Foam core panels are slabs of foam insulation sandwiched between two facings or "skins." These skins include metal, drywall, and/or structural wood composition sheathing, such as plywood, waferboard, and oriented strand board (OSB). Foam-core panels, also called stress-skin panels, sandwich panels, structural foam panels, and structural insulated panels, are replacing stud framing in some residential construction. The greatest advantage of these panels is that they provide ample insulation for a home's outer structure, often with an increased R-value per inch compared to other forms of insulation. When constructed and installed properly, they also are more airtight than stud framing. This inhibits energy loss in the winter and summer, making the home very energy-efficient. Learn more about this building material so you can incorporate it into your own DIY projects.
The two basic types of foam-core panels are structural and non-structural. When used in house construction, structural insulated panels may make up the primary structural support. These panels are strong: a wall with two, half-inch (12.7 millimeters [mm]) thick OSB skins is nearly three times stronger than a conventional 2´4-inch (51´102 mm) stud wall.
Although non-structural foam core panels are not designed to provide primary structural support, they may be used to enclose curtain wall structures like timber frames. Sometimes drywall is the only interior sheathing option for these wall panels.
Foam core panels are a key component of "panelized housing," where specific fabrication occurs in a factory. The builder needs only to assemble the pieces on the site. It takes as little as one to three days to fully erect and weatherproof the shell of a foam core, panelized house. Openings in the panels, such as for doors and windows, may be precut at the factory or cut with standard tools by the builder at the construction site.
In 1993, a structural insulated panel home was built adjacent to an identical, conventional wood-framed house. Scientists at the National Renewable Energy Laboratory (NREL) found that both types of construction performed as expected from their nominal R-values. This contradicts previous testing by the Florida Solar Energy Center (FSEC) that found a 12 to 17 percent energy savings from using foam-core panel construction. The FSEC also monitored side-by-side foam-core and conventional wood-framed structures in Kentucky for about two winter months. The airtightness of the foam-core house (measured at 0.21 air changes per hour [ach]) was marginally better than the conventional wood-framed house (measured at 0.27 ach).
Types of Foam-core Panels
Foam core panels use a rigid insulation core made of one of three plastics: expanded polystyrene (EPS), polyurethane, or polyisocyanurate, a polyurethane derivative. Manufacturers are currently examining ways of using cementitious or fibrous core insulating materials in place of these plastic insulations.
EPS Foam-core Panels
The majority of foam-core panel manufacturers produce EPS panels. EPS, commonly known as beadboard, adheres to the wood sheathing. EPS foam-core panels have a nominal R-value of about four per inch (25.4 mm) and it remains relatively constant as time passes.
Unlike the other types of foam insulations, beadboard is made using a pentane blowing agent. Wall panels made of EPS foam are typically 3 1/2 inches to 7 1/2 inches (89-190 mm) thick while Ceiling panels are 5 1/2 inches to 11 1/2 inches (140-292 mm) thick. A common wall panel is 41´81 inches (1.04´1.06 meters [m]) and weighs 110 pounds (50 kilograms [kg]). Manufacturers can make some panels as large as 81´281 inches (1.06´7.14 m); these require a crane to erect.
Polyurethane/Isocyanurate Foam-core Panels
Wood composition sheathing may be glued to polyurethane and isocyanurate slabs of insulation in the same way as EPS panels. Since it is difficult to achieve a strong bond between polyurethane or isocyanurate and wood, most manufacturers inject the foam between the two wood skins with specialized equipment. This is a technically complex and chemically volatile process, but it produces a strong bond between the foam and the skins.
Polyurethane and isocyanurate foam-core panels have a nominal R-value of around R-6 to R-7 per inch (25.4 mm) of thickness. Both contain a blowing agent that escapes over time, reducing the R-value. In well-sealed panels, however, the release of this blowing agent is slower.
It should be noted that blowing agents such as chlorofluorocarbons (CFCs), and to a lesser extent hydrochlorofluorocarbons (HCFCs), have been identified with the destruction of the earth's protective ozone layer. Regulatory measures now restrict the level of CFC production, and this may adversely effect the cost and availability of polyurethane and isocyanurate panels.
Wall panels made of polyurethane or isocyanurate are typically 3 1/2 inches (89 mm) thick. Ceiling panels are up to 7 1/2 inches (190 mm) thick. The length of available composition sheathings, currently 28 feet (8.5 m), limits panel length. Polyurethane/isocyanurate panels, although more expensive, are more fire and vapor-diffusion resistant than EPS panels.
Inch per inch, polyurethane and isocyanurate panels cost about twice as much as their EPS equivalent. However, polyurethane and isocyanurate panels have R-values nearly double that of EPS. When you purchase an EPS panel with an equivalent R-value polyurethane or isocyanurate panels, it will still cost slightly less, but the price gap isn't as wide.
Foam-core panels normally cost more than competing wall and roofing products, but they require considerably less labor to install. Consequently, a frameless foam-core house may cost nearly the same as a well-insulated stud-frame house with standard 2´6 inch (38´138 mm) stud walls.
Advantages of Foam-core Panels
Foam-core panel walls are superior to conventional walls in a number of ways. Foam-core panels combine a high level of insulation with speed and ease of construction. Unlike certain types of insulation, which lose some of their insulation value when exposed to moisture, normal home moisture levels do not significantly affect the R-value of foam-core panels. The solid foam core virtually eliminates air convection within the walls and thermal bridges through wood studs and insulation voids. The panels also reduce air infiltration that, with proper installation, make a tightly sealed house.
Disadvantages of Foam-core Panels
Fire safety and insect problems are the two greatest problems associated with foam-core panels. The concern over fire safety mostly concerns EPS panels. Polyurethane and isocyanurate burn like wood, remaining intact until it burns through. EPS, however, can begin to deform at temperatures as low as 167°F (75°C), melt at around 200°F (93°C), and flow at around 250°F (121°C). In addition to causing structural failure, melting polystyrene can actually fuel a fire.
Although all plastic insulations emit toxic gases while burning, experts believe that toxic gases released by carpets, furnishings, and foam-filled furniture appear more dangerous than the foam insulation hidden behind drywall and composition board. When installed according to manufacturers' recommendations, EPS panels pass the American Society for Testing and Materials (ASTM) tests and meet national building codes. Polyurethane/isocyanurate supporters nonetheless claim that the ASTM test (ASTM E84) for flame spread and smoke is not appropriate for testing EPS. There appears to be a consensus among the testing community to change the test so that it more realistically duplicates EPS burning patterns. Such a change is likely to raise the EPS smoke and flame levels. EPS panel manufacturers, however, maintain that fire testing of EPS panels should deal with assemblies (for example, the walls) made up of panels.
Several full panel simulation tests have been conducted. In one instance, the Underwriters Laboratory (UL) performed a test on EPS panels with waferboard skins (drywall-EPS-waferboard). UL exposed the EPS wall and ceiling panel room to a wood fire set in the corner of the room. The EPS foam-core melted back about two inches from the skin in the vicinity of the fire, but the panel skins and the EPS foam-core did not sustain notable damage elsewhere.
Winter Panel Corporation, a polyurethane panel manufacturer, commissioned three tests on EPS panels with totally different results. In these tests, each panel failed in just 16 to 19 minutes. The major difference between these tests and the UL test is that Winter's panels lacked the waferboard skins on their EPS panels; the EPS foam was adhered directly to the drywall.
Associated Foam Manufacturers, a trade association of EPS foam manufacturers, conducted two heat endurance tests. Various-sized panels, loaded with heavy weights to simulate a three-story building, were heated in an oven to very high temperatures, then doused with water to simulate fire fighter intervention. In both tests, the core failed to melt and showed no sign of panel bowing, bending, or deflection. Moreover, the bond between the foam-core and the OSB skins remained so strong that they had to use a crowbar to pry the skins off the EPS core.
In actual cases of fires in buildings constructed of EPS panels, the panels have fared well. In one case, a panelized restaurant in Kentucky caught fire in September 1987. Although temperatures are believed to have exceeded 1,000°F (538°C) in the ceiling areas and 200°F (93°C) near the floors, the kitchen wall panels and much of the ceiling remained intact. A limited examination of several wall panels revealed that the foam-core had neither melted nor delaminated from the skins. In similar cases, a lack of oxygen reportedly caused fires in foam-core buildings to extinguish themselves. Fire uses up available oxygen rather quickly. The air supply in a structural insulated panel home may quickly be consumed in a fire. This might suggest an additional hazard for human survival compared to conventionally built houses. The results of these tests and case studies of actual fires indicate the importance of having both drywall and wooden sheathings to protect the EPS foam core.
When installed properly, the fire safety of EPS panels appears dramatically improved. Insects such as carpenter ants, carpenter bees, and termites may become a problem in foam-core panels, however. EPS, polyurethane, and isocyanurate foam provide the ideal environment for an insect nest. In a short period of time, insect colonies can completely honeycomb foam insulation. Some foam-core panel manufacturers issue guidelines for preventing insect infestation. These steps include applying insecticides to the panels, treating the ground with insecticides both before and after initial construction backfilling, maintaining indoor humidity levels below 50 percent, locating outdoor plantings at least 18 inches (457 mm) away from the foundation, and trimming away any tree limbs that may overhang the roof. Boric acid-treated insulation panels are occasionally available in the market. Insecticidal boric acid is a low toxicity insecticide and fire retarder used in other insulation materials.
An increasing number of houses are being built with foam-core panels. They attract many people because of their high insulation value and the ease and speed in construction. Insect infestation can become a serious problem without adequate prevention measures. Although fire safety is also a concern, houses with properly installed foam-core panels appear to be quite safe.
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