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Structural Insulated Panels

A "foam-core panel," stress-skin panel, sandwich panel, or structural foam panel is the same thing as a Structural Insulated Panel (SIP) which is the modern term for this type of construction. A SIP consists of carefully engineered laminate with a foam core 4 to 8 inches thick (10.2 to 20.3 cm) with a structural facing on each side. The most common types of facings are drywall and/or structural wood sheathing such as plywood and oriented strand board (OSB).

SIP construction can replace stud (a.k.a. "stick") framing in almost any construction setting, but are most common in residential construction. The greatest advantage of these panels is that they provide superior and uniform insulation when compared to more common methods of house construction. When installed properly, SIP's also provide a more airtight dwelling. This makes the building more comfortable, improves energy use in both the winter and summer, and makes the house quieter.

The speed of construction when using SIP's is much faster than other types of residential construction, especially if the builder is familiar with them. Shells can be erected quickly, saving time and money, without compromising quality. Testing has shown that a wall panel with two, half-inch (1.3 cm) thick OSB skins is nearly three times stronger than a conventional 2´ 4 inch (5.1´ 10.2 cm) stud wall, even though the SIPs were assembled many times faster than a "stick" framed wall of similar size.

Many SIP manufacturers also offer "panelized housing kits." The builder needs only to assemble the pre-cut pieces. Additional openings for doors and windows can be cut with standard tools at the construction site. Even though SIPs cost more than other construction systems, they require considerably less skilled labor too.


The Florida Solar Energy Center (FSEC) found a 12% to 17% energy savings from using SIP construction. The FSEC also monitored side-by-side SIP and conventional wood-framed structures for several winter months. The airtightness of the foam-core house (measured at 0.21 air changes per hour [ach]) and was better than the conventional wood-framed house (measured at 0.27 ach).

Types of Panels

SIPs use a rigid-insulation core made of one of three plastics: 1) expanded polystyrene (EPS); 2) polyurethane; or 3) polyisocyanurate, a polyurethane derivative. Some manufacturers are also examining ways of using cementitious or fibrous core insulating materials. A company based in Texas has started limited production of a SIP product made with a core of compressed straw laminated to OSB.

EPS and XPS Panels

The majority of SIPs are manufactured with expanded polystyrene (EPS.) This foam is commonly known as beadboard. This type of SIP has a nominal R-value of about 4 per inch (2.5 cm) of thickness. Unlike other types of foam insulation, beadboard uses pentane as the expanding agent. Extruded Polystyrene (XPS), with R values of 5 per inch (2.5 cm) of thickness is also sometimes found. It uses a hydrochlorofluorocarbon (HCFC) gas as the expanding agent.

Standard thickness' for either type range from 3.5 to 7.5 inches (89-190 mm) for wall panels and 5.5 to 11.5 inches (140-292 mm) for ceiling panels. They are available in almost any size, however, common wall panel are 41´ 81 inches (1.04´ 1.06 meters) and weighs 110 pounds (50 kilograms [kg]). Most manufacturers can also make panels as large as 81´ 281 inches (1.06´ 7.14 m), which require a crane to erect.

Polyurethane/Isocyanurate SIPs

Some manufacturers choose to use polyurethane and isocyanurate as the insulating material. The foam is injected between the two wood skins under considerable pressure and, when hardened, produces a strong bond between the foam and the skins.

Aged polyurethane and isocyanurate SIPs have a nominal R-value of around R-6 to R-7 per inch (2.5 cm) of thickness. Both contain a blowing agent (an HCFC gas), some of which escapes over time, reducing the initial R-value of the SIP from about R-9 to R-7.

Wall panels made of polyurethane or isocyanurate are typically 3.5 (89 mm) thick. Ceiling panels are up to 7.5 inches (190 mm) thick. Polyurethane/isocyanurate panels, although more expensive, are more fire and water vapor-diffusion resistant than EPS, and insulates some 30% to 40% better than EPS or XPS, per given thickness.

There are also non-structural panels made of with any of the above mentioned foams. These are far weaker structurally than true SIPs and are only intended for applications such as curtain walls with no loads imposed on them and roofs where there is no attic space for additional insulation.


SIP walls are superior to conventional walls in a number of ways. SIPs combine a high level of insulation with speed and ease of construction. The solid foam core eliminates air movement within the walls and minimizes thermal bridges through wood studs. Together, all these reduce air infiltration and with proper installation, make a tightly sealed/ easily controlled house.

When installed according to manufacturers' recommendations, SIPs meet all building codes and pass the American Society for Testing and Materials (ASTM) standards of safety. Fire investigators have found that in buildings constructed of SIPs the panels held up well. For example, in one case where the structure exceeded 1,000°F (538°C) in the ceiling areas and 200°F (93°C) near the floors, most wall panels and much of the ceiling remained intact. An examination of the wall panels revealed that the foam-core had neither melted nor delaminated from the skins. In similar cases, a lack of oxygen seemingly caused the fire to extinguish itself. The air supply in a structural insulated panel home can be quickly consumed in a fire.


Fire safety and insect problems are the two main issues associated with SIPs. However, experts agree that since the walls are covered with a fire-rated material such as drywall the toxic gases released by burning carpets and other furnishings are far more dangerous. The fire resistance of the drywall covering also protects the foam long enough to give the occupants a good measure of escape time.

Insects and rodents (like with any house) may become a problem for SIPs too. Any foam can provide a good environment for them to dwell. A few cases have been noted where insects and rodents have tunneled throughout the SIPs. Some manufacturers issue guidelines for preventing these problems. Such guidelines often include: applying insecticides to the panels, treating the ground with insecticides both before and after initial construction, backfilling, maintaining indoor humidity levels below 50%, locating outdoor plantings at least two feet (0.6 meters) away from the walls, and trimming any over hanging tree limbs. Boric acid-treated insulation panels are also available. This is an insecticide used in other insulation materials that is relatively harmless to humans and pets.

The airtightness of a well-built SIP structure requires controlled fresh-air ventilation for safety, health, and performance, and by many building codes as well. This is the way well-built modern houses should be anyway. The air in a building cannot be conditioned and controlled efficiently unless it can be contained. SIP's do a very good job of this, as long as the builder pays strict attention to the manufacturer's installation and construction and guidelines. Failure to follows these guidelines could negate the benefits of a SIP structure. A well-designed and installed and properly operated mechanical ventilation system is also very important to achieve the energy savings benefits of a SIP structure, and to avoid indoor moisture problems, especially in humid climates.


An increasing number of houses are being built with SIPs. They are attractive because of their relatively high-uniform R-values, square, flat and plumb walls, and ease and speed of construction. Problems with natural pests can be minimized with adequate prevention measures. Buildings made of SIPs appear to be safer than some other types, even in fire.

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