Structural Insulated Panels (SIPs) are prefabricated insulated structural elements for use in building walls, ceilings, floors, and roofs. They provide superior and uniform insulation compared to more traditional construction methods (stud or "stick frame"), offering energy savings of 12%–14%. When installed properly, SIPs also provide a more airtight dwelling, which makes a house more comfortable and quieter.
SIPs not only have high R-values but also high strength-to-weight ratios. An SIP typically consists of 4- to 8-inch thick foam board insulation sandwiched between two oriented strand boards (OSB) or other structural facing materials. Manufacturers usually can customize the exterior and interior sheathing materials according to customer requirements. The facing is glued to the foam core. The panel is then either pressed or placed in a vacuum to bond the sheathing and core together.
SIPs can be produced in various sizes or dimensions. Some manufacturers can make panels as large as 96×288 inches, which require a crane to erect.
The quality of SIP manufacturing is very important to ensure a long life and performance. The panels must be glued, pressed, and cured properly to ensure that they don't delaminate. The panels also must have smooth surfaces and edges to prevent gaps from occurring when they're connected at the job site. Before purchasing SIPs, ask manufacturers about their quality control and testing procedures. Read and compare warranties carefully.
Types of SIPs
Expanded Polystyrene Insulated Panels
The majority of SIPs are manufactured with expanded polystyrene (EPS) foam board or beadboard insulation. This type of SIP has a nominal R-value of about 4 per inch (2.5 cm) to 5 per inch of thickness.
They are available in almost any size; however, common wall panels are 48×96 inches and weigh 110 pounds (50 kilograms [kg]).
Polyisocyanurate and Polyurethane Insulated Panels
Some manufacturers choose to use polyisocyanurate or polyurethane as the insulating material. Foam board or liquid foam can be used to manufacture an SIP. Liquid foam can be injected between two wood skins under considerable pressure. When hardened, the foam produces a strong bond between the foam and the skins.
Polyurethane and polyisocyanurate SIPs have a nominal R-value of around R-6 to R-7 per inch (2.5 cm) of thickness. Liquid foams contain a blowing agent, 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 polyisocyanurate or polyurethane are typically 3.5 (89 mm) thick. Ceiling panels are up to 7.5 inches (190 mm) thick. These panels, although more expensive, are more fire and water vapor-diffusion resistant than EPS. They also insulate 30%–40% better per given thickness.
Compressed Straw Core Insulated Panels
Straw SIPs are more environmentally friendly than the other types because they're made from renewable, recycled waste agricultural straw. However, straw SIPs offer less insulation per inch of thickness, and they are considerably heavier.
SIPs are made in a factory and shipped to job sites. Builders then connect them together to construct a house. The speed of construction when using SIPs is much faster than other types of residential construction, especially if the builder is familiar with them. Shells can be erected quickly, saving labor time and money, without compromising quality. These savings can help compensate for the fact that SIPs usually cost more than other construction systems.
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.
When installed according to manufacturers' recommendations, SIPs meet all building codes and pass the American Society for Testing and Materials (ASTM) standards of safety. In buildings constructed of SIPs, fire investigators have found that 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.
Areas of Concern
Fire safety is a common concern about using SIPs. However, when the interior of the SIP is covered with a fire-rated material, such as gypsum board, it protects the SIP facing and foam long enough to give building occupants a good measure of escape time.
Insects and rodents (like with any house) can become a problem for SIPs as well. Any foam insulation product can provide a good environment for these pests 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, including the following:
- Applying insecticides to the panels
- Treating the ground with insecticides both before and after initial construction and backfilling
- Maintaining indoor humidity levels below 50%
- Locating outdoor plantings at least two feet (0.6 meters) away from the walls
- Trimming any over-hanging tree limbs.
Boric acid-treated insulation panels are also available. These panels keep insects away while remaining relatively harmless to humans and pets.
Also, the air tightness of a well-built SIP structure requires controlled fresh-air ventilation for safety, health, and performance, and to meet many building codes. A well-designed, installed, and properly operated mechanical ventilation system can also help prevent indoor moisture problems, which is important for achieving the energy-saving benefits of an SIP structure.
Article source: The U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy (EERE). For the most up-to-date information please visit the EERE website.