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.

SIPs Basics

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.

SIPs R-values vary depending on panel thickness and insulating product. Standard EPS SIPs have a nominal R-value ranging from R-4 to R-5 per inch of thickness. Panels made of polyisocyanurate or polyurethane typically cost more and offer a less environmentally prudent alternative, but they also are more fire- and water vapor diffusion–resistant than EPS and have R-values of R-6 to R-7 per inch.

Although some green building advocates object to the use of any petroleum-based building product in ecological construction, SIPA states on its Web site that EPS insulation made of petroleum-derived styrene is actually 98% air. The institute also claims that a SIPs house saves—in its first year—about 19 times the amount of energy required to make the EPS foam used to construct it. EPS foam cores also are recyclable, with some manufacturers accepting scrap material from builders.

“You have to look at the environmental impact of a product over its entire life span,” says Schwind, “with SIPs offering superior energy efficiency over at least 30 years.”

The most environmentally friendly core available today uses recycled waste agricultural straw; however, straw SIPs offer less insulation per inch of thickness, and the panels are considerably heavier, making transportation and jobsite handling a consideration.

The Structural Skin

One environmental concern with SIPs is the presence of formaldehyde in the OSB skins. Although none of the manufacturers we interviewed for this article offer an OSB skin with binders completely free of formaldehyde, some offer alternative skins, such as drywall and cement board. All expressed a preference for OSB as being the most economical and jobsite-resistant cladding. Cement board and drywall are brittle and may break; finished products, such as pine paneling, mar too easily during rough handling.

For those looking for alternative panels, a variety of new materials are available, including light-gauge steel, aluminum, concrete, fiberglass, and a variety of wood panels, such as pine. Some systems offer advantages for particular building needs, especially the cement boards for coastal construction in hurricane-prone areas, where waterproof skins mounted on steel studs offer an advantage over OSB. In the Middle East, SIPs made from a thin layer of Styrofoam sandwiched between about 2 inches of pre-cast concrete have become a popular and economical building material.

Regardless of material, the air tightness of a well-built SIPs structure requires mechanical ventilation to meet most building codes and for common-sense health and safety. With a properly sized and installed passive ventilation system, SIPs will provide an optimum, energy-efficient, structurally superior building system.

Fernando Pages Ruiz builds environmentally friendly, low-cost housing in the Midwest and Mountain States and is the author of Building an Affordable House: Trade Secrets of High Quality, Low Cost Construction and Affordable Remodel.