Adding a layer of insulation to the outside of a house, over the wall sheathing, makes all kinds of sense from an energy perspective. But the thicker the layer, the more challenging becomes the actual means of attaching it to the building.
In a post in the Q&A forum at Green Building Advisor, Burke Stoller shares some of his concerns, as well as a proposed solution. Stoller is working out the details for a 6-inch-thick layer of Roxul ComfortBoard mineral wool, consisting of two layers of 3-inch-thick panels, each 2 feet by 4 feet.
“My concern is that with such a substantial thickness of ComfortBoard, there is a potential for faster ‘sag’ through this, or durability issues during seismic events,” Stoller writes.
“On many of our projects, we have screwed our vertical cedar 1×3 rainscreen material directly through a single, thinner layer of the ComfortBoard directly into the framing,” he continues. “With these thinner layers (2 inches or less), the assembly seems reasonably solid and durable once all the rainscreen strips are attached. It does require some fussing around with sucking screws in and out to keep all of the strips co-planar and flat, but it’s not too bad.”
But a 6-inch layer of insulation poses different challenges. Deflection over the long term may threaten the durability of the attachment.
Stoller’s proposed solution is to insert lengths of 3/4-inch schedule 40 PVC pipe through the insulation and then run long screws through the pipe as he attaches his rainscreen batten.
“What do people think?” he asks. “Would this work, or does it just seem like a lot of work that won’t actually provide any resistance to sag, or deflection? Also, I am wondering if I’d be best served by using 7 7/8-inch-long GRK screws, or 9 3/4-inch GRK screws?…