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EXCELLENT STANDARDS WITHIN COST LIMITS

TECHNICAL & PRACTICE

Designing more sustainable social housing may incur extra costs, but these can be affordable. It's a matter of where you invest resources, as three housing projects by Jon Broome Architects for Greenoak Housing Association show.

These projects - in Woking and Normandy in Surrey, and Storrington in West Sussex (which is yet to start on site) - aim to go beyond the EcoHomes Excellent standard yet remain within Housing Corporation cost limits.

The Woking site is a former housing-maintenance depot on the edge of an estate, comprising 14 two-, three- and four-bed family houses. The scheme was completed in March 2005 and is two-storeys high, rather than the three storeys that Jon Broome Architects hoped to gain permission for.

The rural development in Normandy, comprising 12 affordable homes ranged either side of a straight cul-de-sac, is also at lower density than sought. Terraces would have proved more compact and energy-efficient, and allowed greater density of development, but the planners insisted on semis. This scheme was occupied in summer 2006.

In Storrington, a block of bedsits will be replaced by 12 two-, three- and four-bed houses for Greenoak, plus 12 one- and two-bed apartments for use as temporary accommodation by the council. Construction on this scheme is due to start in July 2007.

On each scheme, two or three dwellings are to be adapted for wheelchair occupants. Car parking is on-plot where possible, with canopies for wheelchair access. There are also substantial stores, including space for bicycles. Porous paving and soakaways for roof-water drainage eliminate off-site water disposal.

TIMBER-PANEL CONSTRUCTION

The construction process is separated into more or less discrete stages carried out by specialist subcontractors: piling, timber frame, mechanics, electrics, plasterboard lining and finishing.

Like the Segal approach to construction that Jon Broome used for his own home (AJ 25.08.94), these dwellings come out of the ground on piles, their lightweight construction avoiding ground beams and the clearance of site spoil.

For Woking, wall, oor and roof panels were to be made off-site, pre-glazed and insulated with fixed breather membranes.

But the recycled-paper insulation was ultimately injected on site because of concerns about settling or damage during transport.

Windows and membranes were also fixed on site.

At Normandy, the practice attempted to follow the American stick-building tradition - a carpenter turns up with a truck of timbers (sticks), and nail-guns them together into a frame.

But the proposal of a tented site factory was not pursued and the usual waste and weather damage of on-site construction ensued.

This was exacerbated by the subcontractor carpenters, whose personnel changed. This meant that houses were built slightly differently, requiring differing solutions to integrating services.

Broome is confident that the panel industry has matured, and plans to use prefabricated timber panels again at Storrington.

DESIGN EVOLUTION The three schemes embody ideas on many fronts, rather than one big sustainability masterplan. These include:

? Adaptability. Jon Broome lays great emphasis on adaptability in the pursuit of sustainability, and it is an aspect he feels is much neglected. All three schemes are designed to Lifetime Homes standards ( www. lifetimehomes. org. uk). In construction, the timber system creates a perimeter box with oors spanning the full width of the dwelling. There are therefore no intermediate structural partitions to inhibit future replanning. In the larger houses, the upper oor and roofspace form one volume, with extra mezzanine platforms for storage and sleeping. Internally, the wall panels are counter-battened to address cold bridging by the main timber studs, and to provide a 50mm-deep zone for installing services and modifying them in future. Vertical runs can be worked into the insulation behind the battens or drilled through them.

Daylight and heat gain. Relatively large window areas, predominantly on the ground oor, provide good daylighting but inevitably increase the risk of overheating. This is countered by open ground oors, including open stairs, for improved crossventilation, as well as daylight distribution. Houses with a combined upper oor and roofspace also have rooights, allowing some stack-effect ventilation. All upperfloor-room windows benefit from exceptionally deep eaves, which provide some shading.

Airtightness. The built schemes achieve a high standard of airtightness - a combination of careful design, clear contractual responsibilities and good on-site training and supervision. At Woking there is a breathable paper, with taped joints on the inside and another breather membrane on the outside. The latter membrane was considered unnecessary at Normandy, and, indeed, test results were better there. Windows are sealed to the internal breather membrane with interior mastic (greener than exterior).

All this was part of the timber-frame contractor's responsibility: delivering a weathertight and airtightness-tested envelope. Initial results were exceptionally good at 1.2m 3/m 2/h compared with the Building Regulations' minimum standard of 10m 3/m 2/h. Following services installation, airtightness was tested again before plasterboard linings were fixed. Inadequately sealed gaps around service penetrations doubled the leakage to 2.5m 3/m 2/h, which was still within the target of 3m 3/m 2/h. This demonstrates the need for commitment and training to be passed down to subcontractors.

Controlled ventilation. Airtightness can be wasted through uncontrolled ventilation heat losses. The houses have main entrance lobbies and whole-house ventilation systems running continuously, plus kitchen boosts. Intermediate oors have open-web beams for running air ducts. At Woking, moisture-sensitive inlets and outlets respond to occupants' space-use. At Normandy, the system is made more energy-efffcient by including ventilation heat recovery.

Initially some complaints were received about noise from heatrecovery units. Systems were purchased in the UK but were in fact imported from Germany, and there was little UK installation expertise - as Broome says, this market is maturing slowly.

Whole-house ventilation with heat recovery will be used at Storrington.

Low maintenance. Externally, through-coloured lime render is used, which avoids painting, plus aluminium-clad softwood triple-glazed windows, untreated timber cladding and clay roof tiles. Using render at full-height prolongs the time that scaffolding is on site, so at Storrington the design is for groundfloor render with untreated timber cladding for the first oor.

PVC and other pollutants. Dwellings are specified as PVC-free throughout, so lino was used, plus polyethylene waste pipes above ground and clay below for drainage. Coated galvanised-steel gutters and rainwater downpipes are incorporated, plus low-smoke, zero-halogen cable insulation. Natural paints, stains, varnishes, mastics and adhesives are used throughout.

Water. Low-water-use WCs, spray taps, water butts and groundoor showers are used, plus on-site management of rainwater.

Domestic waste. There are three-compartment under-sink bins and external spaces for storing segregated waste.

Lighting. Low-energy sources are used for pendants, wall fittings, downlighters and bulkheads. In hindsight, the practice decided there was too great a variety of sources, making replacement difficult for occupants, so rationalisation is planned for Storrington.

Timber. The chief difficulty, for the EcoHomes assessment, was obtaining chain-of-custody certi-cation from suppliers and contractors about the sustainable management of timbers, but Broome observes that the industry is getting better at this.

TO STORRINGTON Measures planned for Storrington, such as whole-house ventilation with heat recovery and a reversion to large timber-panel construction, will be carried through with explanation and training. An early EcoHomes day workshop for Woking and Normandy provided initial design assessments, and this is to be repeated to help set the final agenda for the Storrington scheme.

With falling U-values and controlled ventilation with heat recovery, the need for (and cost of) whole-house wet central heating is questionable. Local emitters placed on the ground oor are being considered for Storrington, along with solar panels to provide some water heating. Otherwise, a bigger push into renewables is not yet cost-effective.

At Storrington, the practice aims to use a higher proportion of recycled and local materials, such as locally grown chestnut for joinery and cladding. Recycled demolition materials from the building which formerly occupied the site are to be used for fill and road bases.

As some brief performance monitoring has illustrated, one household's electricity use can be four times that of another, even for the same building design. Involving future occupants in the design evolution could help, but for these projects tenants are unknown before completion. We all know that occupant monitoring and feedback is useful, though funding it is difficult.

For these three schemes, client Greenoak has a grant from the Housing Corporation for information dissemination and site visits.

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