SIPs – The third method of construction

By Tony Millichap, technical manager of Kingspan Insulation Limited.

Conventional construction methods such as brick and block have two distinct disadvantages: programmes can become unpredictable in our wet climate, and walls are having to become thicker and thicker in order to accommodate the high levels of insulation needed to meet thermal performance requirements. Switching to timber frame can help to speed things up, and will also reduce the carbon footprint of the build if that is a consideration, but for a reliable fast track, highly energy efficient and versatile approach why not consider using Structural Insulated Panels (SIPs), which can now be considered to be the third method of construction.

SIPs have been used for decades in North America and Canada, and have become increasingly popular in the UK as a proven way of constructing very airtight and thermally efficient buildings, both for housing and non-domestic projects. They have been used in some of the most ambitious and cutting edge developments in the UK, providing a reliably energy efficient building envelope for rigorous standards such as Passivhaus, and Level 6 of the Code for Sustainable Homes.

Benefits
SIPs can offer a number of clear benefits over more traditional methods of building, including fast track construction, space saving characteristics, reduced waste, design flexibility and reliable thermal performance.

An example of a typical SIP construction would be two 15mm OSB3 (Oriented Strand Board) facings with a 110mm rigid urethane insulation core, providing a thermally efficient, structurally strong, rigid panel. SIPs can provide a whole wall U-value no worse than 0.20 W/m².K, and U-values of 0.10 W/m².K or better can easily be achieved with the addition of an insulated lining.

The panels are factory cut to the design of the building, ready to slot together using special jointing systems that provide very high levels of airtightness. Finished constructions can produce air-leakage rates as low as 1 m³/hour/m² at 50 Pa. SIPs can also help to avoid problems which may be associated with other common construction techniques, such as air-leakage through poorly sealed sockets or switches, at floor zones through masonry cavity walls, under skirting boards and through poorly sealed loft hatches and top storey ceiling light fittings.

As thermal Building Regulation requirements tighten, and we approach the stringent targets of zero carbon by 2016 for all housing, issues such as thermal bridging become more significant. As well as providing an effective air seal, the jointing arrangements in SIPs mean that the insulation layer is typically interrupted by less repeating studwork than in a timber frame, greatly reducing repeating thermal bridging.

For example, in a property constructed with traditional timber frame the repeating thermal bridges caused by timber studs in the walls and rafters in the roof means that you could typically expect 15 per cent of the walls and six per cent of the roof to be uninsulated. By comparison, in a property constructed with SIPs, as little as four per cent of the walls and one per cent of the roof may be uninsulated.

Because the panels are pre-cut ready to assemble there is very little on-site waste, the risk of defects is greatly reduced and installation is extremely simple. Panels can be 1.22m wide by up to 7.45m tall, and the single fix installation format reduces construction time and is less labour intensive. A quality weatherproof envelope can be constructed in considerably less time than if using some traditional methods of construction, and tasks such as drylining the walls can be carried out at the same time as finishing the roof, making project completion times more reliable and keeping costs down.

Where space is at a premium, the highly efficient integral insulation helps to make the most of the available footprint, keeping wall constructions much thinner than traditional methods such as brick and block, without loss of strength or thermal performance. Using SIPs for the whole construction also generates an intrinsic warm roof space, which can easily be designed as an additional room, or to provide a safe, dry storage area.

From an aesthetic point of view, SIPs also provide a blank canvas to which the desired finish can be applied, from traditional brick to modernist cladding, colourful render to warm timber. This allows huge flexibility and choice in the overall design and appearance of the finished building.

Because the buildings can be so air tight, good, controlled ventilation is an essential part of the design. By introducing MVHR (Mechanical Ventilation with Heat Recovery), heat can be recovered from the old air being expelled and used to warm the incoming airflow, further reducing heating demands without impacting on the end user’s level of comfort.

The very high levels of thermal performance mean that the size of any heating systems can be greatly reduced, keeping both capital and running costs down. SIPs also lend themselves easily to achieving the levels of performance needed to meet the Passivhaus standard, which requires almost no heating.

Reliable solutions
Building with SIPs can bring many benefits to the housebuilder and developer. It reduces reliance on wet trades, which are so heavily weather dependent, and helps to make sure that projects are kept on track, allowing earlier sales, predictable return on investment or an extra month’s rent through timely completion. The factory engineered panels reduce the risk of the finished building not meeting the designed performance and help to maximise space and minimise waste. The results are highly energy efficient dwellings that are equally at home looking like traditional country cottages, suburban semis, or modern urban apartments.