XO Apartments is an all-electric residential block in Stoke Newington, north London. The eight apartments use only renewable energy, are heated via air-source heat pumps and are highly insulated behind their structural frame.
Or rather, frames. The first thing that passers-by will notice about the building is that south London-based studio Dowen Farmer has wrapped the apartments in two: an inner four-storey box clad in pale precast concrete and a two-storey brick outer wall that rises out from a ground-floor plinth.
This interlocking exterior is partly a response to its context. The lower outer wall communes with the bricky residential streetscape while the tall concrete piers behind echo the grander civic architecture of the neighbouring high street. But it is also a means of containing and organising a geometric puzzle of a building. “There were transfer structures at every level,” says Sean Martin of Dowen Farmer, which took on an initial design by Nicholas Kirk Architects. “The whole reinforced-concrete frame was a complex series of steps and pushes and pulls. For a scheme of such a relatively small size, it was exceedingly complex for the structural engineering team.”
This complexity began with the site, a disused car park. This was constrained in terms of both area and depth, due to nearby underground tunnels. Instead of traditional piled foundations, shallow concrete shafts – known as controlled modulus columns – were driven into the ground to transfer loads to the gravel beds below, compacting and strengthening the site’s poor-quality soil at the same time.
Above ground, things didn’t get any easier. The small footprint, and the need to maintain a fire escape stair from the next-door theatre, mean that the building cantilevers out on three sides at first-floor level, forming the outer brick wall. The upper floors then incorporate a series of setbacks, exploiting the space between the two frames to create recessed balconies and private terraces.
The piers and interlocking spandrels are one of the building’s most prominent architectural elements. The designers chose a warm grey mix with red undertones, which would complement the variegated brickwork on the outer wall. A key area of discussion with manufacturer Amber Valley Stone was the placement of the vertical joints. Although the easiest and most transportable solution would have been to cast the units so they stitched together down the centre of each pier, instead the joints have been neatly disguised in a natural crease between the pier and spandrel. The units were cast in wooden moulds and finished by hand, giving a tactile, stone-like quality.
Interior designer Scenesmith decided that the structure should take centre stage, so the concrete frame has been largely left exposed inside the apartments. This required further structural design work, as the location of beams and columns took on an aesthetic dimension. “We had already coordinated the concrete frame,” says Martin, “so it was a second round of coordination, thinking about what could go where, how we could delineate space and make more of a feature of some of these exposed elements.”
In some flats, transfer beams have been cast with chamfered details to frame the open living spaces and echo the faceted precast piers that line the facade. The concrete has been left as-struck, with the soffits in particular bearing traces of the casting process. “The developer, Artform, was very open to the idea of just taking the formwork away, seeing how it was left and celebrating that,” says Martin. To ensure full acoustic separation between flats, an acoustic mat was located directly above the slab with an additional layer of acoustic insulation above this. The floors were then tested to ensure they met the required building regulation performance.
Apart from the skilfully choreographed beams, the soffits are flush throughout – a detail that required more design dexterity around the various setbacks. To avoid an obvious step in the flat below each terrace, the slabs reduce in thickness from 250mm internally to just 150mm below the insulated decks. “So that allowed us to achieve the deck build-ups we required from a thermal perspective,” explains Martin.
However, this had repercussions for the installation of the precast piers. These were cast in sections 3.5m long, 450mm wide and 450mm deep, with some pieces weighing as much as two tonnes. “The 150mm slabs weren’t thick enough to take the load of the piers,” says Martin, “so we had to get the concrete frame contractor to come back and cast an additional piece of concrete – essentially a pad the same shape as the chamfered columns. It was certainly worth it for the flush soffits, but I imagine the residents will never know the complexity that went into it.”