What are SIPs (Structural Insulated Panels)?

Structural Insulated Panels – commonly referred to as SIPs – are a build system incorporating insulation. Insulation is sandwiched between two boards (usually OSB), for a highly insulated build that can be prefabricated off site, and quickly erected on site.

Making a Choice

Although all of the main SIPs suppliers compete on price and service, one of the key material differences is in the composition of the panel itself. According to Simon Hunt from SIPS EcoBuild, there are two different types of insulant that can be used between the panels: “The first one is the closed-cell polyurethane (PU) foam; the second is an expanded polystyrene (EPS). While the PU has a 20% better insulation value, it also costs more — by about the same percentage.” Alex Hyslop from Kingspan TEK concurs: “A Polystyrene SIP will typically result in a thicker full wall construction than urethane due to its inferior thermal conductivity.”

All of the mainstream UK suppliers use the same panel board — OSB/3, which complies to the British Standard BS EN 300 for structural integrity. The importance of this was made apparent when a recent fact-finding mission to the United States (the construction system’s spiritual home) by the UK SIPs association UKSIPS found that the use of substandard OSB could cause failings in SIPs homes.

How Strong Are the Panels?

The strength of the panel is in its composite nature — in the same way that plywood is stronger than solid timber. The OSB itself is strong, but when autohesively bonded with the rigid insulation (as opposed to relying on a bonding agent), it can be up to six times as strong as regular timber frame. According to Kingspan TEK, who supplies the panels used by many of the main UK SIPs retailers, “Axial loading tests on panels 100mm thick, 2.4m in height and 2.4m in width were seen to fail at a load of 440kN/m (kilonewton per metre). Using a factor of safety of 2.0, it is seen that panels will comfortably support a vertical load of 90kN/m. These loads are well in excess of the average found­a­tion loading from a typical two storey house of traditional construction.”

Design and Structural Issues

Because of the inherent strength of SIPs, there are less constraints imposed on the design and structure than you might assume.

Intermediate floors: SIPs can support concrete floors — the abutment of the head of the panel is reinforced with a simple metal angle screwed to it, to cope with the load distribution. However, most SIPs buildings use engineered beams – I beams or glulam – as they are faster and easier to handle. It’s important that the floor decking be taken to the outer edge of the SIP, as the floor forms a structural diaphragm for the building.

Number of floors: SIPs are beginning to be used for three storey houses without problems. According to Simon Hunt from SIPS EcoBuild, “An engineer will usually work out the structure and we ask that glulam, Kerto or similar timber-based beams are incorporated.” SIPs can also be used as external (non structural) walling on multi-storey buildings, wrapped around a steel or concrete frame.

Spans, dimensions: A SIP’s maximum length is usually 7.5m for walls and roofs — for walls a maximum 3m storey height is allowed. Floors can be constructed using SIPs up to 200mm thick with spans of up to 4m. The standard width of a panel is 142mm meaning that, when rendered, a complete wall thickness, including internal linings, of around 220mm (at a U-value of 0.2) can be achieved. Some suppliers offer a range of different thicknesses from 100mm up to 150mm.

Foundations: Alex Hyslop from Kingspan TEK says: “The key part of the foundation system for SIPs is getting the sole plate right. It must be level to ensure the SIP structure proceeds smoothly.”

Ventilation: Owing to the airtight nature of SIPs construction, mechanical ventilation (ideally with heat recovery) is essential.

Mixing construction systems: Don’t forget that you can also use the roof and wall elements as standalones — one particularly effective solution is a SIPs roof with a blockwork wall.

The Process

It’s best to work with a designer who understands the design and structural implications of SIPs, designing the house with these in mind before passing on the drawings to the SIPs company for ‘design conversion’. Most companies will then erect the panels on site after manufacture (craning them into place), with the self-builder responsible for foundations and internal/external finishing and services.

How to Choose a Company

Look for a supplier that understands and satisfies all of the criteria examined within this article. You will also be able to judge them in terms of price and services offered, but there are other things to look out for. According to Andrew Porter from SIPS@Clays, “A successful SIPs house requires a competent CAD designer (roof in particular), good and accurate production on the shop floor, and a first-rate fitting team, who understand geometry. If I were a self-builder, I would check that any company I chose to deal with could provide all three.”

Fixing Cladding

Before cladding SIPs walls, a non-tenting breather membrane (e.g. Kingspan nilvent®) is fixed directly to the wall panels. A minimum cavity of 25mm should be maintained between the breather membrane and the wall cladding. The following are typical wall cladding construction details:

Other issues

Pests: The rigid urethane insulation core and facings used in the manufacture of SIPs resist attack by mould and microbial growth, and do not provide any food value to vermin.

Fire: SIPs exceed the Building Regulations and British Standards requirements with regards to fire resistance. Tests suggest they can achieve up to 73 minutes’ resistance.

Service Runs: SIPs companies prefer services to run through internal partition walls. Wiring is run through the cavity between the panel and the internal plasterboard or, preferably, chased out through a double layer of plasterboard (which removes the need for a cavity and creates a more solid feel and sound to the wall, reducing noise issues). Wiring should be protected with an IEE-approved metal guard.

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