Structural Insulated Panels (SIPs)

Structural insulated panels, better known as SIPs, are becoming increasingly popular as a building system in the UK due to their strength, versatility, fast build times and energy efficiency.

SIPs can provide a quick way of building very efficient walls, which require no additional frame to support them. An entire house, extension, garden buildings or garage may be constructed from SIPs, with panels for the walls, floors and roof manufactured to slot together on site.

Made up of two structural faces (usually oriented strand board – OSB) sandwiching a rigid core of insulating polyurethane foam, the materials are bonded together to create strong, highly insulated panels which are lightweight, quick to erect and free from problems of compression and shrinkage.

By combining the stages of conventional timber-frame construction – framing, sheathing and insulation – into a single unit, a structure can be erected much faster and with less specialised labour. This can lower build costs and ensure quicker occupancy.

SIPs can be used as the inner skin of a cavity wall, as a self-supporting single-skin structural wall – in place of timber frame or blocks – and as a pre-insulated roofing structure.

The diagram shows how the panels make up the shell of the building for the house illustrated at the top of this page

They may be used in conjunction with other building systems, such as timber frame or steel, and make ideal infill panels either inside or outside a frame. Particularly popular for use with oak frames, they provide an external envelope – allowing the oak to remain exposed internally.

SIPs are a flexible product which may be externally clad with – among other things – brick, timber or stone, with render proving a popular and economic choice for selfbuilders. Lighter cladding materials such as timber can be attached to battens, while heavier materials like stone or brick need foundations. Internally, the panels can be finished with plasterboard and a skim coating, enabling wet trades to be kept to a minimum.

Roofs

SIPs arrive on site pre-cut, with the insulation already fitted, and span from wall to ridge beam, usually without any further support. As the structure doesn’t require roof trusses there is space for a well-insulated and habitable room in the roof, which can be left completely open as further accommodation, storage or to form a vaulted ceiling in rooms below. This makes SIPs well suited for roof construction for new builds, extensions or replacement roofs, potentially providing up to 25 per cent more floor space at no additional cost.

Design

Architects and designers are usually aware of SIPs and should be able to modify designs to suit the system. Ideally, choose a CAD designer who has relevant experience of working with SIPs and understands the structural implications of a panel system; or use a package company that can provide a design and SIPs-build service.

The panels’ strength and ease of connection offer designers plenty of versatility to explore their creativity.

Three very different homes, each built using SIPs

SIPs panels are custom designed for each project and cut to order to include any door and window openings, with the advantages of off-site construction. Usually the panels measure 1.2m wide as standard and are available in various heights, from 2.4m upwards.

Designing a house using standard SIPs sizes – usually 1.2m by 2.7m – will be the most cost-effective use of the system.

Utilising a 1.2m grid will reduce the number of cut panels and limit waste, as well as minimising the cost of a fabricator having to cut panels to fit.

By working closely with the SIPs supplier and window manufacturer from an early stage, designers can create exciting features such as glazed gables and other large glass openings, ensuring additional structural steelwork is included where needed.

Costs

SIPs are generally viewed as costing more than standard timber-frame construction and is one of the more costly build systems for selfbuilders. Insulation is already installed in SIPs panels, though, and the increased build speed, coupled with reductions in waste, manpower and site machinery, help to offset this extra expense. Energy-saving benefits should also be taken into account.

The build process

A few SIPs companies offer a full-build package, while others rely on the selfbuilder or project manager taking responsibility for the foundations and fitting out stages. Being fabricated off-site in a factory, they can be delivered pre-cut and pre-insulated, and the shell of a typical house can be erected within a matter of days.

SIPs are quick and easy to erect on site, usually with the aid of a crane or telescopic loader. Smaller panels can be installed by a crew of just two.

SIPs are not dependent on good site conditions and the quick speed of erection can have significant savings on skip hire and scaffolding.

They are relatively light, which can mean a reduced foundation specification. Most types of foundation are suitable but the slab should be accurate and leveI, and there should be on-site access for a suitable crane or telescopic handler if required. A crew of two with no special lifting equipment can easily install smaller panels, which is ideal when access is restricted. Larger panels can be lifted into place with a crane.

SIPs must be protected from the elements and should not be stored in direct contact with the ground. The size of the panels makes them less vulnerable to theft than more conventional building materials – a problem which has sadly become relatively common.

Panels are placed over a sole plate and joined edge to edge by gluing and nailing fillets into the rebated edges. Most manufacturers use timber studs to connect panels, although some will use a mini SIPs panel as a jointing system to minimise air leakage at junctions.

Be sure to follow the manufacturer’s recommendations for sealing joints correctly. Edge openings, such as window reveals, are lined with inset timber to allow frames to be easily fitted, and floors may be sandwiched between upper and lower walls or attached with joist hangers.

The panels can also include service ducting, which makes wiring easy but does mean that the electrical layout will need to be planned at an early stage.

Alternatively, services such as wiring can run through the internal partition cavity between the panel and internal plasterboard, or through a double layer of plasterboard.

SIPs walls have a solid sound when struck and the jointing techniques mean that sound transmittance is reduced between rooms.

The OSB creates a continuous surface onto which radiators, kitchen cabinets and light fittings may be easily fixed with little or no additional support.

Energy efficiency

The nature of the stressed skin panel makes it exceptionally strong, and high levels of insulation can be achieved in thinner walls than other forms of construction.

In timber-framed buildings the spacing of vertical timber studs within walls is usually at 600mm centres, whereas with SIPs panels these are at 1,200mm, which drastically reduces cold bridging.

This makes them an ideal candidate for Passivhaus construction, where buildings need to meet stringent energy-efficiency regulations. A typical Passivhaus home is airtight, with high levels of insulation, and requires virtually no heating or cooling.

The rigid core of insulation in SIPs panels should remain stable over time, with minimal thermal bridging ensuring that the structure is free from condensation and cold spots – meaning a lifetime of reduced heating bills for the owners.

SIPs insulation exceeds Building Regulations requirements, and the panels are ideally suited for use with energy-efficient systems, such as air source heat pumps.

Any heating system is suitable for a SIPs house but may need to be downsized due to the efficiency of the building, and the airtight nature of the construction means that mechanical ventilation is strongly recommended to enhance the indoor environment.

MORE INFO

The Structural Timber Association website offers technical information about SIPs, as well as links to suppliers and erectors.

All images this Guide courtesy SIPS@Clays

See SIPs Case Studies