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Structural Insulated Panels (SIPs) are prefabricated insulated structural elements for use in building walls, ceilings, floors and roofs. They replace conventional stud or "stick frame" construction. They are made in a factory and shipped to job sites, where they are connected together to construct a building. SIPs may be called foam-core panels, stress-skin panels, sandwich panels, or structural foam panels. They were first developed and tested by the Forest Products Laboratory of the United States Forest Service in 1935. Until about ten years ago, they were not in wide use. However, the SIP manufacturing industry has greatly expanded in recent years in response to increasing demand by builders for these labor, material, and energy saving products.
A SIP consists of an engineered sandwich or laminate with a solid foam core 4 to 8 inches thick (10.2 to 20.3 cm) and structural facing or sheathing on each side. The facing is glued to the foam core and the panel is either pressed or placed in a vacuum to bond the sheathing and core together. The most common types of facing materials are oriented strand board (OSB) and plywood, though manufacturers can customize the exterior and interior sheathing materials according to customer requirements. They can be produced in various sizes or dimensions. A SIP has a high strength-to-weight ratio and a high R-value.
SIPs can be used 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 in comparison to more common methods of house construction. When installed properly, SIPs also provide a more airtight dwelling. This makes the building more comfortable, energy-efficient, and quieter.
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 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.
Performance
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 SIP 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 compressed straw core has also been investigated.
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) is also sometimes used.
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 panels are 41×81 inches (1.04×1.06 meters) and weigh 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 insulate 30% to 40% better than EPS or XPS, per given thickness.
There are also non-structural panels made 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.
Advantages
SIP walls are superior to conventional walls in a number of ways. SIPs combine a high insulation R-value 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.
Areas of Concern
The quality of fabrication of the panels is very important to ensure a long life and performance. The panels must be glued, pressed, and cured properly to ensure that the panels do not delaminate. The panels must be completely square, the panel connection surfaces have to be smooth, and the connecting technique well designed to avoid gaps being created when the panels are put together at the job site. Before purchasing SIPs, ask manufacturers about their quality control and testing procedures. Read and compare warranties carefully.
While SIPs offer ease of construction, the installers do have to place very close attention to the manufacturer's instructions to ensure proper installation. Improper installation may reduce many of the benefits of SIPs discussed above.
Fire safety and insect problems are two other issues that are common concerns about using SIPs. As discussed above, SIPs have performed well in combustion tests. When the interior of the SIP is covered with a fire-rated material such as gypsum board, the fire resistance of gypsum board 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) 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 and 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 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. SIPs 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.
Summary
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.
For more information on this subject contact:
Structural Insulated Panel Association (SIPA)
Email: staff@sips.org
The SIPA is a trade association of SIP manufacturers.
Bibliography
Books and Reports
Alternative Framing Materials in Residential Construction: Three Case Studies (PDF 2.44 MB) Download Acrobat Reader, National Association of Homebuilders (NAHB) Research Center for the U.S. Department of Housing and Urban Development (HUD), 1994, 117 pp. Available from HUD-User (see Source List below) in print. Order number HUD-6501.
Alternatives to Lumber and Plywood in Home Construction, National Association of Homebuilders (NAHB) Research Center for the U.S. Department of Housing and Urban Development (HUD), 1994, 74 pp. Available from HUD-User in print (see Source List below). Order No. HUD-6135.
Side-by-Side Evaluation of a Stressed-Skin Insulated-Core Panel House and a Conventional Stud-Frame House: Final Report, A. Rudd and S. Chandra, Florida Solar Energy Center (FSEC). Available from the FSEC (see Source List below). Order No. FSEC-CR-664-93.
Articles and Conference Papers
"Agriboard is Back," A. Wilson, Environmental Building News, (11:7/8) p. 9, July/August 2002.
"Building with Panels," S. Winter, Progressive Architecture, (No. 11) pp. 88-90, November 1993.
"A Compact Timber-Frame Farmhouse," J. Sousa and L. Johnson, Fine Homebuilding, (No. 105) pp. 98-103, April/May 1995.
"Cost Analysis for a Stressed Skin Insulating Core Panel Demonstration House, Springfield, Oregon," K. Aires, et al., The 20th National Passive Solar Conference Proceedings, American Solar Energy Society, Minneapolis, MN, July 15-20, 1995, pp. 156-61.
"Demo Highlights Cost and Efficiency of Panel Houses," Fine Homebuilding, (No. 95) p. 38, April/May 1995.
"Energy Diagnostic Testing and Monitoring of Six Panelized Housing Units," G. Brown, et al., The 20th National Passive Solar Conference Proceedings, American Solar Energy Society, Minneapolis, MN, July 15-20, 1995, pp. 162-67.
"Foam-Core Panel versus Stud-Frame Construction—More Side-by-Side Test Results," J. Nisson, Energy Design Update, (14:3) p. 6, March 1994.
"Foam Sweet Home," C. Wardell, Popular Science, (249:1) p. 48, July 1996.
"The Lowdown on Structural Insulated Panels," A. Cobb and J. Gunshinan, Home Energy, (19:1) pp. 38-40, January/February 2002.
"Nailbase Foam Sheathing," J. Nisson, Energy Design Update, (16:8) pp. 8-9, August 1996.
"NREL's Tests Confirm Good SIP Thermal Performance," R. Judkoff, J. Balcomb, et al., Spotlight on SIPA, (4:1) pp. 2-3, Spring 1995.
"Problems with Juneau's SIP Walls," Ed., Energy Design Update, (23:9), pp. 1-3, September, 2003.
"SIP's Face the Skeptics," P. Sprenger, Home Energy, (15:2) pp. 13-18, March/ April 1998.
"Structural Insulated Panels," J. Kolle, This Old House, pp. 50-54, December 2002.
"Styrofoam-Core Structural Panels," J. Nisson, Energy Design Update, (13:5) p. 11, May 1993.
"Trouble-Shooters Probe Rotting SIPs in Roofs in Juneau, Alaska," Ed., Energy Design Update, (21:10) pp. 4-5, October 2001.
"University of Oregon Studying SSIC Panel Technology," American Plywood Association, Spotlight on SIPA, (4:1) p. 6, Spring 1995.
"Wall-Sheathing Choices," B. Greenlaw, Fine Homebuilding, (No. 106) pp. 50-55, December 1996/January 1997.
Source List
Florida Solar Energy Center (FSEC)
Email: info@fsec.ucf.edu
HUD-User
Email: helpdesk@huduser.org
This fact sheet was reviewed for accuracy in November 2003.
NOTICE
This report was prepared as an account of work sponsored by an agency of the United States government. Neither the United States government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States government or any agency thereof.
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