Concerns Around Zip-R Sheathing
Hi all,
I’m a homeowner who will soon be putting up a new custom house in Southern Ontario (Climate Zone 6).
I’m working with a builder who is PassiveHouse certified, and they panelize their wall assemblies before bringing them to the job-site.
Their standard wall assembly is: drywall -> Intello Plus, 2×6 stud wall w/ fiberglass cavity insulation (~R21), Zip-R9, 0.75″ rainscreen gap, exterior cladding
I’ve been reading/listening/learning all I can about building science in recent months — but am far from an expert and fully acknowledge that a little bit of knowledge can be a dangerous thing.
Reading some of John Straube’s work suggests I need at least 37% of my insulation on the exterior…and possibly more in order to control for possible condensation issues.
I’m a bit worried about the use of Zip in my climate zone.
– OSB sheathing, where I think i’d prefer to use plywood for greater breathability and less moisture sensitivity
– polyiso foam which means the R-value will drop in very cold weather, which is precisely when there’s the greatest risk of condensation. As-is a 21/9 ratio feels a bit iffy to me but if that R9 is actually behaving like R6, then I’d be even more worried
– Also, I’m realizing now that the Zip system has the continuous insulation inside the OSB sheathing, which also seems a bit counterintuitive to me. Now I can’t figure out where the condensing plane will be…will it be at the the point where the poly-iso meets the 2×6 frame?
At a minimum, I’m going to ask to upgrade to the Zip R-12 system if we go down this path to buy me a bit of extra buffer from condensation (21/12 feels like i’m in the right ballpark…polyiso cold weather issues notwithstanding). I’m happy with the use of the Intello smart membrane on the interior, but am inclined to change to a different cavity insulation — possibly Hempwool — for the combination of indoor air quality and its moisture buffering properties.
Zooming out for a second, I wonder if I’m overthinking this? This builder is pretty good at assembling using Zip and air-sealing using this system. Their trades are really familiar with Zip, how to execute all of the taping/sealing needed, etc. We’re aiming at something along the lines of ACH 1.5 or so (the house has some complex geometry where aesthetics win-out over comfort/energy concerns).
But I wonder if I’m better off letting them proceed with a Zip-based wall design, or should I push for a different system using Plywood sheathing and an exterior insulation like Gutex (like the system 475 recommends) or possibly exterior mineral wool?
To summarize, I guess my question is: if they’re less good at air-sealing with unfamiliar products, does that mean I completely undo any benefits I may have gained by switching to a more robust assembly? And if so, should I just hold my nose and stick to the Zip System?
Thanks!
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Replies
Hi Maxwell,
I'll give your question a bump and say that the great majority of builders I know or have talked to really like working with Huber's Zip System. Your mention of careful air-sealing being key is spot-on. But if your builder is PH-certified and is accustomed to using the Zip System products as part of their (very respectable) standard wall assembly, my non-expert opinion is that you are in good hands. If you haven't already, take a look at some of the articles about Zip we have on GBA. They might make you feel more comfortable and spark some questions you can ask of your builder. Here’s one to start with: Working With ZIP System R-sheathing. (The author is building his own house right now and is using Zip R as the primary air barrier.)
Maxwell,
The 39% ratio is for walls with only class III warm side vapor barrier (ie painted drywall). With a smart vapour retader like Intello, the ratio can be pushed without problems. 2x6 walls with R5 rigid and interior poly (much less than 39%) have been built in Ontario for a long time without issues.
I don't think your builder would like R12 Zip. Beside the additional cost, it is hard to nail up as you need to buy a special nailer for the extra long nails needed. If you do an energy model on your place, you'll find that going from R9 to R12 barely changes your yearly energy use, not the best place to spend your efficiency dollars.
If your builder is familiar with Zip R, I would not hesitate to use it.
Maxwell,
I have built two houses with ZipR and really like it. Some people dont like it for various reasons and I understand their concerns. As far as moisture, Akos makes a good point. A few differences with ZipR that are important. 1. The foam is vapor open, not like tradtional foam over sheathing. 2. With the sheathing being the most vulnerable and right next to your 3/4" airgap, it has significantly more drying potential than being buried under foam. If it is what your respectable builder uses, then I would be happy to have it on my house. Best of luck!
Shawn
I don't skimp on the recommended ratios, but like Akos said, the prescriptive ratios are for using painted drywall as the only interior vapor retarder. If someone knowledgeable has done a hygrothermal analysis, I would be comfortable with the proposed assembly. If they have not done a hygrothermal model and are going on recent experience instead, I would be concerned, as moisture problems often take ten years or more to present themselves.
I also agree with Akos on Zip R-12; it requires a special nailer that can shoot 4" nails, but I've heard from multiple sources that it's hard to get the depth correct. As a result, although I like the concept of Zip-R, when I design exterior insulation in zone 6 I use conventional sheathing (usually Zip) and at least 2" of polyiso.
Shawn, the foam isn't vapor open per se but it's slightly more vapor-open than the OSB, according to Atlas, the manufacturer. The facing is vapor-open.
The reduced R-value in cold weather matters, but not as much as you might think, because in an assembly like yours, only the outer half of the foam is significantly affected, and assemblies should be able to handle a small amount of wetting. But the more exterior insulation/R-value you have, the safer your assembly will be.
Michael,
Thanks for that info, I had previously read that the sheet with foam had a 2 perm rating. I beleive it was from Huber. But I trust you are correct.
Shawn
Shawn, 1" of the polyiso they use tests at 1-1.5 perms, so 2 perms is a reasonable estimate for 2" material. Vapor open is usually considered to be over 10 perms.
I'm a bit surprised by that / not sure I understand.
If 1" of polyiso is ~1 perm, wouldn't 2" of polyiso be less than ~1 perm?
Haha, yeah that was a dumb math mistake. Doubling the thickness doesn't halve the perm rating, but that's roughly correct.
Michael,
You are correct, the perm rating of the foam is less than 1, according to Product-Data-Sheet-Wall-R-sheathing-ZIP-System.pdf. Thank
Shawn
Thanks for all of the comments. Very reassuring actually which is incredibly helpful.
This point about using smart vapor retarders like the Intello product making the interior/exterior insulation ratio a bit more forgiving was new to me. Thank you!
Continuing along that vein, I think what I'm hearing is that moving from fiberglass to an interior insulation like Hempwool that has better hygric buffering capabilities is also a helpful upgrade (in the "belt and suspenders" sense of multiple layers of risk mitigation)?
You may find some of the articles on GBA about double stud walls reassuring: https://www.greenbuildingadvisor.com/article/the-mythical-threat-to-double-stud-walls
As far as the sheathing is concerned, a Zip-R wall is not so different from a double stud wall, in terms of the basic assembly:
Interior
- Gypsym
- Class II Vapor Retarder
- Bunch of insulation
- Sheathing
- WRB
- Vented rainscreen gap
- Siding
Exterior
If a double stud wall can work in your climate zone, a similarly detailed Zip-R wall is likely to also work just fine.
Regarding the insulation, many of these articles speculate that the hygric buffering properties of cellulose insulation help, although I don't know that that has been proven.
Hempwool is a new kid on the block. It might be great, maybe not. The specs look pretty good and it is a primarily organic product. But its performance will probably be similar to cellulose, which is far easier to get. Negative net carbon costs are also probably similar. I think the big difference might be how far it has to be shipped to you. If you love the idea of hempwool, go ahead and give it a try. But don't bet the farm on being able to get it. Cellulose would be a good backup plan if hempwool is the chosen insulation. Both are better from an environmental standpoint than FG. both probably have similar hygric buffering behavior. Not sure what hempwool uses to make it bug/mold/fire resistant - not much info on that on their website. In fact, I don't see much in the way of code compliance test data on their site at all. You might need special permission (and a Professional's sign-off) to use it on your project.
How are you separating the attic from the conditioned living area?
I've heard Joe Lstiburek say "Good luck getting proper shear ratings with ZIP-R"., I guess because there's a layer of soft foam between the wall studs and the sheathing. Any conversations on that?
Scott,
https://www.greenbuildingadvisor.com/article/working-with-zip-system-r-sheathing
What did you end up going with? I would assume condensation is a non-issue since ZIP R foam is located on the interior side, and there is a rain screen to allow for drying, with the intello on the interior side to boot.