Whole life costing: Prefabricated structural panels
Prefabricated structural panels are increasingly specified
as an alternative to traditional site construction.
Peter Mayer of Building LifePlans considers the whole
life cost implications.
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 by 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 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 a least 60 years is expected. A recent European Technical Approval Guideline ETAG 19 provides performance criteria and tests for SIPs.
Timber-frame panels may be specified as open or closed panels. In practice, timber-framed panels are an extension of existing timber-frame construction with an expected life of over 60 years.
Light weight 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 275g/mÏ. UK third party certified systems have a design life of more than 60 years.
Engineered laminated timber panels
Engineered laminated timber panels are 50 – 300mm thick and comprise 5 five or more layers of 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 greater than 60 years.
Capital cost range £/m² floor area
Structural insulated panels comprising oriented stand board OSB/3 skins to EN 300 minimum 15mm thick, polystyrene insulation core to EN 13163 or 13164, minimum 100mm thick. Open panels.
170 – 240
Timber frame panels comprising structural grade timbers grade TR26 cross section 47mm x 97mm. Oriented stand board OSB/3 to EN 300 sheathing minimum 15mm thick. Open panels.
100 – 150
Light weight steel frame panels comprising cold formed steel sections mild steel to EN 10147 with minimum of 275g/m² zinc galvanising. Open panels.
Concrete wall panels comprising composite structure of concrete, reinforcement, insulation and facing brick slips. Concrete walls to BS 8110. Closed system including weathering envelope and internal partitions.
450 – 650
• The expected service life of all the structural panel systems is in excess of 60 years
• The cost data gives ranges for materials and installation. Costs are based on the floor area of typical 3 – 4 bedroom dwellings. Costs vary in relation to scheme size, complexity of plan form, number and location of openings, floor to ceiling heights, thickness, size and composition of the panel construction.
• A whole life cost analysis for structural wall panels is complex, as alternatives are not directly comparable. A comparison would require, as a minimum, the complete building structure and envelope to be modelled, including consideration of the comparative costs associated with manufacture, installation and in–use energy costs.
First published in Building 15th July 2005
Building LifePlans provides latent defects warranty for all buildings.
BLP Construction Durability Database at www.componentlife.com provides durability information for building components.