Dow Structural Insulated Sheathing: A Solution for Coastal Walls?

There's a growing buzz among green and energy-efficient builders about Styrofoam SIS, a new "structural insulated sheathing" product from Dow Corporation that offers builders effective wall bracing, significant R-value, and a functioning drainage plane in a single application of material. Dow engineer Doug DeWildt, who led the company's development of the material, calls SIS a "composite member." Says DeWildt, "It's a structural member and a high-performance insulation material. The insulation is a polyisocyanate, one of your highest rigid-foam insulation chemistries. And then combine that with a laminated fibrous board sheathing that's pressure laminated, to get a real dense and yet nailable, mechanically fastenable product." The one-step application gives SIS its practical edge, DeWildt says, compared with a multi-step process using a structural panel (plywood or OSB) topped with a layer of foam insulation. "There's an over-sheathing market, but it's not very big," he says. "It never has caught on with builders or panelizers to put on one layer of sheathing and then go back and put on the next. This allows us to avoid that additional step." DOW tweaked the fiberboard manufacturing process to tune the panels' board facing for practical handling and performance, says DeWildt: "You can find some other products out there that are maybe more highly densified, but you can't get a nail through them real well, and they won't have the dimensional stability." DeWildt also says that SIS turned out to offer an unexpected energy benefit, beyond just its continuous insulating R-value. "It's a rigid board, but it's not quite as rigid as OSB, and so it seals better to the stud. It has a little more gasketing effect. And so we're getting about a third the air infiltration as an OSB house — which is huge, because heat loss is due to both the conductive losses and the convection." SIS has been thoroughly tested as a code-compliant equivalent to other wall-bracing methods that are permitted under the International Residential Code, says DeWildt. That testing was done in accordance with the ICC Evaluation Services' acceptance criteria AC 269; Dow's code report, ESR-2436, is posted at the company's website. According to the evaluation report, SIS has to be applied in accordance with the IRC wall-bracing method 4, "structural fiberboard sheathing." Depending on the number of stories, and which story the wall is part of, sheathing may need to cover anywhere from 16% of the wall area (for a one-story building or the top story of a taller building) to as much as 60% of the wall area (for the lowest story of a 3-story building). Typically, panels must be nailed at 3 inches on-center around the perimeter of the panel; nailing in the field may be either 6 inches on-center or 3 inches on-center, depending on the design wind speed, the stud spacing, and the size and shape of the building. But all that is for the prescriptive IRC, which only applies to relatively moderate design wind speeds (up to 100 mph, in the 2006 code). What if you're building to a wind speed of 120-mph, 130-mph, or even higher, where an engineered design is required? DeWildt says the SIS product still has applications in very-high-wind construction, but he says the details will depend on the building. "The product is used in engineered designs," DeWildt says, "and we will supply design professionals with allowable shear values comparable to other structural panels. It's been tested in a number of ways, with different attachments." Does that mean that you can put SIS sheathing on a house in South Dade County in Florida, and handle the wind loads? "Yes, if it meets your engineering criteria — if you don't have a ton of windows or whatnot," says DeWildt. "We followed the same testing protocols that were done with other panels, and we've got quite a library of testing conditions, so that people can use it the way they like and try to get the design loads that they need." With a requried on-center nail or staple spacing of 3 inches, even in the lower wind speeds allowed by the IRC, there's not much room to increase the number of fasteners. "The variables are the house design, the wind loads, and then the allowable shear values of your product," says DeWildt. "The biggest thing is window area — how many windows there are. But people are engineering with it every week — any time they get outside the prescriptive values, they turn to us for the engineered values, and most of the time I believe they can make it work." For general installation guidelines, view this DOW video.