tp bennett director Doug Smith has built a camera-like Passivhaus in the Kent countryside, with full-height shutters that roll back to reveal an immaculate fair-faced concrete structure
Some people like to capture a view by taking a photo. Doug Smith and his wife Wendy decided to build a whole camera. The tp bennett principal director likens the house he has built in the garden of his previous home to a Box Brownie: its central section, glazed at both ends, looks straight through and down a beautiful valley in the Kent countryside like a lens trained on the horizon. There are even shutters for each aperture – huge slatted panels of Siberian larch that roll across to provide shade and privacy.
But Haus on the Ridge is about more than a view. The four-bedroom villa has been built to Passivhaus standards, with triple glazing and an insulated in-situ concrete structure. This is shown in all its splendour when the shutters are closed over, and the immaculate fair-faced finish of the two wings is revealed.
Smith has long had a professional interest in Passivhaus, but was never convinced by structural systems that rely on layers of membranes, fixings and “sticky tape” to achieve airtightness. “Unless you get that real attention to detail, and a contractor who is obsessed with making sure it’s completely sealed, it’s actually very difficult to achieve the requisite airtight rating.” This is one of the reasons he turned to a more monolithic construction system. “We started to look at it from a concrete point of view and it’s evolved from there. You’ve still got to address the junctions with windows and doors, but it lends itself to achieving a good airtightness. When the Passivhaus assessors did the initial air test, they could not believe what the house achieved.” The initial test recorded 0.29 air changes per hour, although by the end of the project this had changed to 0.46ach (against a Passivhaus limit of 0.6ach).
Smith has long been a fan of the fair-faced aesthetic too. “I’ve always loved the sculptural nature of concrete, the fluidity of it, the almost austere nature of it. I suppose I’m a bit of a minimalist at heart.” The reinforced-concrete structure allowed the team to span the 9.5m-wide glazed central section without columns – a boon for a lover of clean lines and uncluttered spaces.
The external frame comprises 200mm-thick outer and inner leaves, with 350mm of mineral wool insulation in between. The walls were cast without movement joints, which was ideal from an airtightness and aesthetic point of view, but required thorough detailing, including the reinforcement, to make sure nothing but a few hairline cracks appeared. Smith admits that both he and the project architect, Sam Clarke, were “relative novices” when it came to designing fair-faced concrete buildings, so relied heavily on the advice of the concrete contractor, Whelan & Grant, whose CV includes the Stirling Prize-winning Sainsbury Laboratory in Cambridge and the WWF’s Living Planet Centre in Woking.
To improve fluidity around the reinforcement, Whelan & Grant specified 10mm aggregate, rather than the standard 20mm. The mix included a 40% GGBS cement, both to reduce embodied carbon and lighten the tone of the finish. This was cast using a heavy-duty girder wall formwork system, faced with top-grade ply boards, which helped with the huge pressures exerted during the large continuous pours and ensured a sharp finish.
One of the trickiest challenges was designing and casting the fixings for the hefty 3.75m x 2.4m shutters, which are top-hung from the underside of a concrete canopy. “We just wanted to make sure that they didn’t bring the whole canopy down which meant thickening out the top piece of concrete, adding extra reinforcement and, crucially, forming the rectangular slots so we could fix these really thick concealed steel tracks,” says Clarke. “We could have face-fixed them but we decided instead to form a box within the canopy so you can’t see anything.”
To stop the shutters from swaying in the wind, a 27mm-wide guide rail was cast into the projecting lip at the base of the structure. “The channel had to be precisely level,” says Clarke, “and had to be cast into the concrete before we’d even designed the shutters.” From a concrete contractor’s perspective, this meant working to extremely tight tolerances. “Where you’re looking for a high degree of accuracy, particularly over a nearly 4m span, it takes it to another level of checking and making sure everything’s going to work exactly right,” says Simon George, managing director of Whelan & Grant.
Internally, there is a similar level of craft. As part of the Passivhaus ventilation strategy, letterbox-type openings were cast high into the living room walls for air entry and extraction. “The M&E guys wanted to put grilles over them to change the velocity of the air. But once they had been cast they were such a beautiful, sumptuous, sculptural expression, we said there’s no way we can put grilles over those, says Smith. “They ended up putting plenum air boxes on the other side of the wall, which are set within the ceiling of the adjacent rooms.”
The roof slab is 275mm deep over the two wings, which support a roof deck and sun pod. But over the central space, this reduces to a sliver-like 100mm, with deeper beams at 800mm centres. This gives a pleasing sense of height and space, but also enables the services to be carried discreetly from one wing to the other. Above the kitchen, at the eastern end of the open-plan space, plasterboard panels have been inserted between the beams, creating space for cabling and ductwork, as well as helping to soften the acoustics of the otherwise hard surfaces. Another interesting feature of this space is a sliding glass partition, again with an opening and track cast into the concrete, designed to seal Smith off while he’s watching sport on the TV without blocking the panoramic view for his wife – and thereby solving one of the perennial downsides of open-plan minimalist living.
Although this is clearly a highly bespoke, one-off project, Smith believes there are lessons for other Passivhaus schemes, not least the need to make clear design decisions early and pragmatically. “You have to think through the concept, how you’re going to apply it and how you’re going to deliver it from the beginning. It’s fundamental to the DNA of the building.”
He cites as an example the blockwork inner leaves of the side walls, which were finished with a sprayed membrane rather than a visqueen polythene sheet to reduce the danger of follow-on trades compromising the airtight barrier. Likewise, the plasterboard lining was set forward of the structure to make sure that any subsequent services penetrations from the living space, for back boxes for example, don’t disturb the sealed envelope.
Clarke adds that it was vital to have a Passivhaus champion on site, and that they were lucky to have in their main contractor, Robbie Varga of Salisbury Resources, someone who was completely on board with the strategy, making sure that every possible vulnerability was sealed and insulated.
Smith is delighted with his completed home, and reckons he barely needs to use the heating at all. The showpiece shutters provide solar shade but only play a significant role in cooling on the hottest days, while the interior keeps an impressively constant year-round temperature. Smith and his wife don’t even get hot opening and closing them – the tracks were so accurately cast in, he says, they almost roll themselves.