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8 January, 2010
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Fit & forget
Autumn 2009
Published:  22 October, 2009

Existing pitch-roofed, brick-skinned SMTS timber frame houses are more likely to be the model for Sigma II designs than the ‘eco-home’ of the future look of the BRE test-bed building

After intensive research, Stewart Milne Timber Systems’ maxim for eco-housing success is keep it  simple, stupid. Mike Jeffree reports

Among the striking features of Stewart Milne Timber Systems’ Sigma Home at the BRE’s Innovation Park are the three wind turbines on the roof, like giant five-spoked versions of the Mercedes logo. But one day passers-by looked up to see just two. A gust had torn off one of the rotors and deposited it in the car park four storeys below.

This incident alone may not have shaped the thinking behind Sigma II, the closed panel timber frame system SMTS is putting on the market, but it reinforced its view, after a two-year study of the original house, that reappraisal was needed of the role of renewables on the average individual home. The upshot is that the new concept  keeps some elements of its predecessor, but drops others as unceremoniously as the wind dumped the turbine.

Sitting in the living room of Sigma I – all pale timber and white walls – Stewart Milne Group’s product development director Stewart Dalgarno stressed that the prototype is not regarded as a disappointment. Far from it. It did much of what it was intended to do and overall stood up well to the intensive evaluation, which Milne undertook with researchers from Oxford Brookes University.
“We put it through some of the most demanding testing a building has ever faced and took over half a million readings, from temperature and humidity levels, to the number of times doors and windows were opened,” he said. “In some respects it did fail, consuming 40% more energy than anticipated, for instance. But, in terms of providing a healthy, modern living environment, it was a success and finding out where it fell short and why has given us a real commercial advantage for the future.”
Where the prototype also scored was in its use of offsite modern methods of construction, from its modular, pre-cast pile foundations upwards, enabling it to go from ‘grass to keys’ in eight weeks. It was also the first home to hit level 5 of the Code for Sustainable Homes (CSH).

The structure features glulam ring beams at floor zone and double height spaces, a steel portal round a large glazed screen, plus timber frame. The latter comprised Milne’s ‘Prototype Generation 3’ closed panel system, with 140mm timber studwork, 0.035W/m3 glasswool insulation and 9mm OSB3 sheathing, and the inside face including a vapour control barrier and self-adhesive taping. Outside, the house is clad in standard breather membrane, with render and ‘thermowood’ rainscreen applied to a board and vented cavity insulation system. 

The floors are clear span from gable to party wall and ceiling heights are designed to suit standard plasterboard and OSB sheet sizes to simplify construction further. To the same end, the roof used pre-insulated cassettes.

“Overall the house has a U-value of 0.15W/m2K,” said Dalgarno. “And triple-glazed timber windows helped achieved airtightnesss of 1.0m3./h/m2@50pa.”

Then came the shiny eco-house technology. Besides the wind turbines, Sigma I also included solar thermal and photovoltaic (PV) panels and a mechanical ventilation and heat recovery unit (MVHR). 
Putting the whole package to the test was not just a question of the researchers festooning the house with monitors. “The critical aspect was to have a family of four live in the house for two weeks each season,” said Dalgarno. “They were told just to treat it like home.” 

SMTS sales and marketing director Gary Yeoman said the guinea pig residents generally enjoyed their stay.  “They liked the open connection between living spaces, and they also thought the MVHR created a healthy atmosphere and appreciated aspects such as the passive solar stack, which proved very good for venting heat.”

In fact, when they left, added Dalgarno, the family “felt a genuine sense of mourning”. What they weren’t sad to leave behind, though, was the complex new technology.

“We had to set up a helpline for them to cope with all the buttons and knobs,” said Yeoman. “The conclusion was that we needed a K.I.S.S. – ‘keep it simple, stupid’ – approach. People don’t want to worry about how a house works. They want fit and forget solutions.”

Sigma also highlighted other technical issues with the renewables and not just the turbines. Regulating solar thermal outputs proved difficult and the system used went out of production “creating concerns about obsolescence”.

Barratt’s Green House, the eco-home built next door, also overshadowed the PV panels. “And that raises worries about solar rights in built-up areas,” said Yeoman.

The trial prompted questions too about having such an expanse of windows as Sigma I. “The family liked the light, but were concerned about privacy and there were also solar gain issues,” said Dalgarno. 

Having so many technologies in the house also threw up the question of how they interface. “There was a flood at one point and we had 10 contractors investigating to see who should deal with it,” said Dalgarno.

This sort of scenario will not arise with Sigma II,  which takes ‘fit and forget’ as its motto. “The focus first and last is the fabric of the building,” said Dalgarno. “What we learned is that getting the envelope right should be the priority before de-carbonisation through micro-renewables. And that’s the message we want to get to government in its pursuit of zero carbon construction.”

The key element in the Sigma II envelope will be OSB3-skinned 140mm closed timber frame panels using the Nordic ‘space stud’ system. “This decouples studs from the metal connector plates and allows the insulation to fill the cavity, giving a thermal bridging figure of just 0.04 with no loss of strength,” said Dalgarno.

To improve thermal bridging further, the system uses pre-insulated ring beams in the floor, while the greater stiffness of the wider space stud panels reduces the glulam needed in the mid floors and at double height spaces. 

The panel connections have lock joints designed in, with compressible foam seals fitted at the head and foot of the panels and sole plates. There is the option, too, of factory-made floor and ceiling cassette systems, which will  allow builders to undertake immediate airtightness testing –  and Milne’s trials give a figure of 3m3/h/m2@50pa.

Another innovation is the blown expanded polystyrene rigid insulation in the panels.  “This flows into every part of the cavity around the factory-fitted service zones and gives a U-value of 0.15W/m2/k,” said Yeoman.

In a further break from the first house, windows will be “proportionate to wall area” and Milne will offer them factory-fitted in the panels, complete with air seals. “In the first house, we still had three different guys on site involved in installing the windows,” said Dalgarno. “Sigma II is all about simplifying the build process and integrating various trades and suppliers into one package.”

The core Sigma II structure lends itself to a wide range of house designs (up to four storeys), but SMTS expects most customers to go for conventional styles, rather than the ‘eco-home of the future’ look of Sigma I. “We think they’ll mainly be brick-skinned with pitched roofs,” said Yeoman. “There is a market for a more futuristic style, but it’s not yet mainstream.”

SMTS is now presenting Sigma II to architects and developers and the first kits are due on a housing association site at the end of the year. Other projects in the pipeline, including a Carbon Challenge development and RSL housing in Manchester, should then lead to a total of around 500 units delivered in the next 12-18 months. 

The new houses will include MVHRs, with some including PV panels, but other than that, hi-tech micro-renewable energy gadgetry will be conspicuous by its absence. “Our experience leads us to question whether new homes should be individually provided with this technology,” said Dalgarno. “We think greening the grid is the better route to sustainable energy.”

And, he added, without renewables, Sigma II still achieves a very high CSH level 3  – with the next ambition being to nudge that to level 4. “This shows what you can do with a fabric-based building solution,” he said. “Achieving levels 5 or 6 is currently too complicated for most new homes, but if you can achieve level 4 just with the fabric, going to higher levels later with improved communal or individual renewables will be that much easier.”

So, says SMTS, Sigma II householders will have 60-year design-life, low energy, low maintenance, low worry homes – and, they might have added, no fear of low flying wind turbines.

Sigma I in the BRE's Innovation Park

Sigma I in the BRE's Innovation Park

Sigma II uses Nordic space stud timber frame panels which allow insulation to fill the whole cavity, radically improving thermal bridging performance.

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