According to recent report published by APA-The Engineered Wood Association, test results indicate that the lateral load resistance of a structure using ThermoPly Red sheathing could be underdesigned by 23% to 39%.

Although Thermo-Ply panels are less than 1/8 in. thick (nominal value 0.133 in.), the published design values for ThermoPly Red, a structural sheathing product, are stated as being higher than those for 15/32 plywood (or what most of us refer to as "1/2-inch ply").

That's pretty good for a structural sheathing made from layers of what is essentially a Kraft-like paper pressure bonded with a wax-like polymer. Except that the structural design values may not actually exceed 1/2-in. ply, as stated, according to the APA.

Buckling between studs of ThermoPly Red during APA tests to evaluate the panel product's shear resistance
APA-The Engineered Wood Association Buckling between studs of ThermoPly Red during APA tests to evaluate the panel product's shear resistance


Buckling of ThermoPly Red at peak load during APA tests to evaluate shear resistance. This peak load was substantially lower than the  design values published by the  product manufacturer.
APA-The Engineered Wood Association Buckling of ThermoPly Red at peak load during APA tests to evaluate shear resistance. This peak load was substantially lower than the  design values published by the  product manufacturer.

The report points out that the ultimate unit shear capacity of 885 pounds per linear foot reported by the manufacturer of ThermoPly Red is "substantially higher" than the value published in them American Wood Council's Special Design Provisions for Wind and Seismic for 15/32-inch wood structural panel sheathing. 

To verify ThermoPly's claims,  the APA set out to do their own set of tests using ThermoPly Red on the same test assembly used to rate wood structural panels, nailing it off (0.131 inch x 2-1/2 inches nails at 6 inches on center along the panel edges and every 12 inches along studs in the field)  to a section of framed wall (2x4s on 16-in. centers). The test -  the standard test for ASTM E564 required to evaluate the shear resistance of framed walls - subjects the wall assembly to cyclic racking and measures the peak load at failure.

Sheathing tearout at the edge of a wall assembly sheatherd with ThermoPly Red during  APA tests to evaluate shear resistance. 
APA-The Engineered Wood Association Sheathing tearout at the edge of a wall assembly sheatherd with ThermoPly Red during  APA tests to evaluate shear resistance. 

According to the APA tests, a structure relying on ThermoPly Red for shear resistance could be underdesigned 23% to 29%. Also noted in the APA report, Clemson University did its own round of similar tests on ThermoPly Red and came up with results that suggest a wall that relies on ThermoPly for shear resistance could be underdesign by 34% to 39%.


For more information, you can download a copy of the APA report. Registation is required.