Inexpensive exterior insulation stand-off — Critiques?
I am trying to come up with some inexpensive, and sensible details to accommodate a 6″ layer of exterior Roxul Comfortboard IS comprised of two layers of 3″, 2′ x 4′ batts. See attached PDF for details. My concern is that with such a substantial thickness of Comfortboard, the potential for faster “sag” through this, or durability issues during seismic events is problematic. On many of our projects, we have screwed our vertical cedar 1×3 rain screen material directly through a single, thinner layer of the Comfortboard directly into the framing. With these thinner layers (2″ or less), the assembly seems reasonably solid and durable once all the rain screen 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. With a 6″ layer, however, I am concerned that these challenges will multiply, and that the dead load of the siding will cause sag and deflection in the thick layer of Roxul, which will cause durability issues of attachment over time.
My solution, which I am hoping people will both assess/critique or offer alternate suggestions to, is shown in the PDF as well. Essentially, I am thinking of getting a bunch of lengths of ¾” schedule 40 PVC and cutting them into a whole bunch of 5-â…ž” long pieces, and as I install my batts of Comfortboard, I will have a 1″ spade bit that I will use to quickly bore a hole into the Comfortboard every 2′ vertically, and then slide the piece of PVC through so that it bears against my house wrap and sheathing, and then when I lay my vertical cedar 1×3 over it, I will be able to install a long screw directly through into the cedar rain screen, running through the PVC pipe, and into the framing studs.
I could buy something like the Cascadia fibreglass stand-off system (http://www.cascadiawindows.com/products/series/53.php), but I suspect it would be prohibitively expensive, and I am not installing extremely heavy cladding systems. Most likely just Hardi-plank, Hardi-panel, or cedar siding. Perhaps some areas of thin veneer stone or tile.
What do people think? 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 be best served by using 7-â…ž” long GRK screws, or 9-¾” long GRK screws? The former provides 1-â…œ” of embedment, while the latter would provide 3″ of embedment. Any engineers want to tear me to pieces?! I’d be happy for any assessment, good or bad. Thanks in advance for your input.