Three-in-One Insulating Sheathing from Dow

Styrofoam SIS offers its three functions in three layers. A thin structural layer, which is essentially a Thermo-Ply-type composite panel, rests against the studs. Mostly composed of recycled cardboard, with some polyethylene, this layer contributes up to 80% post-consumer recycled content (by weight) for the entire product, according to Dow, although the recycled content is not independently certified. Most of the thickness of the panel is in the middle layer, a closed-cell, rigid polyisocyanurate foam with high compressive strength. A moisture- and UV-protective outer layer, blue in color and composed in part of aluminum foil, provides a drainage plane. An entire 4x8 sheet weighs 16 pounds (7 kg)—significantly less than conventional sheathing.

Proper installation is key to getting the most out of Styrofoam SIS. Dow requires a more aggressive nailing pattern than usual for either standard sheathing or rigid foam: three inches on center for supported edges, and six inches on center for intermediate supports. Although nails will pass through much of the foam, they will provide the necessary holding power in the structural layer. According to Jim Gurnee at Dow, the nailing pattern, with roughly twice as many nails as would be needed for oriented-strand board (OSB), shouldn’t be a problem for builders. “Contractors are saying it’s not an issue because of pneumatic guns,” which make nailing fast, he said. He also noted that nailing would be automated in panelization factories. The nails add some time and expense for a product that otherwise aims to save both, however.