Short spans and slim slabs provide an efficient, rigid frame for England’s largest Passivhaus development
Just to the south of Battersea’s iconic power station, a new four-towered development has risen on the skyline. But rather than generating energy, this one is notable for using as little as possible. Three of the towers have been developed by student housing provider Urbanest Battersea, and together comprise the largest Passivhaus-accredited scheme in England.
All four towers were designed by Allford Hall Monaghan Morris, and step from 11 to 19 storeys in a slightly askew arrangement. The three student towers, which contain 852 bedrooms, are clad in glazed terracotta, in shades of blue, green and purple. The fourth tower – dark grey with horizontal bands of glazing – is an office building. This is not part of the Passivhaus scheme, but it has achieved a BREEAM Outstanding rating, partly thanks to an innovative low-carbon precast concrete floor system.
Passivhaus wasn’t a planning requirement for the student housing, but AHMM and Urbanest, who previously collaborated on the Westminster Bridge Road residential development (2007), made it a goal from the outset. “Urbanest knew that it would probably cost more, but they saw it as an asset they would retain for 50 or 60 years, over which period it would save a huge amount in energy costs,” says James Santer, director at AHMM.
Reinforced concrete was a logical choice for the building frame: “When it comes to student units, which are relatively compact, you need to deal with both lateral and vertical acoustic separation, which is very difficult to achieve without a concrete slab.”
There are relatively small spans between columns, minimising downstand beams on the perimeter of the structure and reducing the risk of thermal bridging through the facade. This enabled the designers to maintain a slim slab of 225mm, reducing embodied carbon significantly across the 53 floorplates. The short spans also limited deflections, which would have posed a challenge to airtightness, a key tenet of Passivhaus.
This structural approach had to be balanced with the planning requirement to provide for future flexibility, Santer adds. “So if it’s no longer needed as student accommodation, it can be adapted to housing, offices or another use. You can’t just place a column every 2m because it’s structurally efficient, you have to show that it can work in another condition.”
In the three-level podium, which is also part of the Passivhaus scheme, much of the concrete has been left exposed, as struck from the wax-coated plywood formwork. “We were keen to stress that you shouldn’t just cover everything with plasterboard, because there’s so much embodied carbon in the process,” he says. Embodied carbon was also kept in check through the use of GGBS: 70% in the piled substructure and lower proportions throughout the frame.
The towers rise from the podium above the third floor, and cantilever on all sides by 4-6m. This was largely due to the restricted nature of the site, hemmed in by rail tracks and the busy Battersea Park Road. But it also enabled them to maintain a decent floor area within a highly insulated envelope. “The challenge with such deep external walls is land take,” says Santer, “but the cantilevers make that less of an issue.” Each facade unit is 700mm thick, with a central 300mm layer of rockwool insulation to meet the target U-value of between 0.10 and 0.15 w/m2K.
Overheating was identified as a risk from the very start of the design process. Each room has a triple-glazed window with a U-value between 0.5-0.55W/m2.K, preventing the inner pane from exceeding 17°C. By splitting the development into four separate volumes, the designers have increased the external insulated area, but gained the freedom to orientate the accommodation more precisely and place more windows on the north and south elevations. Combined with the deep facades, this provides shading from the high summer sun, while maximising solar gains in winter. The frames have a chamfered profile, highlighted in a lighter shade of the facade colour, which makes the windows appear bigger than typical for a large-scale Passivhaus scheme.
Fresh air is provided by mechanical ventilation with heat recovery, supplemented by openable vents in each room. Heating and hot water are generated by air source heat pumps. In all, Urbanest expects energy savings of over 75% compared to average new-build accommodation.
Mace’s delivery role was vital in ensuring buy-in from all of the subcontractors, Santer says. “You have to make sure that the facade team are talking to the slab edge team, and they’re talking to the dry liner guys. Suddenly people have to take far more conscientious ownership of each of those details.”
The communication challenge of Passivhaus starts with design, he adds. “From the start, we have to think about all of the potential barriers that could get in the way, and convey this complex set of ideas in as simple language as possible. It was like having to design a 3D Tetris puzzle and then taking it apart so we could put it back together from first principles.”
Innovation: Low-carbon concrete cassettes
The office tower was not part of the Passivhaus scheme, so had more freedom to innovate with carbon-reduction technologies in order to achieve its BREEAM Outstanding rating. Its precast concrete floor cassette system was devised by contractor Mace with supplier Oranmore, with support from Innovate UK. It has been tested and approved for use with 95% GGBS concrete.
At Battersea – the system’s first use on a live project – a lower proportion of GGBS was used. With a 40% GGBS-limestone mix, the cassettes showed a 38% carbon reduction compared to a steel frame composite deck system. All fabricated blast-furnace steel has been designed out of the cassette frames, as has the metal decking. “The decking, which is needed as permanent formwork on site, isn't actually required off site, so straight away that’s taken off the table,” says Martin Pike, assistant director of construction engineering at Mace. “With the slabs, the challenge on site is around controlling the curing time. We thought, if we can do this in a controlled offsite environment, can we use some of these more challenging mixes?”
Each unit for the large-span office floors at Battersea measured 13m x 3m and weighed about 14 tonnes. They were manufactured at Oranmore’s facility in Brandon, Suffolk. “They were significant precast elements to transport and install, and it’s a very, very tight site, but we wanted to test the product in quite a difficult circumstance,” says Pike. The team tested fully bolted dry connections, to aid disassembly, but decided on a wet stitch solution, which was deemed more robust. “If we wanted to reuse the planks, it’s still very quick to cut through the joint and lift them out.”
Building users should notice the difference from a typical composite floor, Pike adds. “Because the slabs are cast upside down, the soffits have a hand-floated finish, similar to how you would normally finish a floor slab. Composite floors can be almost overly industrial – this has a far more controlled feel.”