he 16 semi-detached houses and 12 sheltered apartments have been developed on a 1.5 acre site by Registered Provider Southern Housing Group, working with PCKO Architects, Passivhaus Designer Warm: Low Energy Building Practice, Contractor Stoneham Construction Ltd, Structural Engineer Conisbee and Employer’s Agent Calford Seaden.

The £4.2m project’s key features are:

All 28 homes have been built to Passivhaus standard – currently 4 are certified, the remaining certifications are imminent

Homes are built using a solid thin-joint aircrete frame construction

The scheme has provided learning opportunities in design and construction of Passivhaus. This was Stoneham’s first Passivhaus project, while this was one of two debut Passivhaus projects for PCKO

The site had previously been occupied by a Southern sheltered housing scheme which was at the end of its life. The site is bordered by existing buildings to the east and by playing fields to the north and west, with the new homes being approached via a cul de sac. A pedestrian route crosses the site.

Stoneham was one of six contractors invited to participate in a two-stage tender process.

Andrew Hulmes, Area Development Manager of Southern Housing Group, comments: “We did not set out initially with the intention of developing Passivhaus. But like others across the industry, the Group was considering how to respond to the zero carbon agenda and so we looked at a fabric first approach, because we wanted to get away from the use of micro-renewable technologies to meet the higher levels of the Code for Sustainable Homes. This wasn’t innovation for innovation’s sake – it was a means to an end.”

He continues: “By the time we had decided to go down the Passivhaus route PCKO had already done the design. It was stepped and staggered and wasn’t suitable for Passivhaus.”

Andrew Ogorzalek, Director of PCKO Architects adds: “We spoke to the Passivhaus consultant and between us we decided that there was no point taking forward the scheme we had – it was a courtyard design and so had extensive external walls and that is something that needs to be limited for Passivhaus.”

The houses are semi-detached rather than terraced, which would be the most efficient form for Passivhaus design. Andrew Ogorzalek adds: “They couldn’t be terraced because we needed to keep an open aspect into the field and the trees. We tried to make as much attached wall as possible to comply with the Passivhaus standard, and that resulted in the creation of quite deep units.” Homes are oriented to provide a feeling of security while maintaining privacy.

Karl Parsons, Passivhaus Designer at Warm: Low Energy Building Practice comments: “The semi-detached house is not ideal for Passivhaus. The scheme has a very distinctive aesthetic and that drove us to push what could be achieved in terms of thermal performance, so that Passivhaus compliance could be achieved. We’ve created a Passivhaus compliant scheme but not lost sight of the architect’s vision for the site.”

Houses are two- and three-storey units ranging from two to five bedrooms. The apartments are all one-bed self-contained units, with the living space incorporating a flexible folding partition to allow for the creation of a temporary guest bedroom for a carer or visitor. The varying sizes of the houses and the flexibility of their plan are designed to encourage a sustainable, cohesive community. Houses have front and back gardens while apartments have landscaped kitchen gardens.

The homes are constructed using H+H’s Thin-Joint system, which combines aircrete blocks with Celfix Thin-Joint mortar.

Karl Parsons explains: “There is no need for Passivhaus certification for a type of blockwork per se – it is like timber, an element that is part of the structure. In some ways block is more straightforward to get airtight.”

Andrew Ogorzalek concurs: “Thin-joint proved to be a very smart method of construction. It provided a clean, nice, fast build.”

David Harris, Director of Stoneham Construction adds: “In our view, Passivhaus is made simpler when constructed from masonry, rather than timber frame. We worked very closely with H+H who were exceptionally helpful. At the time of project planning aircrete blocks were in short supply but H+H pulled out all the stops to ensure that our important project was supplied. We had not used Thin-Joint before but that did not pose any problems. Passivhaus is all about air tightness so if you can reduce the gap between blocks to 2mm instead of 10mm it makes life a lot easier.”

Block laying began in January 2014. The blocks were laid by two gangs of four bricklayers, and Stoneham Construction had two hand saws on site, “to ensure the blocks were cut millimetre perfect” says David Harris, who estimates that the Thin-Joint blockwork went into place 20% faster than traditional block work.

The external solid walls are insulated with phenolic insulation boards and finished externally in white render, a fairly common aesthetic for Isle of Wight homes.

Internally, the block walls have a parge coat, then a services void and finally a plasterboard lining. Karl explains: “This solution gives a speedy internal finish and is a robust way to integrate the services while maintaining airtightness.” Separating walls were formed of two 100mm leaves of thin jointed aircrete.

Another innovative aspect was the use of retrofit joist hangers for the internal floors. This avoided the need to cut blocks or penetrate the walls. This removed any concerns with regards to air tightness performance and sound transfer. All units achieved below 0.6 air changes per hour at 50Pa (n50 tests).

Challenges of Passivhaus

Speaking of the challenges, Andrew Hulmes comments: “We made sure the contractor was on board at the start of the process. A lot of housing associations have built to Passivhaus principles, but have not gone down the full route of Passivhaus certification for all homes – it has meant we have had to take great care with processes and materials.

“There are several elements of this project that we would happily and readily import to other developments irrespective of certification, primarily the specification of H+H’s Thin-Joint System and the mechanical ventilation and heat recovery system.”

Andrew Ogorzalek says of the Passivhaus standard: “It is very stringent and can be quite marginal – avoidance of cold bridging and air tightness are so critical. We’ve taken this very seriously from the point of view of detailing and execution on site. We did a lot of drawings. The project was helped by having a great contractor.”

Karl Parsons: “We had some quite technically challenging details to deal with, because of the semi-detached form and the particular aesthetic. We also had to look quite carefully at overheating. The external sliding shades are a nice feature, but it was a challenge to integrate them into the building.”

Financial outlay

Andrew Hulmes concludes: “We were aware that going down the Passivhaus route would cost more but saw benefits internally. The extra capital cost had the potential to be offset by lower running costs for tenants, which would benefit us in helping tenants to afford to pay their rents and keeping tenants happier and in their homes for longer, and that in turn would reduce voids for us. Isle of Wight Council played an important part in providing some grant aid to the scheme, as it was consistent with their ambitions.”