On the surface, a structural insulated panel (SIP) looks quite humble, resembling little more than an oversized ice cream sandwich. However, the engineered building product—which comprises little more than an insulating foam core adhered to two structural facers—can be the key for any architect looking to build a high-performance project on a tight schedule. And, in many cases, a designer doesn’t have to sacrifice aesthetics in favor of efficiency.
Like an ice cream sandwich, a SIP can come in many “flavors,” the most common of which combines an expanded polystyrene (EPS) insulation core between two layers of oriented strand board (OSB). Variants include extruded polystyrene or rigid polyurethane insulation for the core, and plywood, precast concrete, or magnesium board for the structural facers.
The basic premise behind merging rigid insulation and structural sheathing to create structural insulated panels (SIPs) has been around since the 1930s when the U.S. Forest Service’s Forest Products Laboratory experimented with ways to conserve resources. The system hit the commercial building market in the 1970s when SIP manufacturers began promoting their product as an alternative to standard dimensional lumber framing, primarily for the residential market in the Northeast.
SIPs can simplify and expedite the building-erection process by supplanting traditional dimensional lumber framing and fiberglass insulation with a prefabricated, all-in-one panel. Assembled under controlled factory conditions where waste can be greatly minimized, SIPs are manufactured as completely flat systems that are void of the bends and bows that can encumber wood studs. The panels, capable of handling about 10 pounds per square foot (psf) of dead loads and live loads of up to 70 psf, can also be used in roofs and floors, spanning as much as 18 feet without the need for additional structural support.
In spite of these benefits, SIPs have not made significant headway into the framing market. Lumber has remained relatively cheap—between $15 and $30 per square foot—and framers aren’t all that expensive either. The system’s extra up-front cost—up to 20 percent higher than lumber—doesn’t help make SIPs an easy sell. “It’s not a product you see, so it’s not like upgrading to beautiful cabinets,” says James Hodgson, general manager of SIP manufacturer Premier Building Systems.
But as the conversation in the building industry turns from initial costs to long-term value, SIPs are garnering more attention from the industry. “The system is a high-performance envelope that will save utility dollars and create a more comfortable environment,” Hodgson says. “Now it’s becoming more prevalent due to environmental concerns, the reduction of the carbon footprint, and the rising cost of energy sources.” Even in today’s uncertain housing market, the SIP industry remains strong. A survey conducted by the Structural Insulated Panel Association (SIPA) in 2011 found that the overall production of SIPs had dropped 4 percent, less than half of the 8.5 percent drop in the number of single-family housing starts, the industry’s primary market segment.
Unlike wood or metal studs, which can cause thermal bridging, SIPs are continually insulated walls. They exhibit greater heat resistance and less air infiltration than stick framing with fiberglass batt insulation. According to SIPA, an Oak Ridge National Laboratory study determined that a typical room constructed with 4-inch-thick SIP walls rated at R14 outperformed a similar room built with 2x6 stick-framed walls with R19 fiberglass insulation. Blower door tests revealed that the SIP room was five times more airtight than its stick-framed counterpart—a meaningful metric, considering that as much as 40 percent of a building’s heat loss is attributed to air leakage.
Manufacturing SIPs also uses resources efficiently. OSB comes from fast-growing, underutilized, and less valuable trees than are required for dimensional lumber. EPS consists mostly of air; only 2 percent of the foam is plastic, which can be recycled. Since SIPs come to the jobsite in precut panels that are custom fit to the project, the wastes associated with stick-frame construction are eliminated, leaving almost nothing in the framing process to be trucked to the landfill.