Working With Zip-R Sheathing
We are looking at potentially installing a wall system that uses Zip R-sheathing. We are considering this approach to reduce the labor cost of installing continuous exterior insulation outboard of standard Zip sheathing and the associated detailing required at punched openings, etc.. I want to like this product, but I see some shortcomings. Here are the specifics:
The project is in a mixed heating/cooling region (climate Zone 5). The walls are 2×6 studs and filled with Rockwool batt insulation. Installation instructions for the Zip R-Sheathing recommend direct attachment to the studs. If we do this, the walls would have inlet bracing for shear. I have two questions about this method:
1) If my thinking is correct, doesn’t the back of the OSB in the Zip panel become the first condensing surface when installed directly to the studs? That removes a major benefit of continuous exterior insulation, which is keeping the first condensing surface above the dew point. What are your thoughts?
2) Assuming my logic for the previous question is correct, what’s the most cost effective approach to correct the wall assembly? A). Add sheathing between the studs and Zip R-Sheathing; B). Use a different air barrier/vapor barrier somewhere in the assembly (I’m not a fan of using the drywall as the air barrier…too many opportunities for holes after installation). C). Abandon the Zip R-Sheathing and go back to the original plan of using continuous insulation outboard of standard Zip sheathing.
I’d appreciate hearing your thoughts and experiences using Zip R-sheathing. Thanks!
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Replies
Adam,
I think there is a misunderstanding. Zip-R gets installed with the OSB out. The foam is in direct contact with the studs. Huber also publishes fastener schedules for the panels so that no additional shear-bracing is required - except in some high seismic zones where engineered solutions are probably needed for all projects.
No, I did know that the OSB is on the outside. I'm really unfamiliar with polyiso though. Does it function as an air or vapor barrier? I had read somewhere that it likes to absorb water...please straighten me out!
I know it doesn't technically need the shear bracing (per Huber's fastening instructions), but I like the additional rigidity provided by the inlet bracing. It's very minimal cost to install the bracing, and it makes this engineer feel a little better :)
Adam,
I'm with you. Over-building, rather than skinning the structure until it's just about strong enough, the way some Advanced Framing techniques do, helps me sleep at night.
Hi Adam,
OSB is known to absorb and hold water more than plywood sheathing. But the idea with ZIP is that the applied WRB and tape system keep water from getting to the OBS. Many well-respected architects and builders stand by ZIP as an effective water and air barrier. According to the ICC-ES report, the perm rating of ZIP R sheathing is less than 1 perm, so walls sheathed with ZIP R will not have a lot of outward drying potential. That said, the applied WRB is much more permeable, so if the OSB does get wet, the coating does not prevent it from drying to the exterior. Regardless, if you use ZIP R sheathing, you probably want to avoid an interior vapor barrier or if one is required by your building inspector, use a smart membrane.
> Regardless, if you use ZIP R sheathing, you probably want to avoid an interior vapor barrier
Without much outward drying and without something keeping the sheathing warm, you want a Class II retarder to limit the amount of moisture accumulation. IMO, using "vapor barrier" is asking for confusion.
https://www.buildingscience.com/documents/digests/bsd-106-understanding-vapor-barriers
Jon,
Consider doing a guest blog describing the ideal relationship of interior and exterior perms for various assemblies in various climates, that would summarize the BSC work you linked to. I'm sure I'm not the only one who would find it useful.
There is definitely a need for an all inclusive article on wall/ceiling design for moisture control. The BSC article is the closest I've seen, but there are a few things that should be clearer and it needs to more fully cover the cases of external insulation. Ideal would be if Joe L would produce an updated version (since he really knows the stuff - I just refer to it).
Would also be great if some researcher came up with a formula as to how internal:external perm ratios offset the need for external foam. Or maybe WUFI is the only answer for this.
Jon - I agree with your comment that a class II retarder is appropriate for this specific scenario. Do you have any thoughts about the best way to achieve that (best meaning good performance at reasonable cost)?
I agree, this would be a very helpful reference. I thoroughly understand the basics of the concepts at play here, it just becomes tricky when you start looking at specific wall sections that deviate from the generalized examples. I'd love to see design guidance that gets specific about how to figure out where to put each layer and how that changes when you make a "simple" change like putting the continuous foam inboard of the OSB. It's probably out there somewhere, but the internet is a HUGE place to search!
I just completed my retirement house using Zip R 1.5 over 2X6 framing with fiberglass batts. This is in Zone 4.
Basically, I liked the system and I trust Huber's engineering of the wall assembly. If I were doing it over, I would do much the same again.
But do think through the details, Here is what I learned.
1) Framers will not follow the instructions re nail spacing, and will likely set all the nailheads too deeply - the foam is bouncy!
2) All your wall thicknesses get weird - you will want to spec your door jambs and window assemblies carefully. (In Virginia, I would consider running 2X4 framing with Zip R - I suspect a 4" wall would perform nearly as well as a 6" and it would be simpler to finish.)
3) You will want to spec the airsealing detail at the top and the bottom of the wall carefully.
4) You will want to think through the way the sheathing lands on the foundation. (We held the framing back from the edge of the foundation by 1.5" in order to wind up with a flush surface.)
5) Consider incorporating a rain screen. The ZipR assembly depends on the OSB drying to the outside, so a rainscreen is good insurance.
As I said, I would do it again. The Zip system keeps things simple, it performs well thermally, and we also got an excellent blower-door test on the house.
Thats all really excellent feedback and advice Mark. A few things in particular that jump out at me:
The difficulty setting fastener depth is something that keeps coming up and new installers seem to be having difficulty with. Some framers are adding inline regulators to their compressor/nailer setup to allow more depth control.
And I'm glad that you mentioned the air sealing details at panel perimeters. Otherwise, your doing a lot of work with a premium product and not getting nearly the performance that you should.
Finally, the rainscreen gap is super important with many of the wall assemblies we are building today. It allows drying of the sheathing and the siding, mitigating a lot of risk with vapor drive and allows water to drain, decreasing the likelihood of WRB failures.
Mark,
Thanks for the thoughtful feedback. All good things to think about! I have been convinced for a long time about the performance and value that the ZIP system brings to the construction process. The original design for this project calls for standard ZIP sheathing attached to the studs with 2" of continuous foam outside that. One thing I don't like about that approach is that it puts a LOT of fastener holes in your WRB and buries them inside your assembly. Minor problem? Probably...but it does make me pause.
I geek out on building science topics routinely. However, as a civil engineer I routinely witnessed great designs sentenced to failure during the construction phase. Quality installation is critical to delivered performance meeting the design intent. For this reason, I'm always looking to balance performance with construction methods that don't stray that far from standard construction techniques. That's extra important when you live in a region that isn't accustomed to high-performance building.
I agree with point #2. That's the catch with all high-performance wall assemblies. Detailing the punched openings becomes tricky, and expensive. Personally, I'm good with compromising aesthetics and the increased cost to gain the performance...but
I agree with your point on the rainscreen, we will be using one on this project. We're also trying a new (to us) air sealing technique using Zip sheathing to make an airtight attic deck. I really like how this method makes the air barrier transition from vertical to horizontal surfaces super easy. It also puts the air barrier in a place that is unlikely to get punctured by future subs.
https://www.jlconline.com/how-to/insulation/an-airtight-lid-the-easy-way_o#
Cheers!
> fastener holes in your WRB
A fully-adhered WRB will almost completely address this issue (a hole filled by a nail isn't a hole).
Mark - or anyone else that knows the answer :)
I see Huber specs water vapor transmission at < 1.0 perm for the foam insulation. I've not been able to look at a full sheet of the Zip R-Sheathing, only the small sample they sent. Is there a hard coating on the interior side of the polyiso foam? I don't see how else they could achieve that low perm rating with polyiso. There is a coating on the sample, but I initially thought that was just to keep the foam from crumbling off the sample as it got handled. Thanks!
Adam, polyiso has to be formed against a "skin" of some sort. The most common is foil, but Huber uses coated fiberglass/felt facers. I can't find data for the permeance of the facer but it's likely higher than the ≤1.0 of the foam itself, which they specifically list for the foam itself.
Adam Zahniser said:
1) If my thinking is correct, doesn’t the back of the OSB in the Zip panel become the first condensing surface when installed directly to the studs? That removes a major benefit of continuous exterior insulation, which is keeping the first condensing surface above the dew point. What are your thoughts?
The foam against the studs is the first condensing surface. If it is thick enough compared to the fluffy insulation against it, it will never dip below the dew point. Condensation simply cannot occur. At ≤1.0 the foam is a class II vapor retarder. Moisture should never reach the sheathing. The rest of the system can be vapor open. Everything inside the foam dries to the inside. Everything outside dries to the outside.
> Condensation simply cannot occur
Except that some interior air will flow between the joints and reach the cold OSB and exterior tape. So some condensation will occur with the standard use of Zip-R.
Adam - What did wall design did you end up going with? I'm in zone 5 and plan on starting a new home build just south of Detroit metro this next year. I personally do not like that the Zip R prevents drying to the outside. I also do not like that Huber uses OSB vs plywood. Thinking of just using 2x6 with plywood, tape the seams, rockwood batts in the cavity and Tyvec house wrap in the outside. Yes, I know I will get some condensation in colder winters but the plywood should be able to handle it and the wall can dry both ways. I'd like to add Rockwool insulation on the outside of the plywood but I think the windows and door details would not get built right without a high amount of baby sitting.
Jeff - likewise! I'm near the Toledo area and would also be interested in your wall build. I hope to finally build in the spring. I too like the idea of Rockwool instead of foam but it seems that the integrated Huber system would create less lab0r.
> I also do not like that Huber uses OSB vs plywood.
Huber's OSB products are more consistent and well made than any non-furniture plywood I've seen in recent memory.
It may be more consistent, but if it does not hold up to moisture as well as plywood why do I care? I guess that would only apply to Zip sheathing, not Zip-R which has foam bonded to the backside where water can't get to it anyhow except through the edges. I just think that in most builds there will be some air leaks that will lead to moisture in the wall cavity that will need to get out. The polyiso foam will prevent that moisture from getting out plus I think the insulating properties of polyiso decrease with decreasing temps.
Jeffrey & Drew,
We ended up using the wall system described in my original post. Overall, I am generally happy with that choice. All of the items Mark Harrison pointed out are spot on, and things you should think through before starting. I'll add the following:
1). In Zone 5, the calculated foam thickness is 2". With the 2" Zip-R, you need 4" nails for fastening. Most nail guns are limited to 3-1/2" capacity, so your installer will need special guns. You'll also probably have to special order nails. As others pointed out, my installers had significant problems getting consistent nail set depth due to the foam flexing differently based on the framing behind it.
2). A standard 7-1/4" circular saw can't cut all the way through the Zip-R with 2" foam. You'll need a bigger circ saw.
3). Huber requires blocking at every seam. This resulted in a lot of extra time and money installing extra blocking. Also, all the extra blocking was a complication when it came time to install insulation.
4). If you nail off the Zip-R per Huber specs for shear, it's a LOT of nails (added time and materials). In my opinion, the Zip WRB is completely compromised if you don't fill the nail heads. I personally covered every nail hole with Zip flexible flashing and it was VERY time consuming. But, I believe it was a prudent step.
5). The thickness of Zip-R lead to a lot of exposed nails inside the assembly ("flashers"), that came out the side of the framing or missed the framing completely. These are giant metal wicks for heat and moisture that should be dealt with. I went through the house before insulation, cut them off with a grinder, and liquid flashed the holes in the polyiso backer.
6). Seriously consider how you will terminate the sheathing at the foundation. We flushed the framing at the outside of the concrete foundation wall for finished siding to plane out better with our adhered stone founsation finish. This left 2-1/2" of horizontal foam and OSB that had to be covered. I liquid flashed from the bottom row of fasteners on the face, across the exposed bottom of the Zip-R panel and lapped 3" onto the foundation. This required extra caulking to repair install damage to the foam, and other irregularities that couldn't be bridged by the liquid flashing. This is a very vulnerable location and needs to be installed well. The process was very tedious, time consuming, messy, and required a lot of expensive liquid flashing. Matt Risinger has some potentially good methods on his You Tube channel, which I wish I had seen before our build.
7). I thought the Zip-R method would simplify the install and reduce cost (compared to other external insulation systems). I'm no longer so sure. If your installer hasn't used it before...I'm not sure it will save time. Mine spent a lot of time figuring out how to do it.
Overall, I'm pretty happy with our choice. I'm not sure it was the most cost effective or "best" choice though. In our case, special order materials and tools definitely increased the cost of this method. Advanced wall assemblies are complicated and the "right" one is very project specific. It depends a lot on the abilities/experience of your installers. Talk it through with your builder in advance. Everyone has to be on the same page for any of the methods to ultimately be successful.
> In Zone 5, the calculated foam thickness is 2"
Can you elaborate on what calculation produces this figure for Zip-R?
This is a very good reference for wall options:
http://www.appliedbuildingtech.com/rr/1701-01
I'm going to point out/respond to a couple things for future reference for others
2)Run the 7 1/4 saw at full depth and the foam should snap like drywall. Or you can finish the cut with a long blade utility knife.
3) Did you install the sheathing horizontally?
Install vertically and land on studs for less blocking. I know that statement could start a fight but I've not seen data that there was a big difference in strength and if you need more shear strength you can run metal strapping or other bracing methods.
4) It does use a lot of nails, I've only used the 1.5 version but I use a coil nailer for the sheathing because it uses so many nails.
Huber says the nail heads are not significant but I still cover them. Instead of taping them use liquid flash and a scraper while the wall is still on the deck. It goes fairly quickly if the framing crew is coordinated. One guy nailing and another walking behind him with a sausage tube gun and a bondo scraper.
5) I snapped chalk lines for the studs when nailing off the field. It is kind of a pain but then you don't miss because it is definitely hard to feel the studs behind the Zip-R
6) Yes, Risinger does have some good methods. This intersection is where wide tapes, coil stock or bent flashing really shine. Liquid Flashing this joint works but costs a ton and is slow.
> Huber requires blocking at every seam.
Could you share why you think this is the case? The Installation Manual (https://www.huberwood.com/uploads/documents/technical/literature/ZS_R-Sheathing_Installation_Manual-R3-12v21.pdf) says: "Wall panels that are designed to resist lateral shear forces should have solid framing or blocking behind all panel edges."
That seems to me that seam blocking is only required on shear walls (or "braced wall panel" in the IRC). I would think this is a subset of specific walls, not *all* your walls.
The Code IRC shearwall tables allow unblocked shearwalls but they have 2.5 times the deflection as a blocked shearwall.
It is normal to block all panel joints in high seismic areas. It is even required to install 3x studs and blocking in medium capacity shear walls & higher. It is also normal to specify full diameter nails, not the skinny gun nails.
The testing Huber did likely showed lots of deflection with the foam.
Deflection in a wall assembly = damage to the building