ZIP Sheathing and Vapor Barrier
B3V1156
| Posted in General Questions on
Hello, I was curious to know if a vapor barrier is recommeneded on the interior side of an exterior wall when using the ZIP system in a cold climate? I have heard some mixed reviews on this such as ZIP not being very permeable, so if water was to get in with the use of a vapor barrier, it would have nowhere to go. The climate zone is 6B and planned on including 1×4 strapping on the exterior side of the wall for a rainscreen, rockwool comfort batts in wall cavity and 2×3 horizontal strapping on the interior with 1.5″ polyiso inbetween the 2×3.
Thanks,
Garrett
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Replies
I'm interested in the replies from the pros, just for my own knowledge.
ZIP coating is perm rated to the purpose of getting water out. However, we all know OSB is not permeable at a "good" level. So ZIP has this interesting design vs promoted benefit that doesn't align perfectly.I am also thinking the recommendations here to consider taping OSB seams air tight.
If it weren't ZIP, we'd hear people saying liquid flashing, like Cat5 of MVP, should be applied to both sides of sheathing with a house wrap behind the liquid flashed sheathing. I've never heard that. So if someone suggests yes, I'd be curious to know why.
Good luck Garrett
Liquid applied WRB on both sides of the sheathing? Why?
I recommend using a variable permeance membrane in most projects in cold climates--they have all of the advantages of poly sheeting without the downside of trapping moisture in the walls. The cheapest is Certainteed Membrain. I spec Siga Majrex, Pro Clima Intello+ or Rothoblaas Clima Control, which are more durable than Membrain.
The OSB in Zip sheathing is good-quality OSB but it does have a lower perm rating than plywood, boards or other types of sheathing. If your wall is designed to keep most moisture out (interior membrane, controlled humidity, good WRB and flashing details) and to dry readily (ventilated rain screen, not too close to grade) your walls should be fine. The coating on Zip (and the insulation and facing on Zip-R) are higher perm than the OSB itself.
I always wonder about variable permanence membranes once you have a couple coats of latex paint on the interior walls.
Ive done zip bare and with hydrogap over it. On my own house redo, I'm using hydrogap on gable ends with no overhangs over the zip. No room for a 1/4+ rainscreen unfortunately and hydrogap drains way better with LP Smartside than smarside on zip (sits real tight this way) from my testing.
Latex paint is about 2-10 perms; it's considered a class 3 vapor retarder and doesn't change significantly with humidity levels. It slows moisture from getting into the wall to some degree and is vapor-open enough to allow inward drying.
Siga Majrex is 0.17 perms when dry and 1.3-3.8 perms when wet, so it keeps the vast majority of moisture from diffusing into the wall while still allowing it to dry inward.
Pro Clima Intello is 0.13 perms dry and 13 perms wet so it's even better than Majrex at keeping moisture out and even better than latex paint at allowing inward drying.
To me, those numbers clearly show that using a variable permeance membrane is more likely to result in a durable structure than relying on latex paint alone. It probably doesn't matter much for a wall that meets code minimums (pre-2021 IRC, anyway.) I mostly do walls with 50-100% higher R-values than code minimums; there is very little heat energy available in those walls to push moisture through so I think it's important to keep as much out as reasonably possible. Many building science experts say that we shouldn't worry about diffusion, but I've seen enough data logger results that show elevated moisture levels even in airtight homes that I believe diffusion is still worth worrying about. (Air tightness is much more important but both are important.)
Hydrogap allows liquid water to drain, so if you're installing siding directly over it, it's better than not having a rain screen at all. But it doesn't provide enough air flow to dry the wall so if you have a reservoir cladding with a finish on it, or an impermeable siding, I would be worried about long-term moisture-related issues.
Thanks, Michael. For the variable permeance membrane, I wanted to get your thoughts on if Halo's Interra product would be a good canidate for this? (see attached) Seems to be a 2 in 1 product which I'm sure may not be the most economical approach, but since I had already planned on 1.5" polyiso inbetween 2x3's (OP) it may be worthwhile. As a side note, the option to replace the ZIP with and alternative sheathing and WRB have passed.. Exterior walls will be fabricated panels and are currently in production. At this point it is making sure the wall system described in the OP will be a safe and efffective one. Thanks again.
Halo Interra is not variable permeance, it's impermeable, but because it insulates, installed on the interior in a cold climate it prevents condensation. (It might work in a hot climate as well but I'm not sure what the R-value ratios would have to be.) It's pretty comparable to foil-faced polyiso, which I have used on the interior several times with no problems that I'm aware of, and I've seen it used successfully on projects built in the 1980s.
If you can provide a 1" air space, you'll get an extra R-1 or so out of your wall assembly. Just be sure the foil is shiny and clean when installed; it doesn't act as a radiant barrier if it's dirty.
If I was to use the Certainteed Membrain or Siga Majrex, would that be placed behind the 1.5” polyiso or on top? To me it makes most sense to attach to interior face of the exterior wall frame first then apply the rigid boards, but just would like to confirm.
Your polyiso changes things--I missed that detail on my first reading. Assuming it's foil-faced, it's vapor-impermeable so a separate membrane probably isn't necessary. Ideally you'd tape the seams or at least fit the foam tightly to the framing, but diffusion is largely dependent on area so if you're covering 98% of the wall with polyiso and 2x lumber, you're already blocking almost 98% of the potential water vapor from getting into the wall. If you have air leaks in your assembly that also changes things; you should have at least one really good air control layer. The variable-permeance membranes can serve that role but when using Zip sheathing, that is usually an easier location to make airtight.
Micheal,
Isn't the OP asking how the interior moisture leaves the interior, goes through the insulation, hits the zip sheathing, and then exists the zip sheathing if not through a high perm house wrap like Tyvek? If that's how I read it, it makes me curious too.
Interior moisture that gets behind Membrain or poly doesn't condense into a droplet on the interior side of the sheathing if everything is air tight right? Even if so, Tyvek on the interior isn't going to "drain" outside anyways? Therefor, Tyvek behind the Zip sheathing would never make sense right?
I would say I don't fully trust 30 year rated tape on seams that aren't an issue on liquid flashing or house wrap. Since Tyvek is cheap and no one can guarantee their install taped 100% perfect why not put house wrap on the Zip sheathing on the outside. But that's just my thoughts on it.
Ryan, I'm not sure how your question differs from the OP's or why my response isn't clear. I didn't see a mention of Tyvek on either the interior or exterior in the original post.
When it's colder and dryer outdoors than indoors, which is most of the winter in cold climates, vapor drive is always from interior to exterior. Limiting how much interior air and vapor gets into the wall assembly is the first thing to do. That's where a variable permeance membrane helps, by keeping most air and water vapor from getting into the wall assembly.
For the air and vapor that does make its way into the framing cavity, it will keep pushing toward the outside until it finds a surface cold enough for it to condense (or otherwise accumulate--"condensation" is often a misnomer; adsorption and absorption are more accurate). That surface will likely be the interior of the OSB.
Vapor that has accumulated in and on the OSB will continue pushing outward due to vapor drive, and will eventually make its way through. As the OSB gets wetter it also gets more permeable, up to somewhere between 2 and 5 perms, which is considered vapor-semi-permeable. It will push right through the exterior coating with no problem, since the coating is vapor permeable.
Once the moisture makes it that far, ideally it will reach a ventilated rain screen gap where air flow will quickly dry the moisture. If instead the moisture reaches that backside of painted or impermeable siding, it will have to push through that, which is often the cause of peeling paint.
Adding a WRB to the exterior of Zip sheathing is fine and something I like to do when possible; just be aware that it voids a portion of Zip's warranty. I trust properly installed Zip tape to last a very long time; I don't know why people are afraid of adhesive there when they trust the adhesive holding their OSB and finger-jointed lumber together. But it's often not installed properly, which involves careful rolling or paddling to fully activate the adhesive, and time for the adhesive to set.
Gotcha. Thank you for the detail. So no, never would a house wrap be beneficial behind Zip
Behind, as in to the interior? No, I can't see a situation where that would be helpful.