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Stress Skin Panels, An Emerging Technology For Roof and Wall Systems
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Stress Skin Panels, An Emerging Technology For Roof and Wall Systems
SIP's, or Structural Insulation Panels, and SSP's, or Stress Skin Panels, were born from the invention of a structural grade adhesive that was first developed by the Dow Chemical Company in the early 1950's. Alden B. Dow, architect and brother of Dow Chemicals founder, is credited with first using styrofoam cores sandwiched between plywood. These first panels were used to build three homes in Midland, MI that are still in use today.
In the early 1970's, during the "Energy Crisis" and the renaissance of the timber frame, this technology was reborn. Several manufacturers started to produce these SSP's to use as an exterior shell for timber frame structures. The shear strength and high insulation values these panels provided were a cost savings over conventional infill techniques, and thus it became the method of choice for the timber frame industry.
Note that perhaps not all manufacturing methods for this type of product are created equal. In the beginning some manufacturers were attracted to the higher R-value of the poly-isocyanurate and urethane type injected foams. Others sought out manufacturers of expanded polystyrene which, at the time, was used mostly as packing material. The big difference between the injected and laminated systems is that the injected system depends on the chemical reaction of the foam to bond to the inner and outer facings. The laminate systems use the structural grade adhesive to bond the facings.
In recent years, however, the poly-iso and urethane foams no longer comply with the VOC (Volatile Organic Compound) regulations that are now being enforced. The reason for their higher insulation value is the trapped freon gases within. These gases are unstable and are constantly evacuating by a process called off gassing. The end result is a panel that is constantly breaking down over time.
Expanded Polystyrene(EPS)foam complies with all regulations. Its manufacturing process consists of super-steam heating polystyrene beads in a mold. The block of foam that comes from the mold is then cut with hot wires into panels of the required size. There is generally up to 10% recycled content in EPS. If the recycled content were any higher the foam would become brittle and harder to ship and handle without damage.
About the same time that the panels where being rediscovered, the engineered wood product,Oriented Strand Board (OSB), was being developed. Louisiana Pacific and Weyerhauser found that laminating strands of low grade wood with a urea-formaldehyde resin, a structural panel could be achieved. This laminated product was invented to relieve the pressure on the plywood market and to utilize lower grade, waste, and recycled materials. However, the urea-formaldehyde resins where found to be unstable when exposed to the elements. This product was given such names as formaldeboard, flake board, aspenite, chip board, etc.. It was not well received because it would swell quite a bit when moisture was introduced. Additionally, it was discovered that off gassing occurred, which was unhealthy to the occupants of the structure.
A new resin based on polymers was developed and, while there is still some formaldehyde involved, the result was a far superior product. The polymers keep the formaldehyde in check so that off gassing does not occur. Also, the resistance to moisture was much improved and OSB is now the structural sheathing of choice in most areas of the country. The greatest benefit to the panel industry is that OSB is available in lengths up to 24', and widths of 4' and 8'. There's also speculation that even longer panels will be manufactured at some mills.
The use of SSP's, over time, resulted in their being used in structural applications. Some SSP manufacturers found out about these uses and they sought third party testing to back up the claims. In the mean time, Ken Disch of St. Louis, MO. chose to use stud framing (2" x 4", 2" x 6", etc.) encapsulated within panels and still use all the materials associated with conventional construction. The added benefits of the increased R-value and shear strength resulted in a superior structure called Concept 2000. There were a number of these homes built and are in use today. Disch called his panels Structural Insulation Panels, or SIP's.
As a result of these and the other manufacturers' findings by the third party testing labs, the use of the SIP name was transferred to the SSP. But for a panel to be structural it must have OSB bonded to foam bonded to OSB. With Disch's system, gypsum wallboard, tongue and groove, aluminum, fiberglass reinforced plastics, etc., can be used as the interior skin because the structure of the system still relys on the more conventional stud framing.
Because of his findings, three years ago Disch began developing a panel that would use tongue & groove as the inner face, and would be applied as a finished panel on timber frame and log home applications. His goal was to have a look that was as good or better than conventional methods. That meant basically that the joint between the panels would not be visible, and a continuous paneled ceiling was the result. In cooperation with several log home companies that were willing to try this concept, most notably Pacific Log Homes through their agent, Pat Flanagan, the system was used and evolved into the product it is today.
As with any new product, there were some struggles in the beginning, but now the product is being manufactured and installed at a rate that is cost competitive with conventional built up roof systems. The greatest benefit, perhaps, is the savings in construction time involved. A Panel Roof System (PRS) goes up in days instead of weeks. This means that your log shell is closed in and weather tight sooner so less damage from the elements occurs. The project can be completed quicker, and the overall performance of the EPS foam insulation is superior to conventional fiberglass. There is no need for ventilation in the PRS as there are no air spaces in the system, which helps make it more efficient.
Because the PRS is thinner per R-value than conventional roof systems, much less lumber is needed for rake and fascia trim. That fact, in turn, provides additional cost savings. Currently, some owners are opting to use metal for their rake and fascia. The cost savings are huge on these projects since there isn't even a need for subfascia. The core of the panel acts as subfascia. The roofers install the product and save on carpenter labor for the trim process. The metal is pre-colored and should not degrade as fast as wood, meaning less overall maintenance.
A process is now being developed to pre-finish the tongue & groove so that job site labor is further reduced. This will also help protect the panels' tongue & groove surface during shipping.
Installation of the PRS is not as complicated as some would make it. There are, of course, different methods of installation, but they are not hard to grasp. Valuable installation tools include cordless drills and some sort of a panel saw such as a Prazi or Linear Link. OSHA also requires a safety rope and anchor harness system for the workers on the roof. You will also need a crane or boom truck to lift the panels into position. Currently the best method is to install the PRS directly following the log shell assembly using the same crane.
A 6400 square foot job was completed in 40 hours by 4 laborers and a crane this past summer as an example. This project had two large valleys and a small dormer. Simple gable type jobs of up to 1600 sq. ft. have been completed in a single working day. Since the stress skin panel system is relatively new, the factory offers panel installation training to contractors and also offers full installation of the product by a factory trained crew.
Bob Poor is an area manager of sales for Eagle Panel Systems - a manufacturer and distributer of these Structural Insulated Panels, and Stress Skin Panels. They've proven so successful that Poor is also overseeing the construction of a new manufacturing plant in Colorado. If you still do not have a clear picture of what this product is, visit their web site and get all the details.[LHNZ]
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