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? The former provides 1 3/8 inch of embedment, while the latter would provide 3 inches of embedment.”
That’s the topic for this Q&A Spotlight.
Consider another type of screw
GBA senior editor Martin Holladay steers Stoller to an article describing a project in which Mark Yanowitz attached 6 inches of exterior mineral wool insulation. He did not use standoffs, as Stoller is proposing, but he did use a different type of screw: fasteners made by Heco Topix.
[Editor’s note: For very useful recommendations for the type of screw to use, and how many screws to use, when installing furring strips over 6 inches of mineral wool, see Comment #20 by John Straube at the bottom of this page. Note also Burke Stoller’s useful summary of data and recommendations in Comment #34.]
Heco Topix fasteners are distributed by a company called Small Planet Supply. Michael Maines suggests that a structural engineer on staff probably would be able to suggest a fastener pattern. Some of the fasteners might be installed at an angle, he adds, an idea fleshed out by Charlie Sulllivan, who notes that two screws installed per location, one angled up and the other down, would add strength.
“If you put in two screws, one angled up and one angled down, you can make a triangle with the screw as two sides and a 6-inch length of either the stud or the strapping as the third side,” Sullivan writes. “That triangulation uses the screws in tension and compression to hold the weight easily, rather than using them as cantilevers where the weight is bending them and compressing the insulation. Figuring out exactly what pattern to use is a little complicated, but they have an engineer who has already figured it out available to tell you.”
Or, adds Jim Tyler, Timberlok screws could be used. “When I started planning a wall assembly with 6 inches of exterior foam, the idea of hanging clapboards on furring strips held to the wall with 9-inch screws felt all wrong to me,” Tyler says. “The more I looked into the properties of the fasteners, the more comfortable I became. Timberloks or similar at a slight angle down through your furring and insulation and penetrating 1 1/2 inches into a stud should hold your siding without trouble.”
Supporting the furring strips from above or below
Tyler isn’t sure the sections of PVC pipe would do much to prevent sag, but he introduces another idea: installing a ledger to create a 1/4-inch lip for the rainscreen battens to rest on, transferring the load directly to the foundation.
Malcolm Taylor suggests that the furring strips also could be supported from above.
“What about adding additional support for the battens by blocking out 6 inches at the soffit and fastening the top of the battens to this? I’ve seen this done with 3 inches of exterior insulation — not with 6, though. It wouldn’t be much help where the battens weren’t long enough to span from roof to foundation, or under windows, but two nails should provide somewhere around 200 lb. of additional shear strength to each batten.”
While both options might work, Stoller would rather devise a system that is self-supporting. There are just too many challenges in establishing support at the top or bottom of a wall.
“So, I am left with the rainscreen needing to support itself solely based on its attachment to the framing,” Stoller says. “Reviewing the tables which I have attached [see Image #2, below], it looks like the combined weight of the siding (most will be HardiePlank) and the furring, we are at just under 3 pounds per square foot.
“ComfortBoard’s density is listed as 8 lb./cubic foot so at 6 inches thick, that is 4 lb./square foot. If I have rainscreen battens spaced 24 inches on-center horizontally, and the screws are every 24 inches vertically, that means that each screw is carrying the load of 4 square foot. The total weight of all materials being supported in a 4 square foot area is 28 pounds. Quite a bit.”
With that in mind, Stoller mulls the possibility of beefing up the rainscreen battens by switching from 1x3s to 1x4s, increasing the surface area for spreading the loads by 30% and allowing Stoller to torque the fasteners down tighter, another hedge against deflection.
Tighter spacing for the fasteners, possibly 19 1/4 inches instead of 24 inches, and the use of Cascadia Clips at certain intervals along the wall, also might help. The clips aren’t cheap (about $6 Canadian a pop) but are not as expensive as Stoller had feared.
Roxul offers its own suggestions in a technical bulletin published at its website, Chris M says. For thicknesses over 4 inches, the company recommends input from an engineer.
What about considering other wall assemblies?
Stoller’s ultimate goal is to build houses that will last a century without rotting, and to accomplish that in the rainy Pacific Northwest seems to require exterior insulation.
“I am starting to envy those people that live in cold, dry climates where they can just built a dead simple double-stud wall packed with cellulose and throw a poly vapor barrier on the inside, and not have a thing to worry about!” he says. “In the Pacific Northwest, however, the reports I have read strongly caution against double-stud walls because of the extremely high and frequent potential for prolonged backside of sheathing condensation during the winter.
“So, if we want to build houses that will last 100 years without rotting, we seem locked in to some kind of exterior insulation,” he adds. “If one decides to use mineral wool, for various reasons, it would be great to find some sort of solution that we could present as reasonably affordable to our clients for attaching that product without breaking the budget to do so.”
As attractive as double-stud walls might be, an article posted by Building Science Corporation (BSC) has warned him off because of dramatically higher potential for moisture condensation on sheathing during the winter.
Don’t give up on the idea, writes Kevin Zorski.
In particular, he points to a wall described by a BSC document with 7 1/2 inches of cellulose on the outside of the sheathing, so that the cellulose keeps the layer warm and out of danger for condensation. The taped air barrier is located in the middle of the wall. BSC has warned against the wall in that climate, but Zorski doesn’t understand why.
Stoller says he sees the problem with two double-stud walls discussed in the BSC report, even though they differ in their drying potential.
“I am starting to think I might just put a dart board up on the wall with a bunch of thermally optimized assemblies on there and let chance make my decision,” Stoller says. “Then, when I am grumbling later about how miserable the details are to actually build, I can blame the dartboard instead of my poor decision-making skills!”
There are, however, many variations on double-stud walls, as GBA readers point out, and the more Stoller weighs his options, the more it seems that his original idea of using 6 inches of exterior mineral wool insulation is doomed to failure from an economic point of view.
“It is a fantastic wall in terms of performance, but the fastening of the rainscreen and the incumbent detailing around openings are becoming so complicated that the cost seems to be getting out of hand,” he says. When he adds it all up, the insulation alone will cost some $20,000 and the system of fastening the furring strips over it would add another $5,500 to $10,000.
High cost vs. high risk
“I didn’t want this thread to be about the double-stud vs. exterior Roxul,” he adds, “as they are really apples and oranges. But the more I try to find an economical way to install a thick layer of exterior mineral wool, the more unrealistic that seems to be. It is a Tesla Model-S kind of wall, and so has those prices. The double-stud wall is perhaps more like the Nissan Leaf kind of wall. It doesn’t have all the performance characteristics of the Tesla, but is still pretty incredible at a way lower cost.”
Stoller “wants to love” the exterior Roxul approach, but pauses over its practicality and its cost. He’d love to be more enthusiastic about the double-stud wall, but that, too, gives him pause for its hygrothermal performance.
“Still trying to figure it all out,” he writes.
Our expert’s opinion
Here’s what GBA technical director Peter Yost had to say:
It’s hard to imagine anything more complete than Martin’s recent article on wall design, especially because he focuses so much on double-stud versus exterior rigid insulation in above-grade walls.
And comments posted by GBA readers do a good job of laying out the case for using high-performance fasteners and a fastening schedule for a substantial thickness of exterior rigid insulation, including mineral wool.
It’s also hard for me to imagine anything more robust than a rainscreen over exterior rigid mineral wool assembly in any climate, so long as the water and air barriers are continuous. And for me, there is the rub: the location or plane of penetrations in the assembly — most notably windows — makes the detailing of a continuous water-resistive barrier (WRB) and air barrier challenging. Dragging head or sill flashings to align with the planes of the WRB and air barrier requires clear details and scopes of work and almost certainly a mock-up the first time around.
As to the cost-effectiveness of either exterior rigid insulation or double-stud walls, high performance builders in my neck of the woods (Climate Zone 6) lean towards double-stud walls (and almost always with a smart vapor retarder).
I thought it was worth sharing this wall assembly with my close friend and favorite high-performance residential architect, Steve Baczek. The detail shown in Image #3, below, is detail is for one of Steve’s Passivhaus projects on Cape Cod in neighboring Massachusetts, a climate pretty close to what you folks in the Pacific Northwest are up against. (Image #4 shows a photo of the house.)
What led to the funky but very successful use of engineered floor trusses in this assembly were local code and zoning provisions. They restricted the size of the foundation, but did not apply to the above-grade walls. The use of outboard insulation maximized floor space.
When I asked Steve about the premium for this solution compared to either more conventional exterior rigid insulation or a double-stud wall, he said it would be tough to imagine this as the most cost-effective approach for an unrestricted high performance design, but it solved the problem and constraints for this particular project. He was quick to add the builder loved this solution because it was easy to build, easy to line up detailing consistently, and was high-quality, particularly over the long term.
Like Burke Stoller, I don’t understand why Wall 15 in the Building Science Corporation report cited above does so poorly with the potential for condensation in winter. It would seem the first condensing surface is substantially warmed by the exterior rigid insulation. I have an email into the paper’s authors, but have not heard back yet. I’ll let you know what I hear.