How much do you know about SIPS (Structural Insulated Panels)?

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SIPs (Structural Insulated Panels) are prefabricated, high-performance, lightweight building panels. They’re an excellent system to use in walls and roofs for residential and commercial buildings.

Made by laminating high-density facings to an insulating foam core, their strong structural bonds allow them to bear considerable loads with reduced studding.

The many strengths of SIPs

In addition to their structural strength, SIPs easily achieve roof and wall U-value targets, and provide good levels of airtightness. They are a strong, energy efficient, and cost-effective building system.

Graphic showing a SIP: a panel consisting of a yellow foam insulation core sandwiched between brown exterior and interior sheathing made from OSB

SIPs consist of two high-density engineered timber sheaths – typically orientated strand board (OSB) – bonded on both sides of a low-density, cellular foam core.

Most OSB used to manufacture SIPs derives from UK and European manufacturing plants, and is made from sustainably sourced softwood timber. The insulation core provides excellent thermal properties due to the reduced amount of timber studs required. Air permeability is also typically lower than traditional construction, due to the large format nature of the supplied panels and the small number of joints in the structure.

SIPS have a long design and service life (more than 60 years) and achieve BRE Green Guide ratings of A and A+ for wall and roof elements.

All SIPs constructions are designed to comply with UK Building Regulations for fire safety and structural performance.

The resins used in their production are typically low in VOCs (volatile organic compounds); while insulation materials such as EPS and PUR do not release any VOCs.

Walls and SIPs

SIPs for walls can be of any thickness, although most manufacturers produce panels between 100mm and 250mm. SIPs can achieve new build external wall U-value targets between 0.1 W/m2K and 0.2 W/m2K.

Cross section of an external wall using a SIP

The SIPs structure itself is protected from moisture by both the cladding and the breather membrane on the outside of the panel, as well as the vented cavity between. Cladding can include brick, stone, timber, metal, or proprietary render systems.

In such external sections, SIPs provide the greatest contribution to the overall U-value of the external wall. In most instances, U-value targets will dictate the overall thickness of the SIPs, rather than structural or other requirements.

Intermediate and party walls with SIPs are normally formed using timber studwork and lined with plasterboard. Alternatively, SIPs can form two leaves of a party wall.

These SIPs walls are then lined on each room face with at least two layers of plasterboard fixed onto timber battens to provide the necessary fire resistance required by building regulations.

To ensure that any moisture which does penetrate the cladding does not encounter the SIP, a drained and vented cavity is incorporated behind the cladding. This cavity can range from 20mm (for timber cladding) up to 50mm (for masonry cladding) and will incorporate drainage and ventilation gaps at the base.

BS 5250:2021 Management of Moisture in Buildings – Code of Practice states that an accessible area equivalent to 500mm2 per metre should be provided to the external wall cavity. The base of the building must be kept above external ground level, with recommendations that the lowest structural timber (usually the sole plate) be at least 150mm above external ground level.

Roofs and SIPs

Both pitched and flat roofs can be formed using SIPs. SIPs roof panels can be of any thickness, although most manufacturers produce panels between 100mm and 250mm. SIPs can easily achieve the roof U-value targets of between 0.1 W/m2K and 0.15 W/m2K of most new-build developments.

Cross-section of a pitched roof using a SIP.National regulations and British Standards should be consulted for the size of the ventilation voids required in flat and pitched roofs. These regulations and standards may also impose limits on the size and /or spans of flat roofs for ventilation requirements.

SIPs roofs are classified as ‘cold’ roofs because the structure of the roof passes though the insulation and is subject to a temperature gradient. Cold roof systems (either pitched or flat) must have a ventilation void between the roof covering and SIPs to ensure the structure does not pose a condensation risk.

Pitched roofs will normally be overlaid with a breathable roofing membrane, counter battens and then tiling battens to provide the required drainage and ventilation space.

Flat roofs are overlaid with a breathable roofing membrane, firrings forming a ventilation void, and then a deck and roofing membrane. BS 5250:2021 provides guidance on ventilation voids and ventilation openings for SIP roof structures.

SIPS and sustainability

Many buildings taking a ‘fabric first’ approach to reducing carbon dioxide emissions have favoured SIPs.

A well-designed and built SIPs-based structure can offer exceptional levels of airtightness and minimal thermal bridging.

External wall and roof panels contain minimal timber studs within the depth of the insulation and typically solid timber is restricted to around openings and at corners.

SIPs structures are generally low in weight and so additional savings in embodied energy can be made in the design of foundations and supporting structures – less concrete is needed.

Learn more about SIPS

This article is based on information from two of TDUK’s Timber Knowledge Sheets: Structural Insulated Panels and Using Structural Insulated Panels. TDUK members can read this sheets in full to:

  • learn more about how to design and build SIP-based walls and roofs
  • read how to join and attach SIPs
  • see more diagrams and build-ups
  • learn how to store SIPs and maintain them

You can read these and all the other resources in our huge Knowledge Library by becoming a TDUK member.