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How to get rid of that creep in the wall?

My design challenge has been to apply the concept of “outsulation” , 2.5” exterior XPS rigid board, to a wood structure that will ultimately have 3 coat stucco and real stone veneer as final exterior. The project is in Zone 5, new construction, 2x6” wall, stick framing. Have gotten some good insight from Martin, who has directed to me some valuable resources, as well as fellow GBA readers. Ultimately ran it past a local engineer, between his input and the cumulative experience gained by reading, eventually abandoned the idea. Too risky, too much weight, not a proven assembly, don’t do it.
I started rehashing the obstacles as I prepare for windows. The biggest was wall creep, (not to ignore the flashings that will need to be done correctly and with quality workmanship to keep water out of the assembly). Peter Baker’s article on Buildingscience.com “experimentally” explored the issue with the assumption of 4” of exterior insulation.
BA-1404: Initial and Long-Term Movement of Cladding Installed Over Exterior Rigid Insulation
Martin summarized Fasten Master’s recommendations, with his additional input and commentary. Their fastening schedule is based on weight of cladding and thickness of insulation.
Fastening Furring Strips to a Foam-Sheathed Wall
Despite these resources, and the occasional discussion regarding this issue on this blog and others, I realize these systems assume the weight of the stucco and the stone veneer (lick and stick stone) on the furring strips and their attachment to the wall. The thick scratch coat creates the base of the cladding system, with the lath secured to the furring strips, which are screwed to the wall before being embedded in cement.
Basically all the weight, I think of it as a really heavy blanket, hanging on the exterior wall, is hung by furring strips and screwed to the 2x6 studs with multiple hundreds of high quality (and pricey) fasteners, each with 1½” penetration to the stud. This transfers the weight to the stud walls and foundation, in the process keeping the exterior part of the assembly from creeping down (as well as falling away from the structure).
What if each of those furring strips were sitting on a brick ledge coming off the foundation. Maintaining the same specifications of installation, would this not totally obviate wall creep? The blanket is now hung by the same strips that have a solid footing that can’t go anywhere. I had considered this a while back, but without furring strips, treating the insulation and cladding as a dimensional unit. The wall above will sit on the wall below which sits on a brick ledge, which it is not.
This design should solve the wall creep issue, right? I can now go back to 2.5” of exterior insulation and additional stucco to stone atop that without the need to Mooney strap the interior wall or use EPS under sheetrock to try to combat thermal bridging and add R-value to an anemic 2x6” wall. All the while, sleeping soundly at night, not worried about a risky wall assembly that may creep someday. Makes sense?
Thank you in advance for any constructive input, discussion on the issue or suggestions.

Asked by Sal Lombardo
Posted Jan 10, 2017 5:11 PM ET

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3 Answers

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1.

Sal,
I don't advise the use of stone veneer on the exterior side of 2.5 inches of rigid foam attached to a wood-framed home. You know that, I think.

Your idea of making a wide foundation, and resting the bottom of the vertical furring strips on the foundation, would probably work. But it's still worth running the idea by an engineer.

If you are going to go to all that trouble -- supporting the bottom of your furring strips with a wide foundation -- why not just build a double-stud wall, and support the exterior framed wall on the foundation? That's more conventional than supporting the bottom of the furring strips.

If you really want to combine foam and fluffy insulation with a double-stud wall, you could (although most people don't). If you have a 9-inch thick wall, you could install 2.5 inches of open-cell spray foam on the exterior side of the wall (about R-9) and 6.5 inches of fluffy insulation on the interior side of the wall. (I would include a smart vapor retarder like MemBrain on the interior side of the wall if I took this approach).

Answered by Martin Holladay
Posted Jan 11, 2017 6:48 AM ET

2.

Thanks Martin. Yup, I know it well. Looking at different ways to skin the cat at this point. Walls are up, house is roofed. I became enlightened later in the process, double stud wall is an alien in my neck of the woods. Architect did what he has been doing for decades. The comfort of dogma. Never a word of efficiency. At this point, I am essentially exploring retrofit solutions before the project is even complete. Still hoping to get 27% of the R-value or a minimum R-7.5 on the exterior safely.

Answered by Sal Lombardo
Posted Jan 11, 2017 8:14 AM ET

3.

Are you the owner? I'm sure you've already considered this, but why not change the exterior finish? Ditch the stone and just use plain stucco? Do something else entirely?

Answered by Brendan Albano
Posted Jan 11, 2017 1:15 PM ET

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