Time is Ripe for Structural Insulated Panels

The idea of building with stressed-skin panels is actually an old one, but the progression from idea to reality required a long period of testing and experimentation which has only recently begun to bear fruit.
Development of stressed-skin panels for building construction began about 60 years ago.  Much of the engineering and durability testing over the 60-year period was conducted at the Forest Products Laboratory in Madison, Wisconsin.  The FPL is a federal facility operated by the U.S. Forest Service dedicated to conserving our forest resources through good utilization techniques.
The concept of using skins to carry a portion of structural loads in a building was first implemented in the 1930’s.  In conventional wood frame construction, framing material was designed to resist all structural loads.  The skins, such as siding and interior finish, were considered only to transfer loads to the framing.

From engineering theory, it was reasoned that if skins were rigidly glued to a thick core, they would take most of the structural loads as well.  This also meant that they would become stressed.  With this in mind, a building panel with smaller than usual framing members glued to interior and exterior skins was planned and designated as “stressed-skin” construction.
FPL tested the concept and proceeded to build a small house in 1937.  Wall studs in the panels were 3/4” by 2 1/2” rather than the usual 2” x 4”.  Roof and floor framing sizes were like-wise greatly reduced.  This stressed-skin house attracted a great deal of attention and First Lady Eleanor Roosevelt personally dedicated the house on FPL grounds.  Durability of the stressed-skin panels has been proven by occupancy of the house and exposure to the severe Wisconsin climate for nearly 60 years.  It is currently being used by the University of Wisconsin as a day care center.  

Following the success of stressed-skin construction, scientists and engineers at FPL reasoned that if the skins could take part of the structural loads, let the skins take all of the loads and thus eliminate framing completely.  Engineering theory was developed and tested in the laboratory and complete test structure was built in 1947.  The best core material at the time was corrugated paperboard laid at a variety of angles, glued together and sawn to produce the desired thickness.  A variety of panels were placed in the test structure.  Cores for walls were 2 1/2” thick.  Skins glued to each side of this core included 1/4” plywood, 1/8” tempered hardboard and treated paperboard.  Roof and floor panels were thicker and had heavier skins.  Panels were extremely light but rigid.  This test structure was heated, humidified and exposed to Wisconsin weather over the next 31 years.  Bowing of panels was measured periodically and generally found to be minimal.

The structure was disassembled periodically for testing to observe changes in panel stiffness and some panels were cut in half for loading to failure.  The remaining half was then reinstalled for longer-term exposure.  In 1961 expanded hexagonal paper cores were included in some panels.  In 1969, rigid foams had become readily available so foam cores were installed in the structure.  Even though these changes were made, most of the original panels remained for the entire 31 years of exposure.
In 1978, the FPL stressed-skin structure was totally disassembled and all panels were destructively tested by loading to failure.  Most of the panels retained both their stiffness and strength as compared to panels of identical composition tested at the time of fabrication.  Panels that had some decrease in stiffness and strength had skins such as paperboard that were not suitable for outdoor exposure without better protection from weather.  In all cases, there were no glue-bond failures.  Failures were usually by shearing of the core.
When FPL planned in the mid 1960’s to build a large paper plant to house major research equipment, structural panels were included in the design.  Glue-laminated arches provided the main structural frame with glue-laminated purlins spanning between the arches on the walls and roof.  Sandwich panels were attached to the wall framing to form the building enclosure.  Panels consisted of polystyrene core and paper overlaid with plywood skins.  The building was dedicated in 1967 and the panels have performed well for nearly 30 years.

At the time of its inception, the method was acclaimed as the building technique of the future.  That future has finally arrived as can be seen in the rapidly growing structural insulated panel industry.  Since the current panel industry is using rigid foam cores that have outstanding insulating characteristics, the industry has chosen to call them “structural insulated panels.”  Typical panels consist of a rigid core with oriented strand board or plywood glued to each face.  Because the facings—also called skins– are rigidly glued to the core, they take structural loads and eliminate the requirement for framing members within the panel.
The OSB skins are made from three-to-four-year-old plantation grown hardwoods, not the old-growth timber which is the focus of so much current controversy.  The OSB, which can be made in sizes as large as 8’ x 24’ allows the foam core panels to be produced in similar sizes.  These large panels with racking resistance on both faces result in outstanding shear walls to resist wind and earthquake loads.  Many 4’ x 8’ panels are still used for convenience in handling, but the larger sizes mean fewer pieces to handle when appropriate lifting equipment is available.

The structural insulated panel industry has a tremendous potential for growth.  The high R-value panels with fewer thermal breaks are well able to satisfy the stricter energy requirements of today’s buildings.  Materials for fabricating these buildings are not sensitive to the environmental restrictions and consequent price increases of structural lumber.  Outstanding structural features also offer a solution for buildings to resist the forces of natural disasters such as earthquakes and hurricanes.  Fast erection of the building shell and the ease with which panels can be modified in the field result in a user-friendly alternative to conventional wood framing.  The more than 50 years of patient research and careful testing that have been conducted by several generations of engineers at the Forest Products Laboratory research facility have helped pave the way for the benefits that this system offers to today’s building industry.

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