Prefabricated Structural Insulated Panels
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.
Energy 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 Using Panelized Homes
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
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