SIPs: Faster, Greener, Warmer?

ABOVE: The owners of this house by Build It Green (buildit-green.co.uk) chose SIPs simply for their fast construction time. The resulting house is highly energy efficient and requires mechanical ventilation as it is so airtight

SIPs stands for ‘structural insulated panels’, a method of building that is sometimes likened to timber frame without the timber. Like panelised timber frame, SIPs are made in factories, brought to site on the back of a lorry and then hoisted into position on site using a crane. Where SIPs differ from timber frame is that they gain their strength not from any timber skeleton but from the rigidity of the panels themselves. The panels are essentially a sandwich: the meat in the middle is a solid thickness of insulation, and the bread wrappings are made of rigid building boards such as plywood or orientated strand board (OSB). These layers are all bonded together and this has the effect of making the panels extremely strong and robust. In many ways, the technique is similar to how aircraft wings are designed, with two skins wrapped around a lightweight core and the whole welded together to form a single element.

As with all build methods, SIPs cannot be all things to all men — they have both their advantages and drawbacks, and the decision of construction system should be made by weighing up what is most important to you, be it cost, speed, energy efficiency or perhaps the level of hands-on work possible. So read on to decide if SIPs are right for your project.

The Advantages

• You create a structure with superb insulation levels, few cold bridges and excellent air tightness levels.

• SIPs have the potential for even faster construction speeds than timber frame, as there is less on-site work involved.

• The panelised roof elements also lend themselves to building rooms in the roof, in a manner which is much simpler and quicker than traditional methods.

• Having the insulation built into the system is an advantage because it ensures a level of quality control. This is particularly true on roofs where it is increasingly difficult to incorporate insulation into complex roof structures.

• You can build narrower external walls. This is particularly beneficial on small sites where space is at a premium. With a lightweight wall cladding, such as timber board or a rainscreen, you can build an external wall, still easily exceeding Building Regulations insulation standards, in a width of around 250mm./li>

• Perhaps the biggest advantage is that SIPs offer a relatively simple way of constructing a home which requires very little in the way of space heating. A number of self-builders have now specified homes without conventional central heating systems, relying on the insulation and airtightness of SIPs, combined with a mechanical ventilation system with heat recovery which is used to distribute the heat of a woodburning stove around the house. Supplementary heating is usually specified in just the bathrooms.

"We built an ultra-efficient house"

Joan and Richard Barker built a SIPs house out of necessity. On a tight plot, their East Yorkshire village had no mains gas and there was no room for an oil tank in the garden. So they came to the conclusion that the best option was to build a house so well insulated that it could be kept warm without a space-heating system.

“Our SIPs suppliers, SIPs@Clays, were marvellous. In all, they were on site for two weeks, and were thoroughly professional.” The couple hired a local builder to undertake the brick cladding, roof tiling, internal fit out, plumbing and electrics.

The winter they moved in, the whole electricity bill cost £300 — extremely low for an all-electric house. “We are amazed at how warm the house has been.”

Read more about this project...

The Drawbacks

• SIPs tend to be a little more expensive than normal timber frame, although advocates claim that the cost differential gets clawed back by the quicker construction process. One major supplier, Custom Homes, offers a SIPs version of its designs at the same price as timber frame. And another supplier, Build It Green, has undertaken a study to show that whilst SIPs are marginally more expensive to deliver to site, this extra cost is readily clawed back through lower construction costs.

• SIPs are arguably over-engineered for internal walls where neither the strength nor the insulation capability is required. You can switch to timber frame studwork for internal walls, but this loses some of the simplicity of working with one material.

• Running pipes and cables in and along SIPs walls can be problematic. With open panel timber frame, services can be incorporated into the studwork before insulation is placed in the void. With SIPs, the usual solution is to batten out on the inside walls, thus creating a service void behind the wall board.

• SIPs require very accurate foundations. They need to be level and square, otherwise time is lost making good these deficiencies. In truth, all foundations should be built to such a standard but in reality many aren’t.

• SIPs are better suited to large, simple shedlike forms. The more complex the house shape and, in particular, the roof shape, the more cutting is involved and the smaller the panels will be. This adds disproportionately to costs.

Bear in mind the strengths and the limitations of building with SIPs and that these homes are generally so airtight that you need to plan for a whole-house ventilation system. A good tip is to plan the ventilation system early on and not add it as an afterthought: the duct runs should ideally be short and have as few bends as possible.

Internally, SIPs homes tend to be built with a timber first floor, although it’s possible to use a suspended concrete beam and block system, depending on the loadings. Dr Paul Newman of Potton, whose new Zenit range uses Kingspan TEK SIPs, says, “The range specifies Posi-web floor joists as standard, which in addition to complying to Part E for noise transfer standards, allows plenty of space for running services and ducting. We feel that this is a better solution than beam and block.”

The SIPs Sandwich

SIPs are a way of building that is in many ways similar to timber frame. The walls are built using panels made off site in a factory, but the panels are not hollow, instead a solid layer of insulation sandwiched between two sheets of timber board such as plywood or OSB; the boards simply slot together.

Building with SIPs

1. The SIPs are manufactured to size in the factory before being shipped to site on large pallets.
2. The pallets are checked and the SIPs are laid out over the foundations, which need to be very accurate, level and square.
3. Ground floor walls are erected and supported by acroprops.
4. Once the ground floor and first floor structure have been erected, the first floor walls are built up, supported by acroprops.
5. The second/attic floor under construction.
6. Once the walls are all in place, the roofing panels are craned on.

Further reading:

• SIPs: What You Need To Know
• A SIPs Self-build
• A Guide to Self Build Construction Systems