22 September 2020

Structural insulated panels: inbuilt solutions

TRADA image

Photo: Innovare.


“SIPs are engineered, load-carrying, wood-based structural elements. They are lightweight for ease of on-site assembly and deliver the structural and thermal envelope of the building in one element for walls and roofs.”


Structurally insulated panels (SIPs) are off-site manufactured products that can be used as wall or roof elements to deliver strength, enclosure and thermal performance. While the housing market is an obvious choice for this product, the use of SIPs as an off-site building process has a strong presence in the commercial markets for schools, colleges and offices.


This article provides an overview of the prospective application of SIPs and how the product can be used by engineers and building designers for construction solutions on the broader market applications. The article highlights professional advice available to the market on the assembly of SIP framing.


Technical overview of a SIP

What is a SIP?

SIPs are engineered, load-carrying, wood-based structural elements. They are lightweight for ease of on-site assembly and deliver the structural and thermal envelope of the building in one element for walls and roofs.


Structurally, a SIP comprises an outer layer of wood-based sheathing and is strengthened where necessary with insert timbers.


The strength comes from the nature of a sandwich panel technology in combination with the structural sheathing panels. The thermal properties come from the insulation.


Table 1: The use of SIPs in construction



Walls – structural

To provide vertical and horizontal strength of the building as well as thermal resistance. 

Walls – curtain

As an insulated cladding panel where the external skin is designed for the appropriate weather protection. Where oriented strand board (OSB) faced products are used, secondary weather protection is required. 

Walls – infill


As a prefabricated panel between post and beam or framed structures to provide thermal insulation and wind load transfer. 

Roofs – structural


To provide sarking or a structural deck spanning between rafters or purlins and to serve as thermal insulation. 

Roofs – infill

To act as thermal insulation on top of the main structure. 


SIPs transfer loads through the facing board materials separated by the rigid insulation foam core. The shear transfer to the facing boards is uniform, and the analysis is based on the tensile or compressive stress distribution in the facing boards. There is no EN code for the specific design approach, but the European Assessment Document Guideline TR019 provides guidance for prefabricated wood-based loadbearing stressed-skin panels. Companies should provide third-party approvals for their SIPs. The Structural Timber Association (STA) website offers guidance on design for SIPs under the Technical Bulletin series.


SIPs are typically available in thicknesses ranging from 100mm to 285mm. They are manufactured in panel sizes up to 2.7m wide and can vary in length from 0.6m to 6m. Custom sizes are available, and some manufacturers offer curved SIPs for curved roof applications. For details of different SIPs manufacturers, refer to the STA website.


Fire performance

The fire performance of SIPs is achieved by the application of drylining, such as plasterboard, which protects the structural member. Some manufacturers offer SIPs with factory-fitted fire-resistant boards over the top of or in place of the OSB to reduce the construction stage fire risk and provide inbuilt fire robustness.


The correct specification and installation of the internal linings to both the ceiling and the wall are critical to the performance of the system in a real fire situation.


As for all designs, a fire-risk assessment should be made. The fire robustness of a building is reliant on the strategy and protection being considered in the project fire risk and solution assessment. Regardless of the building material, the quality of workmanship can determine the fire resistance of a building. Junction details where walls meet ceilings and where multiple service penetrations occur require quality fire stopping regardless of the structural element’s construction type.


The ABC of benefits in building with SIPs

The demand in offices, schools and colleges for low-energy buildings calls for the energy-saving ABC of building.



Reduction in air permeability is achieved using the standard SIP building process allowing for airtight barriers at floor junctions. The process of forming structural joints in the panels, tapes and floor zone membranes that are included in designs enable the frame to achieve passive standard air leakage where needed. High performing values 1.5m3/m2hr at 50 pascals are commonly achieved.


Building envelope

With airtightness, the insulation value for superior energy efficiency is required and SIPs offer a range of U-values. The fabric-first approach supports the option to specify a lower specification heating system.


Cold bridging

The total heat loss expected from thermal bridging in all junctions of the building is easily measured. The cold bridging factors to include in a calculation for SIP junctions can exceed the values of accredited and enhanced construction details.


Raising the quality levels in assembly

One of the key benefits of an off-site construction process is realised by the speed and quality of delivery of pre­manufactured factory panels to the assembly of the building. The STA has an assembly workbook for SIPs buildings. The workbook acts as a training tool to raise the quality of erectors and provides increased confidence for customers of SIP buildings. On completion, the workbooks require the candidate to pass a test before being accepted as an STA Assure erector; and every company under the STA banner requires at least two-thirds of the erector workforce to have STA Assure status.


The sign-off from the installer that the as-built frame is as designed is part of the quality improvements for a confident market for SIP building delivery.


An example of commercial application for SIPs

The use of an airtight and thermally efficient panel to provide the envelope around concrete or steel-framed structures is being increasingly employed by designers to deliver projects. One example is the £10m Centre for Advanced Building Technology at Dudley College. The project had a strategic design brief for Passivhaus levels of insulation. The need for an airtight envelope with high levels of thermal insulation values prompted the design team to seek solutions. The benefits of pre-assembled SIPs that could be wrapped around the steel and concrete frame was realised in this build.


About the author

Martin Milner, Managing Director, Milner Associates

Michal Zajic, Associate Director, Milner Associates


This is an extract from the Timber 2020 Industry Yearbook. Download the full article, including supporting images, references and further reading, here