A structural insulated panel, or structural insulating panel, (SIP), is a form of sandwich panel used as a building material in the construction industry. SIP is a sandwich structured composite, consisting of an insulating layer of rigid core sandwiched between two layers of structural board. The board can be sheet metal, fibre cement, magnesium oxide board (MgO), plywood or oriented strand board (OSB), and the core can either be expanded polystyrene foam (EPS), extruded polystyrene foam (XPS), polyisocyanurate foam, polyurethane foam, or be composite honeycomb (HSC). The sheathing accepts all tensile forces while the core material has to withstand only some compressive as well as shear forces. In a SIP several components of conventional building, such as studs and joists, insulation, vapor barrier and air barrier can be combined. The panel can be used for many different applications, such as exterior wall, roof, floor and foundation systems.
History
Although foam-core panels gained attention in the 1970s, the idea of using stress-skinned panels for construction began in the 1930s. Research and testing of the technology was done primarily by Forest Products Laboratory (FPL) in Madison, Wisconsin, as part of a U.S. Forest Service attempt to conserve forest resources. In 1937, a small stressed-skin house was constructed and garnered enough attention to bring in First Lady Eleanor Roosevelt to dedicate the house. In a testament to the durability of such panel structures, it endured the Wisconsin climate and was used by University of Wisconsin–Madison as a daycare center until the early 2000s, when it was removed to make way for a new Pharmacy School building. With the success of the stress-skinned panels, it was suggested stronger skins could take all the structural load and eliminate the frame altogether. Thus in 1947, structural insulated panel development began when corrugated paperboard cores were tested with various skin materials of plywood, tempered hardboard and treated paperboard. The building was dismantled in 1978, and most of the panels retained their original strength with the exception of paperboard, which is unsuited to outdoor exposure. Panels consisting of polystyrene core and paper overlaid with plywood skins were used in a building in 1967, and as of 2005 the panels performed well. SIP systems were used by Woods Constructors of Santa Paula, California, in their homes and apartments from 1965 until 1984. This work was the basis for John Thomas Woods, Paul Flather Woods, John David Woods, and Frederick Thomas Woods when they used a similar concept to patent the Footing Form for Modular homes (US Patent No. 4817353) issued on April 4, 1989. Numerous homes in Santa Paula, Fillmore, Palm Springs, and surrounding areas use SIPs as the primary method of construction. The design was awarded approval from (then) ICBO and SBCCI, now ICC.
Materials SIPs are most commonly made of oriented strand board (OSB) panels sandwiched around a foam core made of expanded polystyrene (EPS), extruded polystyrene (XPS) or rigid polyurethane foam. Other materials can be used in replacement of OSB, such as plywood, pressure-treated plywood for below-grade foundation walls, steel, aluminum, cement board such as Hardiebacker, and even exotic materials like stainless steel, fiber-reinforced plastic, and magnesium oxide wallboard. Some SIPs use fiber-cement or plywood sheets for the panels, and agricultural fiber, such as wheat straw, for the core. The third component in SIPs is the spline or connector piece between SIPs. Dimensional lumber is commonly used but creates thermal bridging and lowers insulation values. To maintain higher insulation values through the spline, manufacturers use insulated lumber, composite splines, mechanical locks, overlapping OSB panels, or other creative methods. Depending on the method selected, other advantages such as full nailing surfaces or increased structural strength may become available.
Methods of manufacturing
SIP's are most often manufactured in a traditional factory. Processing equipment is used to regulate pressures and heat in a uniform and consistent manner. There are two main processing methods which correspond to the materials used for the SIP core. When manufacturing a panel with a polystyrene core both pressure and heat are required to ensure the bonding glue has penetrated and set completely. Although a number of variations exist, in general, the foam core is first covered with an adhesive and the skin is set in place. The three pieces are set into a large clamping device and pressure and heat are applied. The three pieces must stay in the clamping device until the glue has cured. When manufacturing a panel with a polyurethane core pressure and heat are both generated from the expansion of the foam during the foaming process. The skins are set in a large clamping device which functions as a mold. The skins must be held apart from each other to allow the liquid polyurethane materials to flow into the device. Once in the device, the foam begins to rise. The mold/press is generally configured to withstand the heat and the pressures generated from the chemical foaming. The SIP is left in the mold/press to cure slightly and when removed will continue to cure for several days. Until recently, both of these processes required a factory setting. However, recent advancements have presented an alternative with SIP processing equipment that allows SIPs to be manufactured on the job-site. Builders in developing countries may benefit most, where the technology is unavailable but may be best suited to reduce greenhouse emissions and improve sustainability in housing.
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