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The eco-house that Tommy built
Autumn 2007
Published:  30 October, 2007

Architect Steven Willis and Tommy Walsh on the roof of the eco house

Timber frame and masonry marry up in a new TV series on how to build an eco-home. Mike Jeffree reports

You’ve seen the TV show; now build your own affordable eco-house... That’s not actually the programme pitch for the new how-to series fronted by celebrity builder Tommy Walsh, but it is the underlying premise.

Walsh and architect Steve Willis have teamed up with Potton Ltd for the 10-episode Discovery Real Time Channel show which takes the viewer blow-by-blow through the construction of a low environmental impact hybrid block work and timber frame house. The plan is that the self-builder – or professional, developer or housing association come to that – can then buy the design or design and materials package, and create their own version of “Tommy Walsh’s Eco House”.

The thrust of the project was to come up with a construction approach that did not blind the viewer with eco-science. It also latches on to the government challenge to the construction sector to provide sustainable housing at a build-cost of £60,000. And to add symmetry and another viewer hook, the target was to build the house on a £60,000 plot in 60 days.

Visiting the site in a quiet side street of March in Cambridgeshire in August there were still a few loose ends. A bucket beside the loo, for instance, indicated that the rainwater harvesting and mains water ‘interface’ wasn’t quite tied up. Walsh and Willis were also still having good-natured disagreements over how design and construction methods might be tweaked for the final package to go on the market. But they were happy they had got the fundamentals right.

First, said Walsh, they had more or less hit their 60/60/60 targets on the schedule, site and build cost.

“The 200m2 plot cost exactly £60,000, the build cost was £60,450 and it took 61 days – plus a week on the garden,” he said.

Tommy Walsh says the house's eco credentials measure up
Willis acknowledged that there were some add-ons, which bust the budget. The flush-fit PV panel array was £10,000 and the air source heat pump £5,000. “But you could go for cheaper options; a conventional condensation combi-boiler and different PV panels, or none, and you’d still have an environmentally sound house.”

He also said they had met their goal of using building products that would not scare the horses.

“The idea was to use materials that are readily available at builders merchants, but in an innovative way.”

And really the aim of the project, he added, was to be ”eco-friendly rather than eco. To build a proper eco-house you’re talking £300,000-400,000, but you can be eco-friendly for a relatively minor budget. We’re aiming for sustainability, but also something that’s easily achievable.”

This rationale led to the choice of hybrid design, with a concrete block ground floor, timber frame first floor and timber-based room-in-the-roof structure. The team decided that approach gave the best blend of affordability and ease and speed of construction, plus it had a thermal mass solution built-in.

“We could have gone 100% timber frame, but we would have had to tackle thermal performance issues another way,” said Willis.

The ground floor, he explained, acts as the building’s thermal store. The walls comprise 215mm thermal concrete blocks, 50mm of Kingspan Kooltherm K5 EWB insulation and two coats of render. Travertine slabs were chosen for the flooring, with standard under floor heating pipes running beneath.

“Heating the fabric of the building rather than the air is more thermally efficient,” said Walsh. “And, in conjunction with under floor heating, an air source heat pump – which also provides our hot water – is four times more cost-effective than a conventional gas-fired boiler.”

The ground floor acts as the building’s thermal store
The Potton timber-frame walls on the first floor are 140x2380mm panels based on a frame of 38x140mm C16 kiln-dried CLS softwood. These comprise 15mm plasterboard, 50mm Kingspan Kooltherm K12 insulation, a 600-gauge vapour barrier, and a further 100mm of K12 between the studs, followed by an outside skin of 9mm Sterlingboard OSB and Tyvek Reflexive reflective breather membrane. Externally, the walls are part-rendered and part-clad in 15mm chestnut boards.

“The timber structure is super insulated so we haven’t got heating upstairs,” said Willis. “We’re relying on heat rising from the ground floor. To assist this we’ve left gaps under doors and have no extra acoustic insulation in the upstairs floor, which just comprises the 240mm structural Finnforest Finnjoist I-beams, 15mm plasterboard below and 22mm chipboard above. This gives good heat transmittance, while still complying with Building Regulations on sound transference.”

“We’ve put in unconnected under floor heating pipes upstairs as a ‘goal keeper’,” added Walsh. “But we don’t think they’ll be needed.”

Making the building even more energy frugal, its softwood windows use 24mm argon-filled, thermal glazing units, giving a U-value of 1.2.

“With all the components combined, including the PV panels, which can also feed power back to the grid, we’ve got a house, which should cost you about £250 a year for heat, light and hot water,” said Walsh.

Probably the key structural challenge of the building was the juncture between its two building systems.

“We had to work out how to fix and align the thinner timber frame wall to the thicker masonry,” said Potton technical manager Claire Roberts.

The timber frame structure means there is no heating on the first floor
The eventual solution was to fix an 8x3in pressure-treated timber wall plate flush with the edge of the masonry wall using restraint straps and then to nail a 90x240mm Kerto laminated veneer lumber rimbeam on this, offset outwards by 40mm.

“The 240mm Finnjoists of the first floor structure, which was designed for us by Finnforest, were then fixed to the wall plate and for added security also attached to the rimbeam with hangers,” said Roberts. “The result is a very secure, solid structure.”

The fact that the frame is set out over the masonry, she added, also makes the upstairs larger than the downstairs area.

Overall, said Willis, the Potton timber frame helped keep the project on budget and deadline. “It arrived as a kit, so all we needed to do was assemble it.”

Walsh acknowledged that the use of I-joists throughout the house might be a bit of a culture shock to the self-building fraternity, who are still predominantly solid timber users.

“But once they use them, they’ll see the benefits,” he said. “They’re light-weight, strong and simple to use. In particular they’re easy to drill to accommodate services, provided you put the hole in the right part of the web/flange.”

The split-level roof structure was also part of the Potton kit. It has 240mm Finnjoists supporting the larger area at the front of the house, with a 222mm-softwood rafter supporting the back and a window in the rear-facing wall between the sections. Currently the space is an attic, but it could be converted into a room.

Architect Steven Willis in front of the completed building
“We’ve used a steel beam to support the roof space,” said Roberts. “This is shallower than the equivalent strength glulam, which gives us the opportunity to convert the attic with no projection into the ceiling below.”

Externally, the roof at the rear is clad in zinc, matching gutters and downpipes, while the front has Welsh slate mounted on the Australian-made Nulock system. The latter comprises a steel frame, with narrow runnels between each row of tiles and, said Willis, provides a cost-effective way of using slate, as it doesn’t need to overlap. “The tiles are 15in long rather than the usual 20in, which means you use material that would otherwise be rejected,” said Willis. “Our slate cost just £1,200.”

On the aesthetic front, the plan is to leave the rendered walls as they are and also leave the UK-grown sweet chestnut cladding to weather.

“The chestnut doesn’t need any treatment,” said Walsh. “It’s leached a bit of tannin, but I quite like the look that creates, and with time the wood will just silver.”

How the final “Tommy’s House” that goes on the market will shape up remains to be seen. Willis said they were considering a “more modular approach” to parts of the structure and Roberts mentioned that Potton, now part of Kingspan, has access to the latter’s expertise in Kingspan Tek structural insulated panels (SIPs), so perhaps they’ll also enter the equation.

And Walsh believes the design and construction approach also lends itself to formats other than the two-bed house built for the programme, including three and four-bed versions. He even has a vision for a Tommy’s terrace.

Keywords: eco-house Potton Kingspan timber frame Finnforest I-joists