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.
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
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.
Advantages
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.
Disadvantages
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.
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.
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