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Building Systems

 

GEMDUS - The FACTORY BUILT SIP HOME

Green • Expandable • Modular • Dwelling • Unit • System

THE HISTORY OF SIP's

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.

 

 

 


 
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