aesthetically pleasing than the wall-mounted heaters. They are a nuisance in the smaller rooms. Underfloor heating distributes heat more homogeneously and can also provide cooling in summer.
Did the fact that the building was intended for seniors influence the design?
The client requested as many small dwellings as possible in order to increase the number of units. The house was planned as a retirement home. Essentially, it features subsidised apartments. Today, a mix of people lives here – young, old, or with mental disabilities. Compared to other subsidised housing projects, the budget was adequate. It was developed at a time when projects remained unbuilt because budgets weren’t realistic and contractors didn’t submit a bid. The client responded by moderately increasing the budget.
Where were the highest savings possible?
We were able to reduce the quantity of construction elements. The loadbearing structure assumes multiple functions: It transmits loads, insulates the building, and provides a pleasant indoor climate. Once we repeat the process of waste stone recycling and prefabrication of block, it can become competitive. One of our new projects benefits from this experience. This is an economic advantage. Savings also take place across the entire building lifecycle. For this reason, it is important that passive systems work well. Lower maintenance costs and energy consumption can balance high construction costs.
Was recycling motivated by sustainability aims most of all?
Not primarily. It was a rational decision to repurpose existing material. Sustainability followed. We are interested in the result: a space that recalls the traditional art of marès stone masonry construction, yet retains its unique character. Architecture should develop specifically from its historic origins. Sustainability is just as indispensable as the loadbearing structure. This is no added value. Following the crisis in 2010 we needed to save energy due to economic reasons. This led to sustainable strategies. Such measures also improve the feeling of comfort among users. If clients demand this, all the better.
Please find the project documentation related to the interview here.
