Stewart Milne cracks code for sustainable homes.

TiC online talks to timber frame specialist Stewart Milne to find out how the Group's five-star sigma house cracks the code for sustainable homes.

THIS year’s Offsite exhibition at the Building Research Establishment (BRE) revealed the extraordinary lengths taken by the construction industry to achieve carbon-efficient housing.

The race is on to meet the Government’s affordable housing targets and reach its Code for Sustainable Homes (CSH) agenda. But what new materials are helping contractors to meet the grade?

Timber frame manufacturer Stewart Milne exhibited its four storey Sigma Home, one of the UK’s first five-star, near-zero carbon homes. It uses renewable energy by way of heating water from solar panels and electricity is generated from roof turbines and photovoltaic tiles.

“We wanted something that could be realistically taken into commercial production,” says John Slater, group managing director of homes for Stewart Milne. “Many of the houses and products showcased at Offsite were exemplars of design but were one-offs. I think what was interesting about Sigma, is that people could actually visualise themselves living in it and builders could see themselves building it.”

Cracking the code affordably

Developed by the BRE, the code will takeover the requirements of the current EcoHomes System. In doing so it will become the national standard for sustainable homes.

The CSH will assess the sustainability of a home by awarding points in nine design categories. Points achieved from each category are totalled, and the total translated into a star rating for the home. There are nine categories ranging from water use to waste generation.

So can the Sigma home be produced affordably? “Certain elements of it, absolutely,” says Slater. “What I think is achievable is to get to code three and four of the CSH affordably. However, we are going to need to engage with our supply chain to ensure that we can get the raw materials at an affordable price because they’re quite high at the moment.”

Slater admits that as soon as you consider codes five and six, you enter micro-renewable energy, which can be challenging, he says. “We need to be considering things such as combined heating systems and those that are most deliverable. The cost of putting wind turbines, or photovoltaic panels or a mixture of all of those items in the home at once becomes challenging. If you put one item in, it becomes a lot easier.”

Thermal mass

The structural build-up for the walls of Sigma uses an advanced closed-panel timber frame, timber rainscreen cladding, and glass wool insulation between the studs. Slater expains that the recent technological advances in insulation for light-weight construction has solved the issue of thermal mass in low-intertia structures like timber frame.

The concrete lobby continues to laud thermal mass on the back of last year’s Arup study, in which it concluded that a timber frame home would emit 15 tonnes more CO2 than a masonry home over a The concrete lobby continues to laud thermal mass on the back of last year’s Arup study, in which it concluded that a timber frame home would emit 15 tonnes more CO2 than a masonry home over a period of 60 years.

Although thermal mass is not directly linked to the CSH, Stewart Dalgarno, managing director of Stewart Milne for England and Wales says the two can be related. “Thermal mass isn’t mentioned in the CSH, what it sets out clearly is the carbon emissions required through the energy aspects of the CSH. How you do it is not defined. That’s up to the industry to come up with ways to reduce emissions.”

SMG’s Sigma home uses chemical manufacturer DuPont’s innovative Energain phase change panel, a tool designed to dramatically increase thermal performance. “Some of our competitors in traditional construction talk about thermal mass as being one of the problems with timber frame,” says Slater. “By using the DuPont product and looking at different technologies then we’re able to identify a product that can perform as well as any other product from a thermal mass point of view.”

Phase-change materials (PCMs) are substances that melt and solidify at certain temperatures and are capable of absorbing and emitting large amounts of energy. In that way they behave much like ice cubes in a drink. They absorb large amounts of heat when melting, effectively keeping the drink cool for a long time.

DuPont’s Energain product is a paraffinbased material placed between two sheets of aluminum and fixed behind standard drylining. “The paraffin wax is incorporated into a copolymer which stops the product from leaking,” says Dr Loic Rolland, DuPont technical programs manager. “In terms of thermal performance, the research undertaken by the architectural and building professions suggests that the most comfortable temperature in a room is 22°C and so the copolymer is designed with a melting point of 22°C. The material is designed to absorb heat at that temperature and then begin releasing it again at 18°C.

“It has been a product we have been very pleased to engage with and consider,” says Slater. “We’ve had very positive feedback from the product so far.”

Dalgarno admits that the thermal mass and lightweight construction debate will go on for some time. But, he says, it’s only picked up speed since timber frame has become more prominent in the market place. “Our solution to the CSH is quite clearly looking at the performance of the fabric of the building, which is first class in timber frame. In the case of Sigma you can build airtight buildings in a factory and connect the panels simply to achieve an airtight performance,” he says.

Test bed

Dalgarno explains that the next stage for the sigma home is to use it as a test bed to monitor it in terms of air quality and thermal comfort levels. “We’ve just let the Post- Occupancy Evaluation (POE) contract to Oxford Brookes University,” he says. “A family is going to move in during Christmas and New Year. We’ll have one going in the wintertime and the same family back in the spring, autumn and winter so we’ll monitor the cycles, comfort levels and air quality levels.”

Dalgarno says that once the results of the POE are complete Stewart Milne will have far greater confidence to begin developing the first Sigma villages.

“We’re actively looking at sites now and looking at how we can put these homes down,” Dalgarno says. “We’re looking at between 30 and 40 in each village.” The development will support the group’s growth agenda with the villages pigeonholed for the South West and the South East of the country. “A lot of people at the BRE exhibition saw a glimpse of the future,” he says. “We are now tasked with delivering it.”