SIP System History & Overview
SIP (Structural Insulated Panels) The Preferred Building
System by Quality Builders, Developers and Home Owners
|
THE
HISTORY OF SIPs
SIPs are a “new” building material that has
actually been in use since the 1940’s. SIPs consist
of two outer skins and an inner core of insulating material
to form a monolithic unit. Most structural panels use Oriented
Strand Board (OSB) for their facings. OSB is the principle
facing material primarily because it is available in large
sizes (up to 12’ x 36’ sheets). Manufacturers
use OSB facings on structural panels due in part to the
rigorous testing needed for code approvals. The core
of a SIP is made from Expanded Polystyrene (EPS), Extruded
Polystyrene (XPS), and Urethane Foam.
The insulating core
and the two skins of a SIP are nonstructural and insubstantial
components in themselves, but when pressure-laminated
together under strictly controlled conditions, these
materials
act synergistically to form a composite that is stronger
than the sum of its parts. Panel fabricators supply
splines, connectors, adhesives, and fasteners to erect
these systems.
When engineered and assembled properly, a structure
built with SIPs needs no frame or skeleton to support
it.
Structurally, a SIP can be compared
to an I-beam: the foam core acts as the web, while
the facings are
analogous
to
the I-beams flanges. All of the elements of a SIP
are stressed, the skins are in tension and compression,
while the core
resists shear and buckling. Under load, the facings
of a SIP act as slender columns, and the core stabilizes
the
facings and resists forces trying to deflect the
columns.
The thicker the core, the better the panel resists
buckling, so larger-core SIPs offer more insulation
and are stronger
as well.
Stock SIPs are produced in a thickness
from 41⁄2" to
121⁄4" and in size from 4’ X 8’ up
to 9’ X 28’. Their R- Values range
from R-19 for a 41⁄2" EPS to higher
than R-25 in a 61⁄2" panel
and up to R-52 in a 12 1⁄8" panel.
From
a material standpoint, SIPs take the place of
a whole assembly. Instead of separate pieces
of framing,
insulation,
and sheathing, a SIP panel incorporates all of
these components and comes ready to install.
Panels come
from the SIP fabricator
with door and window openings, rakes, and blocking
precut and assembled in the panels.
The exterior
envelope of a building creates a barrier from the
elements for the comfort of
the inhabitants.
Many materials can be used to
form the envelope, but none can do it as efficiently,
as fast, as
economically, and
with as much design flexibility as SIPs. SIP
system technology offers a number of advantages
over conventional
framing
methods.
SIPS ARE CODE COMPLIANT AND STRUCTURALLY
STRONGER THAN STICK-BUILT SYSTEMS, MORE ENERGY EFFICIENT
AND FASTER
TO CONSTRUCT
Wall and Roof Systems: Structural Advantages:
SIPs are stronger than stick framing due
in part to
having dual
shear panels vs. one-sided on stick framing.
SIPs have undergone exhaustive testing by
third-party testing
firms. The National Evaluation Service, Inc.,
is
the BOCA, ICBO,
and SBCCI code authority. In the real world,
SIP houses have survived earthquakes and
hurricanes when the stick-built
houses around them were destroyed.
1993
- Earthquake in Kobe, Japan, devastated a large section
of that city, but SIP houses
built
with panels
came through
the earthquake virtually unscathed.
1998
(March) -Tornado struck Clermont, Georgia, the
tornado destroyed 27 houses,
including
7 homes near
the SIP house.
The Owner lost 25 mature trees to the
storm and half of the homes roof shingles, but the
house suffered
no structural
damage.
SHEAR RESISTANCE
Shear resistance is the ability of an assembly to withstand
horizontal forces applied to a structure by earthquakes
and high winds. This is where the most important difference
between SIP construction and conventional framing methods
shows up. The standard ICBO and BOCA approved test
is ASTM E-72-80, “Conducting Strength Tests of
Panels for Building Construction,” section 14.
In this test, two 4’ by 8’ by 41⁄2" SIPs
are assembled with no studs and are connected by OSB
surface splines. After the assembly is locked into
place, force is applied to the top corner. In a series
of tests conducted in 1995 assembly failure occurred
at an average load of 10,700 lb. Here, failure is defined
as the point where the fasteners pulled out of the
bottom edge of the panel and along the center seam,
dividing this figure by the safety factor of 3 gives
3,566 lb. This figure is again divided by 8 (the length
of the assembly) to arrive at an allowable load of
446 plf before failure. The average allowable racking
resistance for the tested panels was about 400 plf.
As
a comparison, the APA-Engineered Wood Association offers
test results for the following assembly, which
is the standard wall construction currently used. It
consists of an 8’ by 8’ wall composed of
2x4 bottom and top plates, double end studs, and studs
16" on center along the assembly. Using 8d nails,
1⁄2" plywood was applied at 6" spacing
around the perimeter and at 12" spacing throughout
the field. This assembly reached the failure point at
4,744 lb of applied pressure. When this figure is divided
by the safety factor of 3 and the length of 8’ an
allowable load for this type of wall is calculated to
be 197 plf.
Right off the truck and installed in the
basic configuration, a SIP has an allowable load factor
of
446 plf compared
with the standard wall value of 197 plf. A SIP wall
has twice the structural strength of a standard framed
wall. This difference is clearly evident in a SIP structure
that
is exposed
to high winds. The absence of creaks and groans is
very noticeable. This is also why a SIP building has
fewer
or no drywall callbacks due to cracking or fasteners
backing out.
TENSILE STRENGTH
Another test to determine the strength of the lamination
and of the core material itself is the ASTM C-297 test
for dry tension, which determines how much force it takes
to pull the SIP apart. The test selects and removes a
2"x2" cross section of a panel. These samples
are then surface-attached to the testing equipment and
pulled until either the core shears or the facings delaminate.
The dry tension result on a 41⁄2" urethane
core was an average of 87 lb. In all of the 10 tests
performed on these samples the core sheared before the
skins delaminated, clearly demonstrating the structural
integrity of the panels.
COMBUSTIBILITY
The issue of how a material performs in the presence
of fire is a primary concern to the code authority
as well as to the manufacturers of products that are
used in the construction of homes. Test results indicate
that a SIP system meets current codes for fire-resistant
wall assemblies. A SIP wall with 1⁄2" drywall
on the interior surface will meet the mandated 15-minute
resistance requirement for residential structures.
A 1/10" thick, factory-applied “fire finish” coating
will also meet the 15 minute fire requirement. A two-layer
surface of 5/8" type X drywall will meet the requirements
for a one-hour-rated wall assembly. Tests indicate
that a SIP structure performs safely in a fire situation
because the airtight construction will quickly starve
a fire of oxygen.
Another concern in a fire situation
is the toxicity of the burning material. Current
BOCA codes have deleted
requirements for combustion toxicity because there
is no acceptable test protocol simulating actual fire
conditions.
But some studies that have been done on combustion
toxicity suggest that most SIP core materials are no
more hazardous
than other common building materials.
Combustion Toxicity-Comparisons:
toxicity factors are due to the following elements:
CO, C02, HCI, HCN,
and miscellaneous other toxins. Listed below are materials
and the maximum sum of the toxicity factors. (The National
Research Council of Canada supplied this information
to determine health risks associated with various combustible
materials. The United States currently does not have
a standard because there is no acceptable test protocol.)
|
Material |
Maximum Sum of Toxicity Factors |
|
|
|
|
Polystyrene (SIP Core) |
20 |
|
Polyester (fabrics) |
20 |
|
Phenolic resin |
30 |
|
Wood (white pine) |
50 |
|
Cotton |
60 |
|
PVC |
360 |
|
Wool |
390 |
|
Nylon-6 |
950 |
ENERGY EFFICIENCY
61⁄2" SIP wall is rated at R-25 compared to
stick framing at true R-14. The R-19 rating in a 2x6
wall is given due to the rating of the batt insulation
capability. 2x4 and 2x6 stick built walls have large
fluctuations in temperature at the stud locations as
a result of thermal bridging. The continuous insulation
of a SIP wall means even, comfortable interior temperatures,
as well as more dollars saved over a stick built home,
due to increased heating and cooling requirements in
order to compensate for the constant energy loss.
In 1998
the Oak Ridge National Laboratory in Oak Ridge, Tennessee,
completed thorough testing of various wall
configurations. Results showed that a SIP wall with a
31⁄2" EPS core had a 31% better insulation
value than a conventional wall framed with 2x4s and insulated
with fiberglass batts. The basic 31⁄2" core
SIP wall also performed better than the 2x6 stick-built
wall with fiberglass insulation.
Testing done in October 2000 using a standard
framed home made of 2x6 construction vs a 61⁄2" EPS
Core had results indicating that the 61⁄2" panel
rating is over 40% more efficient than 2x6 constructed
home. Also, tests show that the 2x6 framed construction
using R-19 batt insulation has a reduced R-rating of
R-14 in real life. Much of the energy loss is due to
stud placing at 16" on centers, creating thermal
breaks in the framed wall system.
THE FASTER CONSTRUCTION TIME OF A SIP BUILDING
A
SIP can cut one to four weeks from the construction cycle.
Builders across the country are finding that they
can save time and money by erecting a ready-to-install
wall assembly instead of having their carpenters construct
the assembly on-site. In addition, SIP systems allow
the use of less-skilled workers during erection. This
factor has become increasingly important as the skilled
labor force dwindles.
The fact that SIP structures can be effectively
built by unskilled labor has resulted in the increased
use
of SIPs by Habitat for Humanity International, a nonprofit
organization that produces affordable housing using
mostly volunteer labor, which has more than 1,200 affiliates
in the United States. Some of these affiliates use
SIPs
for their affordable housing projects not only because
of the quick construction but also because the energy
efficiency means that the occupants will be better
able to heat and cool their homes in economically tight
situations.
The University of Oregon conducted extensive
tests on SIP panels. They monitored the labor required
to erect
one of these structures as well as its energy performance.
They found that their SIP house was completed in 161
fewer hours compared with industry standards for a
stick-framed house and that the SIP house required 34%
less on-site
construction time.
HOW SIPS ARE CONSTRUCTED
SIPs consist of two OSB facings pressure-laminated with
adhesives to an EPS foam core. Various sized presses
are used to apply this pressure to allow the panels to
cure properly. The industry standard is an 8’ x
24’ panel. But now there are machines that can
produce 9’ x 24’, 10’ x 24’,
and even 10’ x 36’ panels. In addition, there
are designs for a new generation of roller presses that
will utilize fast-set reactive hot-melt urethane glues
that will essentially be able to produce a continuous
panel. The new fabrication technologies are moderated
by the realities and limitations of material shipping,
handling, and placement. See SIP
Fabrication for more information.
|