WSP Walls with Rigid Foam Backing

[SteelPE]

I have a project that is literally days away from being completed. The architect client had specd out a plywood sheathing with a foam backing to be placed on the outside of the wood framing. We identified this early on as being an issue with regards to lateral force resistance. We called the proposed manufacturer to find out that there was no published shear values for their proposed wall system. We then offered to the client the ability to sheath the inside of the building (which has it's own issues related to inspections of blocking and such). We both agreed that this wasn't a viable option.

Then, the other day, this video pops up on my YouTube feed (skip to 11:45):

https://www.youtube.com/watch?v=VYqINjqKafY&t=54s

I don't necessarily like these systems as they just don't pass the smell test, but according to their website, they do have published shear wall data which can be found here:

https://www.huberwood.com/uploads/d...ng-Single-Family-Sales-Sheet-REV-1_20-SEC.pdf

ICC appears to also have published data:

https://www.huberwood.com/uploads/d...Insulated-ZIP-System-R-sheathing-ESR-3373.pdf

Now, I understand that the shear values are not great when compared to standard WSP panels applied directly to the face of the stud. Has anyone used these products before?

 

[XR250]

I am forced to use these systems as well. I don't like them because I feel the published values probably only work in laboratory conditions. I use them anyway.

 

[phamENG]

I did a high wind, single story, public building with this a couple years ago. It's nearly as strong as plywood, but more flexible than gyp shear walls. There's another testing group that had a report on it that meshed nicely with the SDPWS tables and made it really easy to incorporate into my spreadsheets. I'll look for it later this evening.

For a single story, I'll use it begrudgingly. For two or maybe three story, I might use it if I have a whole lot of wall space and nothing too fancy going on. Beyond that...no thanks.

I'm only half joking when I suggest this....let-in braces? Depending on the size of the building, it could actually work.

 

[SteelPE]

phamENG,

The comment I received back from the architect is that the principals are not happy with the decision of using WSP walls. This all has to do with the details the architect is selecting. They are requiring a rigid foam on the outside of the building. If we use a WSP then they are going to need to sheath twice. I understand their concerns but I'm not the one wanting to put insulation on the outside of the building in spite of structure.

This particular structure is an category III building with a Vult of 152mph.... so we don't exactly have light loads. Is it single story.... yes, but they do have one area of the building that is 18' tall.... so might was well just say it's a two story building.

 

[jayrod12]

WE fasten rigid insulation to the outside face of wood walls all the time. Then we have them install vertical strapping to the studs with long screws at whatever spacing is dictated by the finishes. Sometimes it's every stud, other times it's every second stud. The strapping goes on after the insulation and waterproofing. The nice part about the vertical strapping is it provides a rain screen type drainage plane between the finishes and the waterproofing.

The strapping usually consists of PT 1x4 or strips of PT 1/2" plywood. The screws, depending on the weight of the finishes, are usually something like the GRK RSS at around 16" o/c.

 

[phamENG]

Ouch. The one I did was still RC2, so the loads weren't that high. Have you attempted to do the design using the insulated Zip system, or are you still investigating them?

Here's a slightly more nuanced look at my opinion of them:

On the plus side, the foam is actually pretty strong (for foam) and as I recall it's dimensionally stable. So once the panel is in place, you don't have to worry about of out of plane movement in the foam working the nails loose.
On the negative side, even strong foam is pretty weak. It's essentially a void through which the nails can bend. That's the reason (as I'm sure you're well aware) for the added flexibility.

So it's obviously not as strong as directly applied WSP, but as long as you're not using too much foam it's not that bad of a reduction. With your wind loads, of course every inch of wall counts so even a small decrease could make things difficult for you. Where the biggest trouble will come in is the serviceability checks. You may have a hard time getting the building stiff enough to prevent damage of interior finishes during "the big one." How's the interior? Wide open spaces or somewhat chopped up? I'm no fan of gyp shear walls, but you may need to look at them to justify it in the end.

I think this kind of system is pretty neat - I like it for houses, small commercial buildings (vanilla box type stores), and that sort of thing. Maximizing energy efficiency, while usually not a direct goal of structural engineers, is important all the same. It's just important that we make the architects, owners, and anyone else who read the marketing brochure understand the very strict limitations under which this stuff is practical. Sounds like you tried, but it fell on deaf ears.

 

[Craig_H]

I've already had a couple of architects kick the tires with me on this product. It sounds attractive to them a first blush, because they are sold the idea that they can get energy efficiency without the added labour of fastening exterior insulation with strapping and long screws. I haven't used this yet, so I am just sharing thoughts, not experience.

When I saw the original Zip-R product, I had an internal debate with myself on the additional flexure that the fasteners would be exposed to. I doubt that it has a strength impact, but the flexibility and deflection under load would certainly be different from the typical code calculations. I suspect that the deflection of these is controlled by nail slip, Pham? Does Zip give an alternative equation for calculating the nail slip in this scenario? I would imagine that our typical equations for nail slip are dependent on direct contact between the sheathing and the stud.

The thoughts going through my head are:
- What happens to the foam under cyclic loading? Does it disintegrate or crush where it is in contact with studs, as they rub back and forth? That could leave the panel loose after a time.
- These foams are known to shrink over time. Generally the shrinkage is noticed in the panel length rather than the thickness, but I suspect that the shrinkage strain is isotropic. That would further leave a gap and your sheathing panels loose.
- Normal sheathing attachment, with the sheathing directly against the studs, results in significant fastener penetration in the studs (generally ~ 1.5-2 in). If you used their thickest insulated sheathing at 2-1/2" thick, I suppose that you could potentially still power-nail with 3-1/2" nails. That's the largest size that I know of that can go in a power-nailer, and it meets their tabulated requirement of 1" penetration.
- With only ~1" of fastener penetration, is withdrawal under cyclic loading a concern? Under tight contact between the sheathing and the stud, I wouldn't be as worried, but I imagine a lot of relative movement between the two due to the foam-gap, which makes me wonder.

The aspect of this product that makes me laugh is the "building science" perspective. By using this, your air/water barrier is now at the extreme outside of your building. In a cold climate like mine, and where you would be inclined to use the thicker insulated products, you generally want your air/water barrier within the wall, at a warmer location, so that any water vapour that makes it into the wall doesn't condense as it passes through to the outside. This product puts the cold condensing surface right in the worst location - at the extreme outside of the wall. I'm willing to bet that water would condense at the panel joints against their proprietary tape, and swell the edges of the bonded OSB. Perhaps it has a place in warm/humid climate zones where you are worried about water vapour moving in the opposite direction.

Edit: I now see that the sales sheet table specifies 1.5" of nail penetration. Not sure where I saw 1". This means that the thickest sheathing/insulation boards need to be hand-nailed. That's gotta eat of a lot of the supposed labour cost savings.

 

[phamENG]

Craig,

I believe they capture it in the apparent stiffness (G_A) by using the 3-term deflection equation, which combines the nail slip and WSP shear stiffness terms from the 4 term.

They cyclic loading is a concern I would have as well. Fortunately, I'm not in a seismic zone so reliable cyclic response isn't as important. Though wind can ravage a structure over time, too. For low rise structures, at least for the first earthquake it might actually do better than standard WSP, since it'll dance around like a wet noodle - just hopefully not far enough to rack over.

To your last point, I'm in the Mid-Atlantic, and we spend more of the year in the hot and humid category than in the cold category (I don't think we ever quite qualify as "dry"). SteelPE might have an issue in Mass, though - hopefully the arch checked that?

 

[jittles]

I think this kind of system is pretty neat - I like it for houses, small commercial buildings (vanilla box type stores), and that sort of thing. Maximizing energy efficiency, while usually not a direct goal of structural engineers, is important all the same. It's just important that we make the architects, owners, and anyone else who read the marketing brochure understand the very strict limitations under which this stuff is practical. Sounds like you tried, but it fell on deaf ears.

Agreed.

It's been suggested on projects of mine before (typically high wind, high seismic), but once the reality of the design values set it, it never went past the "neat idea" stage, mainly due to the nature of the buildings we were looking at.

 

[Brad805]

It would be interesting to see more of the testing setup and results. It looks like EPS foam, but that is not clear from what I can see. I expect more info could be found in the SIP industry. This is a fairly common thing for them and I expect that could be where the idea originated from.

RDH has done some interesting screw testing for exterior applied insulation without girts or other thermal breaks, but those do not apply here.

https://www.rdh.com/wp-content/uploads/2020/01/ASHRAE-2019-Structural-Testing-of-Screws.pdf

 

[SteelPE]

So, I misspoke, the architect was not actually asking to use this specific system (Zip system). They had specified a different system that is similar. We rejected it based upon conversations with the manufacturer who pointed us in a different direction. We relayed this information to the architect who ultimately sided with us in using a traditional WSP system.

I talked to the architect about this Zip system (well the arch is on vacation so I spoke to his underling). He said they didn't like the Zip system because of tendency to overdrive the nails creating issues with the exterior skin. So for now, we are keeping the two skins on the structure (one at the face of stud and one at the face of the rigid insulation..... as they are now creating a field build SIP).

 

[XR250]

He said they didn't like the Zip system because of tendency to overdrive the nails creating issues with the exterior skin

Curious why this is an issue with ZIP-R and not sheathing in general.

 

[phamENG]

XR - I see it all the time on houses. House framers never adjust the air on their guns to get penetration right. The Zip panels have an adhered membrane on them that, when properly taped, makes house wrap/Tyvek unnecessary. If you overdrive on that, it tears through the membrane and exposes the OSB underneath, creating lots of gaps for moisture to move through the barrier.

 

[SteelPE]

XR250, what phamENG said is pretty much what the architect said.

When I spoke to him I agreed with his concern.... I also went on to say that it would be nice to have someone else go to the site and complain about the over driving that typically occurs as no one give a s**t about the structural guy.

 

[phamENG]

Last shear wall design I did I went so far as to put it on the plans:

"Over driving of nails (nail head embedded below the surface of the structural sheathing) will be grounds for rejection of work and require replacement of the wall at the contractor's cost."

Whether or not it holds water will be anyone's guess...

 

[XR250]

Ahh, makes sense.

 

[dhengr]

PhamENG:
An interesting turn-of-phrase…, “Whether or not it holds water will be anyone's guess...,” since that is exactly the part of the potential problem which you are trying not to have to guess at. You guys who have responded to this OP seem to have a pretty good handle on the facts of the matter/product/system, and have given it some serious thought. Good for you guys.