The Changing Shape of Construction

Despite the government's support for modern methods of construction to meet the UK's housing needs, the conservative construction industry has been slow to give up what it knows works best and embrace new technologies and working methods. Richard Stirling visits the UK's largest offsite construction housing project to hear how this is changing.

MAKERS of Structural Insulated Panels (SIPs) have faced an uphill battle to convince architects that they can use their products to design attractive homes instead of plain boxes. They have also struggled to convince clients that SIPs will perform better than traditional construction materials during the lifetime of the building. Kingspan Century hopes to change the construction industry’s traditional and conservative approach to building and is supplying SIPs to what will become the UK’s largest housing project to use offsite construction methods.

The company is currently supplying its Tek panels to Lovell for The Way, an affordable housing scheme in central Manchester. The company has worked with architects to develop some intricate features in the 147 homes it has delivered so far, and has convinced Lovell that the extra cost of SIPs over traditional build will be recouped in the energy performance of the buildings over their lifetimes.

Lovell has just completed the first phase of the £75m scheme to build 550 homes. The scheme came about through a partnership between urban regeneration company New East Manchester, and is about to enter its second phase to build 223 homes.

Kingspan has supplied SIPs to build 80 of the 147 homes in the first phase, and is gearing up to supply the second phase of the scheme. Kingspan has had to accommodate some unusual features in the houses when planning its schedule of work. Architect Bowker Sadler has designed some innovative features into the homes, such as curved roofs and “TV-style” boxout windows.

“For a social housing build, it’s quite an interesting project,” says Kingspan national sales manager Jeff Tomlinson. “The housing association wanted something a bit different. It’s a landmark project for this area, and to get the funding in place it’s got to be a bit special. It’s got to have the wow factor.”

The economies of scale

Tomlinson says the prospect of incorporating curved roofs and cantilevered buildings into the design before the panels have even left the factory was at first quite daunting. “When the designs came in, I think everyone ran a mile,” he says. “But once we’ve looked at the designs we have an understanding of the volumes required. Designing and engineering isn’t that difficult because there are only about 12 house types in the development. Once the design is done we have to do panel layout drawings so once it gets to site everything fits.”

The panels themselves are made from two sheets of Orientated Strand Board, which sandwich a core of polyurethane foam. These “blanks” are manufactured in Germany, and arrive at Kingspan’s UK factories 4ft by 24ft sections.

To cut down on wastage, contractors create openings, such as doorways and windows by piecing different sizes of board together, as opposed to cutting them out of whole panels.

Panels are labelled and flatpacked in the factory and taken to the construction site where they are bolted together.

Tomlinson explains that contractors get faster at erecting the houses as the job proceeds. “Houses arrive in blocks, so a pair of semis are built together.” He says. “The first time a contractor builds a pair of semis it usually takes five weeks to complete the structural shell. But because of the repetition involved in the offsite construction, this comes down to about three weeks.”

To build the houses, the contractor has set up something akin to an automotive factory, where workers are set specific repetitive tasks such as putting together the floor joists or decking.

“By breaking it down the guys eventually don’t have to look at the drawings any more,” says Tomlinson. “That’s when you get the real speed and economies of scale. When you’re doing 120 houses, it then comes down to a fine art.”

Kingspan had supply-only contract for the first phase of The Way, and Lovells employed Chamberlain and Flood for the erection. The company employs a 25-strong team of full-time joiners, and Kingspan gave the firm inhouse training to erect the homes using its SIPs system. However, Tomlinson says, Kingspan plans to both supply and build phase two of the project itself.

Opening up new horizons

Some homes on the development are cantilevered to give a greater floorspan upstairs than they do on the ground floor. Tomlinson says this is one way that designers can benefit using SIPs over timber frame. “From an engineer’s point of view, it opens up new horizons for them,” he says. “It doesn’t quash the freedom of the design. We also have more timber in the product than there would be in timber frame.”

Timber frame floors rely on studs to take the vertical load, while a sheathing material takes the lateral load. However, the panel is a composite of different materials and all the loads are transferred along its length and bredth. This makes the homes in The Way feel very solid, with little or no give in the floors.

“One of the irritants in modern housing is you have got springy floors,” Tomlinson says. “We use I-joists, which guarantee to be squeak-free and the floor feels very solid. Most people struggle to differentiate between this type of construction and masonry because it’s a very solid building.”

The structure of SIPs also avoids settlement. In a timber frame building, this could be between 6mm and 12mm for each storey.

The design of the houses also makes use of the properties of SIPs when it comes to making space, and the houses feature high ceilings, which slope to give a light airy effect. Because the roof panel is already insulated, the designers don’t lose any space putting in extra insulation.

One aspect that does make SIPs desirable for architects to design with is their thermal performances, as they come well within the Uvalue of 0.27 required under the revised Part L of the Building Regulations.

Their low U-value contributes to the overall score for the energy efficiency of the house, which means SIPs compensate for other elements of the building. “This product scores 0.2 straight out of the box, which is 30% better that the requirement of the Building Regulations,” says Tomlinson. “When you have got over 22% glazing in a building these days, you have got to compensate the thermal performance in other areas. Because the Uvalue is so low, the windows have already been compensated for.”<

Weighing up the cost of SIPs

Flexibility and energy efficiency don’t come cheap though, and designers have to weight up the properties of SIPs against their cost.

Kingspan’s Tek product costs 10% more than constructing with timber frame. “You have got to be careful into which market you sell the product,” says Tomlinson. “If the client doesn’t want energy efficiency or reduced settlement and wants to build as quickly and cheaply as they can, then they would be better with another product. We pass these enquiries onto our timber frame business.”

However, he explains, the extra construction costs are dwarfed by the overall price the buyer pays for the house.

“Although this is a Housing Association project, all the doors are bespoke and the kitchens are good quality,” Tomlinson says. “What adds cost to a house is not what it’s made from, it’s the fixtures and fittings. There’s a £15,000 to £20,000 premium on the kitchen alone. If this was constructed out of masonry, you would save £1,500 on each house, but you wouldn’t have the air tightness or the energy efficiency you can achieve with SIPs.”

He adds that energy efficiency is a big factor when constructing for Housing Associations, and suits the requirement for “affordable homes”. “It’s important to remember that running cost is a big issue for people on low or zero incomes,” Tomlinson says. “The Housing Associations themselves can’t recoup the extra cost by charging more for an energy efficient home, but they do find it is easier to let so it stands empty for a shorter period of time.”

However, Tomlinson says the offsite construction industry still has a way to go before contractors and architects no longer associate prefabricating buildings for social housing with failures in 1960s council properties. He adds that SIPs tends to come under heavier criticism than traditional methods of housebuilding.

“The problem with MMC is that people are distrusting of it,” he says. “For most people who are in their late 40s and early 50s in the construction and design industries, there’s a doubt there and they tend to look for its Achilles heel.”

Tomlinson says meeting the tough standards for energy efficiency might be just what the offsite industry needs to attract designers’ attentions.

“Housebuilders are in for a kick over the next 18 months,” he says, “because when they start getting planning permissions rejected they will have to look at energy efficiency properly. We’re competing against old attitudes and ideas. We just have to convince designers to get out of the box.”