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Structural Insulated Panels
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
The most common types of SIPs use insulation made from expanded
polystyrene orpolyisocyanurate, a polyurethane derivative. You can also find
SIPs with a compressed, insulating straw core.
Some manufacturers are examining ways of using cementitious and rigid
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
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
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
• Applying insecticides to the panels
• Treating the ground with insecticides both before and after initial construction
• 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 airtightness 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.
Plastic Fiber Insulation Material
Plastic fiber insulation material is primarily made from recycled plastic milk bottles
(polyethylene terephthalate or PET). The fibers are then formed into batt
insulationsimilar to high-density fiberglass.
The insulation is treated with a fire retardant so it doesn't readily burn; however, it
does melt when exposed to flame.
The R-values of plastic fiber insulation vary with batt density: R-3.8 per inch at 1.0 lb./ft3
density to R-4.3 per inch at 3.0 lb/ft3 density.
Plastic fiber insulation is relatively non-irritating to work with, but the batts reportedly
can be difficult to handle and cut with standard tools.
In many areas of the United States, plastic fiber insulation might not be readily available.