Continuous Exterior Insulation: Cladding Support and Penetrations
Thought I had already submitted one of the questions below, but do not see it listed on the GBA website. Sorry, therefore, if this is a repeat, but I have another couple of questions anyway.
My goal is to build a zero energy house in Climate zone 5A. In considering alternative wall designs, I like the concept of continuous external insulation (ComfortBoard 80, either 3″ or 4”, and either as one or two layers) over a single stud wall (2×6 with rock wool inside the stud cavities) with OSB & WRB between the inner and outer mineral wool, and a rain screen between the external mineral wool and the exterior siding (most likely to be vinyl siding).
I have read numerous discussions about the strength of long screws that will be used to attach the rain screen to the studs through the ComfortBoard and OSB sheathing. I have not decided whether I will be using wood firing strips or using Cascadia clips and Z-girts to create the rain-screen between the ComfortBoard and the vinyl siding. The consensus seems to be that the long screws through the ComfortBoard will be strong enough to hold the total weight of the wall assembly, including the vinyl siding.
My first question has to do with using a heavier external cladding, like engineered stone (not brick), rather than vinyl siding. Although the vinyl is clearly within my budget, I prefer the looks of the stone but it may not be in my budget. Nonetheless, should the engineered stone be supported from underneath by a ledge/shelf that is attached to the foundation so that the screws holding the insulation and rain-screen tight are not also carrying the extra weight of the stone (compared to vinyl siding)? The foundation will be poured cement. Would such a ledge/shelf actually decrease the downward torque on those screws or not? Could the ledge/shelf be a metal L bracket that is bolted to the foundation or could it be a masonry ledge that is built into the top of the foundation? Can you point me to any references or projects that illustrate this being done? As I understand it, the stone would be mortared onto a screened/mudded back-surface that is built as part the wall assembly (i.e. mounted on the outside of the rain-screen gap, the same surface to which the vinyl siding would be mounted). Since the stone cladding is mortared onto wall assembly behind it, it is part of that wall assembly. Would the idea of support from underneath actually conflict with the fact that the stone is also attached to the wall? I understand that the proposed support from underneath must not block the flow of air from the bottom up through the rain-screen, so this ledge/shelf cannot be completely solid or continuous. I welcome any feedback on these ideas. I have been unable to find any discussion elsewhere.
My second question has nothing to do with what the exterior cladding that is outside of the rain-screen gap, but rather has to do with with the penetrations of the sheathing and WRB caused by all the screws that must hold the exterior insulation and firing strips to the wall. I assume that some of the screws that help hold up the sheets of rigid insulation need not be in line with studs but will nonetheless penetrate the WRB and the sheathing. The screws holding the furring strips must in fact penetrate the WRB, the sheathing, and the studs with sufficient depth to hold up the entire exterior wall assembly. Since such attention is placed on the detail of sealing the WRB and taping seams of the WRB and the sheathing before adding external insulation, I wonder how much that effort is compromised by all the screw penetrations involved in mounting everything that is outside the WRB. Is there a way to seal all these crew penetrations, or do they not significantly compromise the air tightness of the house? Are there any scientific studies that address the question? I have never seen this discussed.
A third question relates to both of the previous questions and regards the use of Cascadia Clips. I would think that the rigidity of the fiberglass clips would lend strength to the outer wall assembly or at least decrease the downward torque on the screws that attach the furring just outside of the insulation and that are driven into the stud. I saw a thermal image, by the way, of a wall assembly with Cascadia clips. From the color topography, it appears that the screws themselves are definitely a thermal bridge despite the clip being made of fiberglass, and despite the fact there that they are a bridge only at the specific points where they penetrate and not over a continuous area such as a stud surface. This thermal bridging has to do with the screws, not the Cascadia clips.
Any feedback on these thoughts is greatly appreciated in advance.
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Regarding the cladding weight, screw size/length, and deflection - those concerns are addressed in this building science corp article. The executive summary is that 4" #10 screws spaced 16" OC vertically over essentially any insulation can support a lot of weight, like engineered stone or traditional 3 coat stucco. Vinyl weighs so little that you could easily use 24" OC and still have a lot of reserve strength.
Regarding the penetrations - There are fewer penetrations in a wall with furring strips and insulation than there are with a normal installation of lap siding. If you think about the lap siding being rows of nails 16" OC every 7" vertically, that's a lot of nails. In most cases it's fine. There's no exposure to the weather, and water / air can't leak through a hole sealed by a nail, it just doesn't happen. That's why you shouldn't pull a shiner, even if it makes you sad.
A wall with furring strips will have screws vertically 24" OC has significantly less penetrations, for whatever it's worth.
As far as fastening the insulation goes, I would only use maybe four fasteners per 4'x8' panel temporarily, the furring strips will hold it otherwise in place. well placed screws of appropriate length are practically self sealing and shouldn't be a concern. For mineral wool I wouldn't even use fasteners if you can get away with installing it all at one go. Start the bottom row with maybe two screws and insulation washers per 2x4' comfortboard to get everything aligned and steady, and then install the furring strips loosely. As you slide the comfortboard in place behind them, you can screw the furring strips in place and it will hold the insulation in place just fine. Make sure you dont butt the comfortboard behind the furring strips. The edges are weak in compression, and it'll smash unevenly. Butt them in the field between two furring strips at least 6-8" from an edge if you can help it.
If you're still with me, I think you can relax somewhat on the WRB penetrations. 99.X % of the rain will be diverted by your siding, any that makes it through will run down the back or down the furring strips. Practically none will hit the insulation, and even if it does, it'll just bead or run until it evaporates into the void. You'd have to have such a torrent to reach the actual WRB behind any thickness of insulation that you may actually be under water.
You wrote :
"I think you can relax somewhat on the WRB penetrations. 99.X % of the rain will be diverted by your siding, any that makes it through will run down the back or down the furring strips. Practically none will hit the insulation, and even if it does, it'll just bead or run until it evaporates into the void"
A good explanation for why open-cladding is such a poor choice in most climates.
My concern about the WRB screw penetrations was less about rain water following the screw shafts through the WRB but more about the WRB as an air barrier being compromised at least by penetrations that are not buried in a stud. Granted, if there are very few penetrations between studs, then perhaps it is no big deal. And perhaps for the the screws in between studs, they are a hole that is plugged with the screw as opposed to being an open hole. Again I was thinking of it from the perspective of an air barrier being penetrated versus a water barrier being penetrated. We make a big deal about air sealing and the slightest drafts that result around electrical wall outlets, windows, the junction of the floor and the wall, etc. That is why I wondered about the punctures to the WRB air barrier from the outer wall assembly.
I appreciate your confidence, however.
I agree that it's good to be concerned. I think you could make a very quick test with a few pieces of scrap to rest your concerns. Make a small box with OSB and caulk all the inside joints. Put a screw in from the bottom and fill the box with water. I'd wager a large amount of cash that you wont see it leak. If it's water tight, I think it's a safe assumption that the air leakage through any such penetration is minimal, especially those that terminate into studs.
Thank you also, Kyle, for the warning not to butt comfortBoards behind furring strips if the furring strips are wood. I wondered about that and you are the first I have seen to offer that advice. If Cascadia clips are used, then by definition, the board seems are in line with the Cascadia clips, but then I would think that compression force is not being applied to the edge of the board by the z-girt strip because the z-girt strip attached to the face of the rigid clips.
David, I agree with all above, and depending on the weight/thickness of your engineered stone this response might be superfluous. But... The ideas you mentioned for supporting heavy stone cladding are viable options and are both done frequently out here (Bozeman, Montana) to support true stone veneer (think 6" or thicker stones - that's a lot of weight).
Bolting angle iron to the foundation (titen screws or similar) creates a good ledge to support stone, and you can use something like Armatherm strips between the angle iron and the concrete to get at least a minimal thermal break (continuous insulation would stop above the angle iron and continue below it.
Possibly cheaper than a concrete ledge/shelf would be building up a CMU wall on top of the footing to support the stone, with the CMU stopping just below finished grade. So, CMU on a footing, with continuous insulation behind it, drainage mat & waterproofing against the foundation wall.
If using the CMU approach, you must have weep holes through the CMU joints at the footing level to allow the space between concrete and CMU to drain. Also, a through-wall flashing with weep holes at the transition between the CMU and stone, or better yet through the stone itself slightly above grade. MTI makes some very useful products for the weep holes and flexible self-adhesive flashing tapes can be used to conform to the uneven stone surface.
Thanks Peter, I neglected to mention weep holes / drainage.
Thank you for your reply. I'm not sure why a CMU wall in addition and outside the poured foundation wall would be cost effective, but I am not a mason.
I looked up Armatherm strips and see that they offer a z-girt that is attached to the back wall by screws at the back plane of the z-girt (that abuts the back wall). This contrasts with the Cascadia clips that require screws going all the way from the outer plane of the z-girt into the wall - the screw thereby creating a thermal bridge through the insulation. If the Armatherm z-girt is structurally just as strong as the Cascadia clip, do you think that the Armatherm z-girt (mounted vertically) provides a thermal advantage over the Cascadia clip or wood furring?