Insulation has played a major role in helping to realise the Lancaster Co-Housing scheme, the biggest Passivhaus development in the UK
Lancaster Co-Housing, a development of 41 riverside homes just outside Lancaster, is a project of many milestones. Not only is it the first group project to achieve both Code Level 6 and Passivhaus standards, it is the largest Passivhaus scheme in the UK and the first fully Passivhaus development by architect Eco Arc, which has specialised in low-energy architecture for nearly 30 years.
With those credentials, it’s no surprise that this housing is packed with insulation to help meet Passivhaus’ stringent requirements, contributing to overall u-values of 0.12 for exterior wall, 0.09 for roof and 0.14 for floor.
A major factor in this success was the unusual co-housing nature of the scheme, and the commitment of owners, architect and builder to cutting-edge sustainable design and living. Owners benefited from a 1.25% mortgage rate reduction from Ecology Building Society for achieving Passivhaus certification, and zero stamp duty for Code for Sustainable Homes Level 6 as well as vastly reduced energy bills. Contractor Whittle Construction, an experienced Passivhaus builder, was secured on a two stage partnering contract to encourage a non-adversarial process. All parties enjoyed a profit share if they were able to come in under the agreed maximum price.
The 2ha site sits on the north bank of the River Lune at Halton. It was bought from receivership after the previous developer went bankrupt by a group of like-minded individuals keen to develop eco-housing collectively. Eco Arc took on a Ralph Erskine community architect type role to facilitate a participatory design process with the future residents, using Christopher Alexander’s ‘pattern language’ technique. Decisions were agreed by consensus on the general site layout, organisation, and design for the house types – limited to five to help simplify the collaborative process. These ranged from one bed flats to 3/4 bedroom houses. Even so, Eco Arc produced no less than 378 contract drawings for the project.
It is a mainly linear development of houses with a pedestrian street to the north and private space facing the river. A few houses make up a second row, and there is a community building in the centre as well as communal and car pool facilities. A renovated industrial building provides optional, rentable, workspace.
Using the Passive House Planning Package, Eco Arc analysed seven wall options including SIPS, insulating concrete formwork, masonry and timber frame.
After cost analysis with the builder, it chose a rendered masonry cavity wall structure with a truss rafter roof. Walls have two skins of recycled aggregate blockwork and a super wide (300mm rather than the conventional 75-100mm) insulated cavity with recycled glass insulation. This structural option worked out £77,000 cheaper overall than a timber frame.
For the 300mm wall insulation, Eco Arc specified Knauf DriTherm Cavity Slabs 37 & 32 on grounds of cost and ease of installation. This is used with Teplo basalt fibre wall ties that minimise thermal bridging.
‘We’ve done super-insulated buildings for 28 years, so it wasn’t a big stretch for us. The challenge was avoiding cold-bridging,’ says Andrew Yeats, principal partner at Eco Arc, adding that this was particularly true for the depth of the plywood cavity closer boxes around the windows. For this reason, these were changed from full to half depth.
Residents could chose from two roof designs, which required different insulation systems. An open, ‘cathedral’ style, design used a 350mm I-beam filled with recycled glass insulation blown in with a jet by a specialist installer to give a full fill around the beam. For this, the architect specified Knauf Perimeter Plus.
The other roof option was a bobtail truss with 500mm depth of Knauf Loft roll laid in criss-cross layers of quilts. On the underside is 12.5mm of plasterboard and skim-on battens on an OSB airtightness layer and insulation support. This had to be reinstalled in many of the houses after thermal imaging revealed that it had not been properly installed the first time. To minimise air leakage, it was important that residents understood there would be no loft hatches or access to the loft spaces. Additional storage was provided elsewhere on the site to compensate for this loss of capacity.
For the floor, Eco Arc used a combination of Jablite insulation products. Most of the houses have 300mm of Jablite Classic beneath the 150mm GGBS cement replacement concrete slab, which is itself topped with timber floorboards with Proclima airtightness tape at the junction with the walls. In Terrace E, however, the Premium product was used instead. The same depth of Jablite’s Premium expanded polystyrene gives an even higher thermal performance below the damp course in the walls.
GreenSteps supplied the triple-glazed, low-e timber windows and also the doors. The windows have 75mm of EPS expanded polystyrene insulation. Over this, 8-10mm thick Wetherby external wall render is applied to wrap around the 170mm deep window reveal and to the 175mm wide window surround detail, again to reduce thermal bridging. Proclima Contego airtightness tape is used internally around the window screw fixings. Windows – including the glazed doors – achieved 0.89 W/m2k air tightness.
Including site demolition and preparation, the project cost £5.4m at £1,825/m2. The housing itself came in at £1505/m2 – a ‘very reasonable’ price says Yeats, bearing in mind the long-term benefits of the 150kW biomass boiler, 40kW solar thermal and the 50kW PVs that this includes. Energy use for heating comes in at less than 15kWh/m2 a year, and the scheme has a 100 year lifespan.
According to Yeats, one of the main lessons taken from the project was the need to embrace the rigour of Passivhaus from the outset, and the benefits of working collaboratively with a committed client and contractor.
Lancaster Co-Housing this year won a Galvanisers Association Galvanising Award and was shortlisted for an RIBA Regional Award. Eco Arc is now using some of the designs and techniques employed at Lancaster Co-Housing in several of the many Passivhaus projects it has under way.
Client Lancaster Co-Housing
Architect Eco Arc
Project manager and quantity surveyor Turner and Holman
Structural civil engineer Ramboll
M&E engineer and certified Passivhaus designer Alan Clarke/Nick Grant
District heating system designer Pettit Singleton Associates
Passivhaus certifier WARM
CSH & Life Time Homes consultant Eric Parks
Main contractor Whittle Construction
Insulation Knauf; Jablite • Doors and windows GreenSteps Ltd • Timber roof James Jones Joists; C R Taylor Timber Ltd; Metsä Wood • Window insulation EPS by Wetherby render installer/supplier • Clay roof tiles Sandtoft Cassius • Roofing membrane Pro Clima Solitex Plus • Basalt wall ties Ancon/MagmaTech • Weatherboard cladding Marley Eternit Cedral Boards