As houses get tighter and energy codes stricter, it has become increasingly common for builders to apply one or more layers of rigid foam to the outside of wood-framed shells, then install horizontal siding over vertical strapping. The method adds R-value to the walls, cuts thermal conduction through the framing, and when done carefully, creates a flashed drainage space for any water that makes it through the cladding layer. The technique's been covered in several JLC articles over the past few years; after reading "Building a High-Performance Shell" (5/10), my colleagues and I decided to test the strength of the connection between the strapping and the framing - especially given the fact that the fastener is being driven through several inches of foam, which acts essentially the same as an air gap.

Using our hydraulic testing equipment, we set up an experiment to measure the real performance of the type of assembly seen in the JLC article. Our test specimens consisted of a No. 2 Douglas fir 2x6 stud, 7„16-inch OSB attached to the stud with three 10d nails (.148-inch diameter), two layers of 2-inch foil-faced polyisocyanurate foam...

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