STRUCTURAL INSULATED PANELS

STRUCTURAL INSULATED PANELS
Source: DOE Consumer Energy Information: EREC Reference Briefs
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 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 thick OSB skins is
nearly three times stronger than a conventional 2×4
inch 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 precut
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 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, is also sometimes used.
Standard thicknesses for either type range from 3.5 to

7.5 inches for wall panels and 5.5 to 11.5 inches for
ceiling panels. They are available in almost any size;
however, common wall panels are 41×81 inches and
weigh 110 pounds. Most manufacturers can also
make panels as large as 81×281 inches, 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 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 inches thick. Ceiling panels are up to
7.5 inches 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 in the ceiling areas and
200°F 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.
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.