Costs: Prefabricated structural panels

Structural panels whole-life-cost issues

Structural panels come in many forms, but can be divided into two principle types. “Open” structural panels consist of pre–assembled wall framework to which other elements are added on site. “Closed” panels are complete pre-assembled wall panels, which typically include insulation, moisture control layers and the weathering envelope. They can also include prefitted windows and doors internal services and finishes.

The general cost benefits of pre–assembled panels lie in the improved speed of construction, reducing site costs. Faster construction times should also result in earlier income streams from the building.

Quality may be improved due to more co-ordinated supply chain processes and production in controlled factory environments with reduced site snagging. These benefits are enhanced where closed panels are used and there is a high degree of duplication.

Detailed design needs to be worked out early in the building process. Cost advantages are readily lost, however, if there are inaccuracies in panel tolerance, delays or late changes to design.

The whole-life costs of structural panels are largely related to capital costs. Structural components by their nature are expected to last at least 60 years with no cost input. Because all structures are at risk of deterioration or damage in the long term, this depends on the risk of failure being carefully managed. This relies on appropriate material selection and treatment to resist mechanisms of deterioration; structural calculations that ensure the structure can withstand expected loads; and the system’s design protecting the structure from the causes of degradation.

Structural panel options

The integrity and accuracy of connections between panels, floors and roofs is a key consideration for long term performance. The detailing of the base of the panel where it meets the ground level should ensure there is no risk of moisture accumulation. Some systems do not benefit from specific design standards and there may be limited evidence of long term performance; in these cases construction tends to be based on first principles, results from testing and generous safety factors.

Structural insulated panels

Structural insulated panels (SIPs) typically comprise a sandwich with insulation as the “filling” and timber panels as the “bread”. Typically, in the UK, timber panels are made from oriented strand board or particleboard, plywood panels or cement-bonded particleboard. Insulation tends to be expanded or extruded polystyrene, polyurethane or polyisocyanurate foam. Non–rigid insulation may be used where the skins are connected by intermediate studs or ribs.

Thermal insulation properties are very good with high levels of continuous, integral insulation. However, some systems may require additional mechanical ventilation to control moisture-vapour levels.

The adhesives used in SIPs have a strong track record of more than 25 years, and SIPs have been used in the USA for more than 40 years. Where the SIP system has been tested and is protected from moisture, a life of at least 60 years is expected. A recent European Technical Approval Guideline ETAG 19 provides performance criteria and tests for SIPs.

Timber-frame panels

Timber-frame panels may be specified as open or closed panels. In practice, timber-frame panels are an extension of existing timber-frame construction with an expected life of over 60 years.

Lightweight steel-frame panels

Mild steel panels tend to be of the open type. Locating insulation on the external side of the frame overcomes the risk of cold bridging.

Protection against corrosion is provided by galvanising, usually to a minimum of 275 g/m2. UK third-party certified systems have a design life of more than 60 years.

Engineered laminated timber panels

Engineered laminated timber panels are 50-300 mm thick and comprise five or more layers of cross-laminated, solid timber strips. Variations include panels with OSB or gypsum fibreboard.

These panels originate in mainland Europe and have a life expectancy of at least 60 years.

Concrete wall panels

Concrete panels have been in use for 50 years, and the science and engineering of durable concrete has made great progress since the pre cast concrete non-traditional housing of the 1960s and 1970s.

Brick-clad concrete panels should have a service life of more than 60 years.