If you’re old enough to have worn bellbottoms, you were probably in the building business back when newfangled walls made from two slices of plywood and a foam core first hit the market. They were called stressed-skin panels, created by Dow Chemical and the forestry service to help slow the depletion of wood resources. Nowadays, the same basic sandwich goes by “structural insulated panels,” or SIPs. (Perhaps the word “stressed” was not good for marketing.)
Though the name has changed, the basic design of SIPs—panels made from a plastic foam core sandwiched between two sheets of engineered wood—has remained the same.
As an alternative to traditional framing methods, SIPs install more quickly and are more energy efficient. Still, the components have had a difficult time taking market share. While construction with SIPs has grown 30% since 2003, the overall market in residential construction remains at about 1%, says Chris Schwind, a spokesman for the Structural Insulated Panel Association (SIPA) in Gig Harbor, Wash.
Although manufacturers can make SIPs with a variety of materials, most consist of either 7/16-inch or 5/8-inch OSB skins, factory glued to an expanded polystyrene (EPS) insulation core. Factory-installed conduit or holes drilled through the foam provide raceways for electrical wiring.
SIPs are available in various sizes and dimensions, including panels as large as 96 inches by 288 inches that require a crane to erect. Most panels are produced in an automated, factory environment using computer-controlled equipment that transfers panel-cutting instructions directly from digital CAD drawings, providing precise quality control and virtually no waste. On the job, panels assemble quickly, reducing labor costs and allowing the builder to dry-in without weather or labor delays.
A study conducted by BASF and RSMeans Business Solutions concluded that builders could reduce their framing labor as much as 55% by using SIPs instead of traditional methods.
What’s more, SIPs’ high levels of insulation and inherent air seal provide an advantage over other panelized systems, such as component wall framing and modular construction. On average, a 4-inch SIPs wall can provide R-16 to R-26 of insulation for a 6-inch wall with no air infiltration or thermal bridging. SIPs also provide a smooth, stiff subsurface for siding and drywall, along with all the advantages of factory construction, including a virtually waste-free framing package because panels are made to order based on the floor plan.
In fact, following a study by Oak Ridge National Laboratory, the DOE’s Energy Star home certification program now exempts SIPs-built homes from the 17-point thermal bypass insulation inspection and blower-door tests required for qualification in the program.
Builders usually combine exterior-wall SIPs with traditionally framed, factory-built interior walls, and truss floor and roof systems. But many SIPs manufacturers also provide SIPs floors and roofs. SIPs roof panels offer an attractive alternative to conventional truss framing, especially for slab-on-grade or raised-floor construction, providing the means to a highly insulated, non-ventilated attic or crawl space that forms part of the conditioned environment, efficiently accommodating mechanical systems without having to drop interior ceilings or build bulkheads for ducts.
SIPs also can simplify some architectural features, such as rounded roofs and arched windows. The computer numeric control (CNC) system–guided routers used in the manufacturing process are capable of cutting almost any shape or size of panel, making it easier to render complex architectural details. Many of the modern, perforated shear-wall design engineering methods apply to SIPs, making it easy to add large swaths of daylight in restrictive hazard design areas, such as California and South Florida.
Interest in SIPs as a superior structural component grew with the success of SIP-constructed buildings still standing after the 1993 Kobe earthquake in Japan and more recent hurricanes in the southeastern United States.
The Insulated Core
Although expanded polystyrene (EPS) is the most common insulation material used in SIPs, both it and extruded polystyrene (XPS) have been questioned over the chemicals used in their production and their potential effects on health. Alternatives include polyisocyanurate or polyurethane foam, and compressed straw and mineral wool.