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Structural insulated panels

Feilden Fowles takes a fresh and rational approach to timber construction with its Ty Pren residential project, writes Felix Mara

London architect Feilden Fowles’ Ty Pren, a 170m2 four-bedroom house in Trallong, South Wales, was on site for 15 months, which means you could guess it was built at a rate of 2.6 m2 per week.

This is surprisingly slow for a building envelope constructed with SIPs (structural insulated panels), whose manufacturers trade on enhanced erection speeds. But this long construction period has more to do with Feilden Fowles’ thoughtful approach and commitment to research and high-quality craftsmanship. Fergus Feilden and Edmund Fowles began working on this project as RIBA Part 1 graduates, and their fresh and thoroughly rational approach tells us a lot about timber construction.

‘We spent two years researching and developing this design through site visits, models and prototypes’, says Fowles. ‘It evolved into a crisp extrusion, using skilled craftsmen to deliver a high-tech building.’ His comprehensive technical study includes a rigorous evaluation of the timber construction options, which included platform frame, balloon frame, solid timber panels and SIPs.

High standards in environmental design were top priority for both architect and client, so the project was largely performance-driven, although its context, on the edge of the Brecon Beacons National Park, was both a constraint and an inspiration. Feilden Fowles thought a pastiche of the local vernacular would devalue it and instead aimed for a subtle reinterpretation.

‘Ty Pren translates literally as “House of Wood”, as timber drove the design strategy throughout’, says Fowles, who worked for Hopkins Architects for two years on Kroon Hall at Yale University, the AJ100 Building of the Year (AJ 27.05.10). ‘Ty Pren was clad in larch, sourced and felled from the client’s estate two miles away, and subsequently milled on site.’ The untreated cladding has a predicted life of 25 years and, when removed, the plan is to burn it to heat the house.

Internally, locally sourced oak was used for the fit-out, and the entire north wall was constructed from sustainably sourced birch-faced plywood, articulated by oak studs, with shadow gaps running along the datum lines of the house. The pop-fit internal doors provide a seamless wall and Ty Pren’s 1.2m grid corresponds to standard SIP and sheet material sizes. The east, south and west ‘solar elevations’ have a filigree larch skin. ‘The use of SIPs coupled with high performance Danish windows [approximately eight per cent of the construction cost] has resulted in a super-airtight building.’

This enhanced airtightness, which is prioritised in the new Approved Documents for Part L of the Building Regulations (conservation of fuel and power) that will come into effect in two weeks’ time (summarised in AJ 01.07.10), could also be seen as the Achilles’ heel of SIPs – because they can’t breathe, the house relies on openable windows and mechanical ventilation.


ArchitectFielden Fowles
Architect’s appointment July 2006
Planning submission April 2007
Planning permission granted April 2008
Start on site July 2008
Contract duration 15 months
Gross internal floor area 170m²
Form of contract No building contract
Total cost £290,000
Cost per m² £1,700
Client Gavin and Davina Hogg
Architect Feilden Fowles
Structural engineer Momentum Engineering
M&E consultant Green Earth Energy
Main contractor Martin Forsyth Builders
Estimated annual CO2 emissions 5.5kg/m²

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