Conceived at Ecobuild in 2010, and born at the show this year, the StramitZED ‘affordable’ ecohome concept is now set to take its first steps into the market. By the time it’s a few years old, its ambitious parents hope it will be producing thousands of offspring a year.

StramitZED aims to provide Code for Sustainable Homes (CSH) Level 6 housing at a comparable price to standard non-zero carbon masonry build and it’s the fruit of a joint venture between Bill Dunster’s ZEDfactory and straw insulation board specialist Stramit UK.

The former probably needs less introduction. Describing itself as a “leader in zero carbon architecture”, it first hit the headlines in 2002 as the creative force behind the BedZED sustainable living development in London and now ZEDfactory office in China.

No resins
Stramit to date has been less high profile. The UK company actually makes the 90m lines for producing strawboard, but also imports the material from a French company using its technology, in which it has a stake. The board itself is a no-added ingredients product. The straw (derived from wheat or rice stalks) undergoes a patented process, first developed in Sweden in the 1930s, combining heat and compression, which produces physical and chemical changes that bond the fibre without the need for added resins.

The finished product, which is made from 35-60mm thick, is then cut to size and sealed in craft paper. It’s billed as an alternative to plywood or plasterboard and, in StramitZED houses, made into highly insulating panels or cassettes. These comprise two Stramit sheets attached to a timber frame infilled with Warmcel or sheep’s wool insulation, the inner face impregnated with water-repellent Surfapor from NanoPhos, the outer clad in a Tyvek waterproof breathing membrane and airtightness barrier.

According to Stramit UK managing director Bruce McVicar, the board can be sawn, glued, nailed or drilled and finished with render, plaster, paint or other decorative covering. It also boasts good fire resistance and acoustic performance. Most importantly for the StramitZED project, its thermal performance in its own right meets the CSH “Heat Loss Parameter” and it scores big-time on carbon. It’s high on sequestered CO₂, absorbed during the raw material’s growth, and minimal on embodied due to the low energy production process. Dunster himself extols it as the “ultimate zero carbon, low environmental impact material”.

“It has the best carbon sequestering profile in the Green Guide, at 63kg per m²,” said McVicar.

Joint venture
But why the marriage between architect and building products company in the StramitZED joint venture? Why not stick to the conventional specifier/supplier relationship, with Stramit simply providing strawboard to ZEDfactory projects?

“We started talking with ZEDfactory at Ecobuild in 2010 and they described how they work closely with materials suppliers so their buildings make optimum use of the product, making its properties integral to the design,” said McVicar. “That chimed with our view that you derive maximum benefit from Stramit by designing with it in mind from the outset, rather than adding it as a generic. It’s also another sales and marketing route for Stramit. Each StramitZED house represents a value-added sale, then provides a flagship demonstration of Stramit’s properties.”

The StramitZED concept emerged from ZEDfactory’s earlier RuralZED design, which is based on a laminated timber barn frame. A development using the latter in Northamptonshire was billed as the first in the UK to hit CSH Level 6, but Dunster acknowledged it was too expensive for current market conditions. And the use of Stramit in a stud frame itself plays a key role in bringing down the cost of StramitZED, reported to save an estimated £15,000 per unit over alternative solutions.

“StramitZED has been designed to start at £1,000 per m² for a 4-bedroom house,” said McVicar. “That makes it very competitive, with the average conventional build CSH 3 house ranging from £900-1,100 per m².”

The structure
The core of the structure, he explained, is a solid softwood timber stud frame, with C16 material for most of the structure and heavier-duty C24 for main supporting elements and the ring beam on each storey. For convenience the timber for the demo building at Ecobuild this year, which featured cut-aways to show visitors how it worked, was bought from a merchant and probably came from a range of sources. But to keep the supply chain carbon footprint to a minimum the plan is to use only home-grown material, from Wales and perhaps Scotland, when StramitZED houses are built for real.

“The ultimate aim is to engineer the frame so that we only need to use C16 – the grade most readily available in the UK,” said Dunster. “This could involve the Stramit cassettes playing more of a structural role in the building. Currently they provide racking strength only.

Screwpile is the foundation type of choice for StramitZED as it minimises concrete use and further enhances its houses’ green credentials, but it accepts that terrain may dictate other types. Terracotta floor tiles and ceiling bricks will give the buildings added thermal mass, wood pellet burners will provide the “minimal” winter heating required and Rationel windows wrap up the energy-efficient performance of the envelope.

Bruce McVicar (right) explains the StramitZED concept to government chief construction adviser Paul Morrell at Ecobuild

The ventilation/heat recovery system is intended to be predominantly passive, including a roof ridge vent and Velux roof windows. “But that may be reviewed and MVHR used in parts of the country where wind speed is insufficient,” said McVicar.

ZEDroof
The other eye-catching feature of the StramitZED house is its ZEDroof, with one whole roof plane comprising solar electric and photovoltaic panels from Himin, the giant producer ZEDfactory has worked with extensively.

McVicar acknowledged that this slab of solar panels was “quite in-your-face” and may prove “too much for some consumers to live with or even overlook”. “But this represents one of the value changes in construction aesthetics we must all go through to achieve sustainability,” he said. “The roof will not only provide heat and power for the house, but enable the occupant to sell electricity to the grid and benefit from the feed-in tariff.”

Externally, he added, the walls could be conventional, even brick clad if customers wanted, although for ease of CSH-compliance and cost, they’ll be “encouraged” towards natural materials, such as timber or render.

The StramitZED prospectus presents two, three and four-bed detached variants, in north-south and east-west configurations, plus semi-detached, terrace and integrated urban-block formats. And it says it can also work with other architects’ designs.

Construction options
There are also construction options; turnkey design and build; a self fit-out package, where a StramitZED team erects only the core structure; and a ‘flat-pack’ kit alternative, where customers use their own builders.

StramitZED is initially targeting the private sector, but that may change. “The price level is not currently where the public/social housing sector wants to be,” said McVicar. “But further on there may be ways of taking out cost, maybe even supplying a CSH Level 5 version, if that’s what the customer wants and they could otherwise only afford CSH Level 4 in other build systems.”

The self-build market is also of interest, although McVicar hesitates to recommend StramitZED as a wholly DIY product. “We see it more suited to the self-builder working with a contractor or QS engineer.”

At Ecobuild, where the StramitZED team had just four days to build the demo house as opposed to the 10-12 weeks construction time expected in the field, interest came from across the market. The upshot to date is several client projects which are now at design and planning. The first, a private house, is expected to be completed by the year-end. A number of bigger developments should follow soon afterwards.

The company is also in talks to build a house at the BRE Innovation Park, a showcase and test-bed for eco-homes and other modern methods of construction buildings.

And if all goes according to plan and interest continues to build, it could lead to the construction of a UK strawboard plant. “It would be a facility primarily for producing Stramit, which we’d also supply to other UK customers, but possibly prefabricate the cassettes too,” said McVicar. “We’re probably looking at around 2015 for this and, ultimately, see prospects for it providing material for 3-5,000 StramitZED houses a year.”

The StramitZED prospectus presents various configurations and the company can also work with other architects’ designs

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