[Sustainability in Practice] Sunnybank: How Venner Lucas Architects opted for Passivhaus midway through construction of a new home in Coldingham in the Scottish borders, writes Oliver Chapman
Architects generally try to dissuade clients from last-minute changes once a project is on site. So imagine my surprise when I learned that Venner Lucas Architects had recommended to its client the adoption of the rigorous Passivhaus standard for its new home in the Scottish Borders four months into a 12-month construction period. A good rapport with carbon-conscious clients Jim Lucas (no relation to architect Tony Lucas of Venner Lucas) and Jennifer Mole, developed through the prior refurbishment of their Greenwich home, helped smooth over what could have otherwise been a very bumpy transition.
The ability to even consider such an about-face was largely due to the architects’ intelligent passive design early on in the project. The site’s former bungalow, appropriately called ‘Sunnybank’, had been demolished. Verner Lucas adjusted the orientation of the new four-bedroom rectangular house to direct its main two-storey elevation to face due south, parallel to the contours of the south-facing open site. The north elevation has only half as much exposed surface area because the lower floor is partly built into the slope of the site. ‘It designed itself,’ says Lucas.
The house is undeniably an ‘object’. Its simple form and ‘servant/ served’ organisation could easily be misinterpreted as an architect’s imposition on the clients’ brief. Between the parallel ‘servant’ and ‘served’ elements of the plan lies a linear void with a rooflight strip which connects all the circulation to the main family area on the lower ground floor. Entered at the upper level, this void is generous and visually arresting, contrasting sharply with the mute entrance courtyard.
One might think this organising principle better suited to the era when Louis Kahn relegated functions like cooking and washing to secondary parts of the home. Not so here. Owner Jim’s cooking hobby means that parts of the kitchen less suited to open plan areas, like space for food prep and larder storage, are tucked away in dedicated rooms of their own. The larder is built outside the insulated envelope and is large enough to dispense with the need for the standard big family fridge, leaving the main living area free from kitchen gadgets and clutter.
All the bedrooms are neatly aligned and share the view south over the garden, with ensuites at either end of the bedroom row. The bedroom landing is open to the central void and circulation flows down a slight ramp to connect with the stairs. This reaches a datum level before dropping a couple of steps further to the lowest level; subtle manipulation of levels gives the lower ground floor galley kitchen, study areas and dining area, a privileged view of the sitting area around the stove. The wall behind the stove is separated from the ceiling by a glass strip and screens the ‘flexi-space’, currently used as a playroom, which occupies the equivalent space to the dining area, mirrored in plan.
Switching to Passivhaus meant dropping plans for a biomass-powered underfloor heating system which, together with its bulky fuel store, occupied useful floor plan area. Instead, the clients embraced the concept that there would be no fuelfed heating system at all. The window specification was ratcheted up to triple glazing with insulated frames. Testament to the wisdom of this choice is the fact that the living room’s log stove was only used four times during the recent harsh winter, and more for pleasure than necessity.
The most onerous challenge proved to be the Passivhaus airtightness target of 0.6 air changes per hour. The design already included a heat recovery ventilation system with ground source pre-heated air, so considerable attention had been paid to detailing for airtightness. To help achieve this, the entire shell is protected from services penetrations by the inclusion of a service void between the shell and plasterboard finishes.
Thermal insulation thicknesses were increased on the walls, roof and, despite the concrete slab and retaining walls already having been poured, it was still possible to add to the floor as the insulation was originally planned to go within the shell. Venner Lucas’ decision to manage separate trade packages with only a site manager and no main contractor meant that these changes were directly in the practice’s control with the architect’s dual role as architect and project manager all masterminded from their office in West Sussex.
The house failed the stringent airtightness test for Passivhaus certification due to faulty seals on the windows. This must be galling for the cat that was denied the convenience of a humble cat flap, and the client, who was refused a letter box. Instead, the Passivhaus standard has worked as a benchmark to aim for, and if the house can’t quite attain the Passivhaus badge, no one seems too worried, though the owners’ inclination is to try again once the windows are properly sorted.
Tony’s business partner, architect Susan Venner, who has completed training in the Passivhaus Planning Package, endorses the discipline it places on designers to get the detailing, specification and performance modelling to the highest standard. She is skeptical, however, that the greater objective of low carbon emissions needs to correlate so directly to an airtightness target. The structure was fabricated largely on site. Pitch pine timber posts were connected to laminated timber beams by steel angle brackets, and the frame was in lled with insulated timber frame panels to form external walls. The stone was salvaged from the family’s derelict farm outbuildings nearby and the timber cladding is a heat treated hardwood from an FSC certified West African forest.
The switch to Passivhaus was a gamble that paid off in environmental terms without compromising the architectural design. Great care has been taken to prioritise the spatial experience of the house, the sophisticated changes in levels and the penetration of natural light, in the face of taxing Passivhaus targets. This success challenges the latent prejudice against setting technical sustainability standards which are often seen as running counter to architectural intentions and adds to our own practice’s eagerness to apply Passivhaus to our work. Sunnybank’s quiet pride in avoiding the display of its extensive technology, concealed in secondary areas and hardly visible, is a sign of the success of this delightful low carbon family home.
Start on site September 2008
Completion December 2009
Gross internal floor area 270m²
Form of procurement Direct labour with specialist contractors for glazing PVT and EPDM
Total cost £600,000
Cost per m2 £2,222
Client Jim Lucas and Jennifer Mole
Architect Venner Lucas Architects
Structural engineer Gyroury Self Partnership
Electrical consultant CB Associates
Site manager Alex Birne
Estimated annual CO2 emissions 19kg/m²