Summary
• Sigma II Build System has BBA certification.
• Its closed panel wall solution is available in solid stud or C-stud versions.
• The C-stud was developed and is manufactured in-house.
• The wall panels are designed to achieve a U-value as low as 0.1W/m²K.
Sigma II Build System, the low carbon construction system developed by timber frame fabricator and builder Stewart Milne Timber Systems, is about to come of age.
While the system has already been specified for many projects where high levels of energy efficiency combined with speed of build have been the main criteria, its profile has been relatively low. But recent certification by the British Board of Agrément (BBA) has been the catalyst for rolling out the system to a wider range of sectors – it will be formally launched at Ecobuild in March.
Fabric first
Sigma II is an example of the “fabric first” principle of achieving high energy performance ratings through the building envelope, rather than through bolt-on renewables. This is very evident in its closed panel wall solutions, which are available in two specifications, a solid stud and a “C-stud”.
From the outside in, the prefabricated closed panel comprises reflective breather paper; 9mm OSB sheathing; solid or C-stud timber studs; bonded EPS bead insulation; 9mm OSB sheathing; and 25mm service zone strapping. The site-fitted outer leaf can be brick, or a lighter weight option, like timber cladding; and the site-fitted inner face can be 15mm foil-back plasterboard or 35mm thermal laminate comprising rigid PU insulation and plasterboard.
No sealing tapes are required on site as gaskets are fitted around the perimeter of the panels, which click together.
“The solid stud, which is available in 89mm or 140mm options, adopts conventional timber, skinned either side with OSB,” said Stewart Milne Group director of product development Stewart Dalgarno.
As it offers U-values more likely to be required for apartment buildings and commercial applications, it is well suited for these types of buildings. The wider C-stud (195mm and 235mm options) comes into its own where a higher thermal performance is required, such as in Code for Sustainable Homes (CSH) level four and five housing, or where single wall build-ups are being used.
The C-stud, which was developed by Stewart Milne Timber Systems, comprises two pieces of timber connected by a sheathing board, currently OSB, although the company is looking at alternatives that would provide adequate strength but at lower cost.
British timber potential
Using smaller sections of timber – 63mm and less – results in a 23% reduction in wood content over a conventional 140mm stud and could open the way for British-grown timber.
“The C-stud isn’t too different from an I-beam but it’s a lot more cost-effective for us than metal webs or I-beams as we can produce it in-house and value engineer the component to suit end application,” said Mr Dalgarno.
With the C-stud the company has engineered a fit-for-purpose structural element that it can make to any width it wants, he said. “It’s an endless production process – we just chop it to length and fabricate it into the panels.”
Another benefit of the C-stud is that because it has a thermal break in the middle, it has a very low thermal bridging factor.
“We focus on getting a balance between a reasonably good U-value, a good thermal bridging value and a good airtightness value. Those are the three dynamics you have to get right and it’s why we’ve gone for an integrated build system approach,” said Mr Dalgarno.
Wall insulation
The wall panels are designed to achieve a U-value as low as 0.1W/m²K and the insulation in both the solid stud and C-stud versions is fire retardant expanded polystyrene (EPS) beads with a lambda (thermal conductivity) value of 0.032.
“We chose EPS because it has improved thermal conductivity over fibre; it’s BBA-certified; it’s weather resistant so you can store it outside without any additional protection and it’s got a 60-year durability assessment,” said Mr Dalgarno. In addition, he added, because the beads are blown into the panels along with a water-based polymer glue, they bond together hard so there are no problems with slump.
It’s important to note that the wall solution is just one element of the Sigma II Build System which also comprises a choice of two roof solutions – trussed rafter or cassette – and a range of floor cassette solutions in open, closed and insulated formats.
Junctions between these elements are critical. “We know what the thermal loss is at all of these junctions and are engaged with Constructive Details Ltd [a joint venture between the BBA and Robust Details Ltd] to get those Psi values accredited,” said Mr Dalgarno.
“We’re about to do a Passivhaus scheme with Sigma II and are targeting an airtightness of 0.64/hr@50 Pa,” he added.
“The Sigma II Build System has allowed us to erect a home in a day, insulated, weathertight and airtight ready for testing,” said Mr Dalgarno. “This helps build in assured fabric performance and quickens the construction time-frame, leading to reduced development costs and a faster cash cycle.”
