Contractor Osborne Homes is the only punter prepared to put money into both sides of this contest, which is taking place in the village of Sompting, near Worthing. The firm is building a timber-frame terrace of four houses just a few yards from a three-house terrace using a shell made up from SIPs – structurally insulated panels.
Let's take a closer look at the outsider's statistics. A SIP is a type of sandwich panel made from two boards called orientated strand board with an expanded polystyrene filling. Osborne has invested a lot of money and effort into these panels and has turned them into a whole-house system. The reason for that investment is that demand for timber frame is outstripping supply, and constraining the number of units that Osborne can build. If the SIPs prove themselves, the company will have widened its choice of modular systems. The firm is also hoping SIPs will be faster and cheaper than timber frame to build.
To develop the whole-house system, Osborne teamed up with SIP maker Vencel Resil in May 2002. SIPs are not particularly new; indeed, they have been used in the USA for 40 years. What Osborne has done is work out how to fit together the Vencel Resil product, called Jablite, to create a whole-house system suitable for terraced homes that is quick and easy to build and conforms to the Building Regulations. The partners have worked out how all the different parts of the building fit together and are detailed, and have trialled the solutions with full-scale mock-ups. "Most of the problems had been ironed out before we got to site," explains Colin Mitchell, Osborne Homes divisional director.
In February this year, Housing association Southern Housing Group agreed to Osborne using the whole-house SIP system, but only after the contractor promised to rebuild the three-house terrace at its own cost if the SIPs failed. The construction process is being watched carefully by timber research and promotions body TRADA and BRE, who want to see SIPS in action.
Building Life Plans will offer a warranty on the whole-house SIP system, provided it is up to scratch when completed this November – which means Osborne and Vencel Resil will be able to market it to other housebuilders.
The two systems were given an even start in the heavyweight contest. An insulated concrete base was constructed and mechanically smoothed for both homes. "We started with a familiar system that we have found the best for timber frame," explains Mitchell. "It gives us a hard surface to work from and you don't have to bring back any wet trades." The next stage was to put up the scaffolding for both terraces. And it's seconds out, round one.
It’s quicker, it’s better for the client, it’s simple and it doesn’t require new skills
Colin Mitchell, Osborne Homes
Unfortunately the SIPs team had several frustrating obstacles that slowed them down. Two late deliveries lost the team a day, and they wasted another sorting out lifting arrangements with the National Grid – there are power lines adjacent to the site. "We also had people checking every detail of what we were doing," points out Mitchell. The result: timber frame nine days; SIPs nine days. The two lost days could just as easily have struck either system, so if they are discounted that cuts the construction time of the SIPs down to seven.
"I think we could better that by 25%," says Mitchell. "We are aiming to do it in five days next time, but timber frame would still take nine." So, round one to the SIPs.
SIPs were very popular with the team working on site. "The construction techniques are not difficult to learn," says Mitchell. "Our erectors were delighted with the SIPs. The panels are rigid compared to timber frame, so there is less checking and adjustment needed to get it right." There is another clear advantage. "SIPs are very simple," says Mitchell. Design teams spend less time detailing the SIP design than the complex arrangement of timber frame studs making up each timber frame panel. Furthermore, each large SIP section is much easier for workers to identify on site then timber frame sections. That's round two to the SIPs.
Osborne believes the adaptability of the SIPs have made them a hit with its client, Southern Housing Group. The panels arrive with window openings already formed, but it is also simple to cut openings on site if necessary. "This allows much more flexibility," he says. "It gives you the advantage of being able to freeze the final design later on in the process. More time makes for better decisions. That's important for housing associations." Round three to the SIPs.
From the contractor's viewpoint the preference for SIPs is becoming evident. "It's quicker, it helps us meet our client's design requirements, it's better for the client, it's simple and it doesn't require new skills – we could even use less skilled people to put it up," Mitchell says. He also reckons it is more environment-friendly than timber frame, as the orientated strand board is made from waste timber, and the whole system, both strand board and expanded polystyrene insulation, can be recycled into their respective parts.