Time and cost remain the driving forces in the delivery of any project. However, sustainability also continues to be a key factor in the specification of building materials.

Traditional timber frame has always fared well against these criteria, providing an efficient and competitive build solution, which offers a low carbon footprint and materials from sustainable sources. At the same time, it is no secret that timber frame has come under some scrutiny because of the potential fire risks during construction. Importantly, these risks have not gone unnoticed by the insurance or fire safety/protection industries.

While the use of structural insulated panels (SIPs) with timber frame presents a semiprefabricated solution, in some cases the working of the panels is still required on site and so introduces a margin for workmanship error.

It is these challenges that sometimes lead developers to favour fully prefabricated or modular systems, particularly for the construction of high-rise buildings. To prevent the timber industry missing out on these opportunities, manufacturers need to continually push the boundaries of what can be achieved with timber system construction. That was a major driver behind the development of Insulshell – a new fire-rated engineered structural cassette panel system which incorporates robust insulation technology.

Insulshell is a composite structural cassette that integrates superior levels of insulation with high-performance panels to create an innovative fast build system for floors, walls and roofs.

Suitable for use as a stand-alone structural balloon frame system or as an infill to structurally framed buildings, the system has been designed to provide comprehensive protection as the cassette panels already provide a minimum of 60 minutes through-wall fire integrity before installation – which arguably eliminates the risk of spread of fire during and post construction.

Designed for buildings which aim to achieve high levels of thermal insulation while maintaining speed of construction, Insulshell, incorporating its patented jointing system, delivers prefabricated panels to site which minimise thermal bridging and maximise airtightness (typically less than 1.0m3/hr/m2 @ 50Pa recorded).

From a U-value perspective, Insulshell will achieve a minimum of 0.2W/m2K but can be as low as 0.09W/m2K, depending on the system specification.

Finally, Insulshell delivers a total through-wall construction, which makes it more predictable on site as the design is less likely to be compromised by the interfacing of multiple trades.

Opening new markets

As a timber-based and fully prefabricated cassette system, Insulshell is not designed to compete with traditional platform frame construction. Instead, its engineering is aimed at opening new markets for timber-based construction.

In the words of Simon Tuddenham, associate director for leading real estate business Colliers International: "We believe the technology offers a real opportunity to help meet the UK’s current housing shortage due to the speed, quality and cost of construction that is possible. What we have found is that this new system allows for very competitive build costs and has helped make a number of previously unviable sites financially viable by significantly reducing the build costs without a deterioration in the quality of residential accommodation provided."

While traditional timber frame is widely used in the residential market, the engineering of Insulshell makes it a viable specification for retail, education, multi-storey residential and high-performance residential projects. A key factor in this is the total cost of build as Insulshell offers cost parity with existing methods such as structural framing system (SFS) and SIPs, but with the additional benefit of flexible design as well as integrated thermal, acoustic and fire insulation.

As a result, it has already been specified by two major UK retailers over traditional build methods.

Large panel technology

Unlike standard platform frame construction, which uses typical panel sizes of 2.4×2.4m, Insulshell incorporates large panel technology of up to 4.5x24m. In doing so, this significantly opens up the market opportunity for multistorey construction.

While this is facilitated by the large panel technology, from a technical perspective the interfacing of the floor details is also a major contributor. This is because the intermediate or separating floor cassettes do not penetrate the external structural core panel, they are simply lowered into position and fitted to a bespoke locking ledge. This removes the platform junctions that are necessary in a traditional timber construction and, as a consequence, significantly reduces the cross-grain shrinkage potential.

"Although designers are aware of cross-grain shrinkage when using platform frame, there always remains the risk of human error," said Peter Steer, independent consulting structural engineer. "The engineering of Insulshell means there is very little cross-grain and this is only apparent at the sole plate. A six-storey building at 18-20m high would be expected to shrink by a nominal 5mm when constructed with Insulshell, as compared with an approximate 30mm with traditional platform frame. Insulshell significantly reduces cross-grain shrinkage."

Although there is high demand in the market for prefabricated systems that increase the speed of construction, reduce the risk of workmanship error and offer superior levels of insulation, there is also some resistance based on the perceived lack of flexibility. Prefabricated is generally confused with ‘modular’, which often evokes visions of linear construction (which will not necessarily fit with the client’s design vision). This is where the flexibility of Insulshell’s timber-based cassette panel system comes into its own, as by its nature it facilitates virtually any design.

Moreover, as the performance of the system is engineered at design stage, manufacture and installation remain extremely simple and straightforward. A further benefit of this is the potential for local assembly units to be established to support the delivery of projects nationwide or even globally. Given community engagement and supporting the local economy are emerging as considerations in the awarding of contracts, the use of Insulshell has the potential to support this.

As a specifier with experience of these circumstances, Andy Partridge, associate partner at architect Roger Stirks Harbor + Partners, commented: "Part of our social initiatives within new schemes is to use local labour wherever possible. The simplicity of Insulshell means it would be very easy to create local jobs to specifically deliver the panels required for projects. Moreover, it is virtually limitless in terms of design potential which makes it an ideal system for us to use."

Official launch

With Insulshell due to be officially launched to market early this year, its design flexibility was put through its paces during the London 2012 Olympics as ‘The Pavilion’ – a temporary construction at Lancaster House. It was conceived as an extension to the existing building so that the site could accommodate UK Trade & Investment delegations during the Olympics, and architect Waugh Thistleton specified a vast glazed area with a timber floor and roof. Tasked with delivering this on site, events contractor Innovision was faced with the challenge of constructing the single-storey 4x50m structure against tight timescales and an agreed budget.

Offering a full turnkey solution to this challenge, Insulshell designed and manufactured a fully demountable system that could be redeployed after the Games.

Manufactured in 18 sections, the system was fully prefabricated off site with the glazing installed on site. Taking just four weeks to manufacture, two weeks to install and four days to demount, the flexibility and speed of construction of Insulshell were well demonstrated.