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jon-manuel

We are settling on a wall assembly consisting of 5/8" gyp board, 2x6 studs, open cell spray foam insulation (in the cavity), zip system sheathing (pre-applied air/weather barrier), and vinyl siding.  We are getting conflicting opinions on whether or not this is a competent wall assembly.

The guy who represents tyvek has a different opinion than the zip system people while the contractors are ok either way.  The tyvek rep says don’t use zip sheathing unless you are insulating on the exterior side of the sheathing.  Conventional thinking says this is accurate ‘if’ zip sheathing acts as a non-permeable air/vapor barrier.  So the tyvek guy insists that the zip sheathing is not permeable.  The zip people say their sheathing is permeable however third party testing shows once the zip system is taped it is a pretty tight seal (as long as the tape is functioning).

My issue is, if the zip system does in fact act as an air/vapor barrier it will trap moisture behind the sheathing.  I do see it as a valid point since standard plywood sheathing has gaps at all butt joints, is not taped and does not have an applied barrier on the exterior face.  I worry that these walls, once constructed, will start to produce mold as a result of moisture being trapped which is why I am intentionally not calling for a warm side vapor barrier.

I’d love to hear people’s experience on this topic.

 
Jan 13, 16 1:55 pm
gwharton

Vapor barrier goes on the warm side. If the zip system as installed has a low enough perm rating, it's a vapor barrier, regardless of what the manufacturer says. So, I'd put specific performance criteria in the spec for vapor permeability and insist that the manufacturer provide independent test results showing their system as installed meets those criteria.

Jan 13, 16 2:01 pm  · 
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gruen

number of perms defines if it's a weather barrier or vapor barrier. Zip system has 12-16 perms, which means it's a weather barrier. Tyvek is much higher about 50 as I recall, but this gets to the point where it's a bad weather barrier. 

you'll be putting a vapor barrier under your gypsum board anyway, which will block the vapor at the inside, just like you are supposed to. A vapor barrier is much lower 1-2 perms. 

I'm assuming you are in a relatively cold climate, as most of the US is. 

If you don't put a vapor barrier on the inside, then the zip system won't do much anyway, because you are letting the vapor into the wall, where it will condense before it gets to the exterior sheathing anyway.

BTW - your open cell foam is ALSO a vapor barrier.

you are way overthinking this thing. Vapor barrier goes on the warm side - which is the inside of the home in cold climates. 

Jan 13, 16 2:21 pm  · 
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We need to know where this project is located, if it is in the northern climate then all the advice is correct, if it is in the south the warm damp air is on the outside.

Mar 18, 21 9:43 am  · 
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Wood Guy

Jon-manuel, there are two issues here, air barriers and vapor retarders. The short version is that open cell foam in 5 1/2" thickness is both, so unless you live in a very cold climate, you won't have moisture movement within the wall assembly. The edges of the assembly (plates, corners, etc.) are a different subject.

Some of the finer points depend on your climate zone. When vapor drive is from outside to inside, some moisture will accumulate within the sheathing. Using a rain screen and good flashing details helps a lot. Now for the long answer....

 

Zip sheathing, taped, is a very good air barrier. I design, among other things, houses with R-60 walls and less than 0.3 ACH50, which is extremely airtight, and we use ZIP as our primary air barrier.

As for the vapor portion, resistance to water vapor movement is measured in Perms. ZIP, like any OSB, has a perm rating in the 1-3 range, which is considered "vapor semi-permeable". It actually opens up a bit when it gets damp, which helps assemblies dry out, up to a point. "Vapor Barrier" is specifically a product with a perm rating less than 1. "Vapor Permeable" is something with a perm rating over 10. So OSB (including ZIP) is on the low-middle end of things. It can work just fine in forgiving assemblies.

ZIP's weather-resistant coating perm rating is in the teens, and Tyvek is even higher, so they aren't contributing to trapping moisture in the assembly, unless you do something stupid like installing wood siding or another reservoir cladding directly over the WRB (aka housewrap). With higher end sidings you need to add a rain screen to create a forgiving assembly; vinyl siding is plenty leaky on its own, so you don't need to worry about it slowing down water vapor on its way out. (But you should worry about water getting in through the vinyl.)

Jan 13, 16 2:39 pm  · 
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danielzajic

Late to the discussion... I'm wondering if a 2x framed wall with ZIP and exterior foam will work in a very cold climate, without closed cell inside the walls. Seems like it would allow drying to the outside (and inside) but would energy efficiency be too compromised?

Mar 18, 21 7:56 am  · 
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Non Sequitur

Is there any insulation is the stud cavity daniel?

Mar 18, 21 8:21 am  · 
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danielzajic

There could be, but I'd rather have empty wall cavities, if possible, for various reasons (running wiring, etc). Seems better to use insulation outside the framing, more bang for the buck, and would keep the entire structure at a more consistent temperature year round.

Mar 18, 21 9:01 am  · 
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Non Sequitur

then follow-up question, how cold is very cold?

Mar 18, 21 9:02 am  · 
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danielzajic

Climate zone 6, so consistent drops below 0F at night.

Mar 18, 21 9:09 am  · 
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Non Sequitur

... speaking from a zone 6 with -40C winters, and unless you add several layers of exterior insulation, you will need some, if not most, of your thermal layer within the studs. Use deeper studs, or add a chase, if you need service spaces. For example, bare minimum code wood stud wall in my area is R13 inside stud and R15 continuous. It can vary, but that's the starting point.

Mar 18, 21 9:23 am  · 
1  · 
danielzajic

I was thinking to use maybe 8" of foam (R40?) and then a couple inches of closed cell within 2x6 walls, leaving plenty of cavity space, OR, 10-12" of exterior foam, and 2x4 walls with no insulation. Is R50 not enough, or am I missing something?

Mar 18, 21 10:30 pm  · 
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Non Sequitur

That’s a shit-ton of extra width to that wall. Loss of interior space, extra deep windows, structural support for foam, etc. Plenty of bad ideas.

Mar 18, 21 11:16 pm  · 
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atelier nobody

https://www.youtube.com/watch?v=E_ngmbANeOg

Mar 19, 21 12:49 am  · 
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danielzajic

Thanks, I've watched that, and will watch again. I also got the BSC Building in Cold a Climates book. I guess if there's at least *some* exterior insulation, the thermal bridging of the walls is already reduced quite a bit, so cavity insulation is more effective. Thanks for the replies, all, appreciate your input! ✌️

Mar 19, 21 8:53 am  · 
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pghaec

The exterior-only approach is fine as long as you can hit your desired R value for your climate and it is arguably the most hygrothermally forgiving assembly you can build. Whoever said it's plenty of bad ideas doesn't seem to understand moisture management.

The only issue I can think of is if you have any fasteners penetrating the sheathing, you'll want to add a little bit of insulation on the backside of the sheathing to cover those fasteners because during very cold periods condensation could in theory form because they're thermal bridges. But it seems pretty unlikely, to be honest.

With exterior-only insulation, if you're not working with extremely tight setbacks, it's arguably actually better than a split insulated wall. I don't even think it's that much costlier because the labor to cobble in all the interior insulation, put up an additional vapor throttling 'smart' membrane behind the drywall, and seal all the outlets etc is probably a wash compared to the extra 4-5" of exterior insulation. Hell 3" Rockwool is cheaper than 2" Rockwool most of the time.

And in extremely cold areas, exterior insulation is the ONLY way to safely build: https://www.buildingscience.com/documents/insights/bsi-031-building-in-extreme-cold

Nov 27, 22 5:55 am  · 
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danielzajic

Thank you for that reply. Good point about the fasteners. I have zero setbacks with either of my project ideas, so that's not an issue. It's fun to push the envelope (sorry, couldn't resist) and think about what layers/components can be removed, and how building something ultra-efficient and long-lasting can be simplified if we can remove things we think we need but really don't (e.g. finished walls).

Nov 28, 22 8:59 am  · 
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pghaec

People mostly add insulation to stud cavities because it's convenient and lines up with sequencing that they're used to. But from a technical perspective, it's mostly worse than continuous exterior insulation in cold climates. There's nothing wrong with extra deep windows in cold climates -- they provide additional protection from rain and wind washing if you set the windows at the sheathing plane.

You definitely don't want most of your insulation in the stud cavity as things get colder. A higher proportion of continuous exterior insulation in cold climates or with humidified interiors is always more forgiving.

Nov 30, 22 6:35 am  · 
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danielzajic

Another good point about window placement, I hadn't considered that before, either. That's good, because it means no special window support is needed, as it would be if installed outside the framing. Also creates some shading for high summer sun. Good discussion!

Nov 30, 22 12:22 pm  · 
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pghaec

Also, ZIP is definitely good enough and probably the easiest to construct, but if you did want to go the extra mile you could substitute in plywood with a fully adhered, monolithic (not microporous) membrane.

Nov 30, 22 2:05 pm  · 
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jon-manuel

I thank everyone for the feedback.  It's all very useful.

I am recommending that we do not install a 6 mil poly VB on the inside face of stud.  My understanding is that closed cell spray foam is a true vapor barrier where as open cell foam is vapor permeable; so technically not a vapor barrier.  Despite this I still feel that this assembly should be treated differently than a traditional stud cavity wall with batt.  Beyond that there is a popular and growing conciseness that poly vapor barriers in stud cavity insulated walls are not ideal and are being phased out (unless your in a climate that is always hot or always cold).  The simple solution is to 'only' construct walls that are insulated on the exterior side of sheathing; unfortunately this is not always feasible and presents constructability issues in simple wood framed construction.

I'd like to know if anyone is of the opinion that I should absolutely add the 6 mil poly to the inside face of stud in this wall assembly.

Jan 14, 16 2:51 pm  · 
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gruen

open cell spray foam is not as good of a vapor barrier as closed cell, but it is a vapor barrier when it gets thicker. 

Honestly, the paint on the drywall is also a vapor barrier, but the vapor will get in around the openings and outlets anyway. 

I think you should download WUFI and play around with various assemblies. 

Jan 14, 16 3:43 pm  · 
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bowling_ball

Since we're nitpicking, paint isn't a vapor barrier, but it can be a vapor retarder.

Mar 18, 21 11:11 pm  · 
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atelier nobody

True for regular latex paint, but some high-performance and other specialized coatings, such as epoxies and elastomerics, actually will block a lot more vapor (which can be a very bad thing if you put them in the wrong place - ask me how I know).

Mar 19, 21 12:48 am  · 
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jon-manuel

Great, thanks again!

Jan 14, 16 5:55 pm  · 
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Wood Guy

jon-manuel, unless you are in a very cold climate, you should absolutely not add the 6-mil poly. Poly should not be used with batt insulation, either. In fact, the only place in a house where poly belongs is under the slab. 

Painted drywall is a vapor retarder, not a vapor barrier. The difference is important because a vapor retarder allows the wall to dry if it gets wet. A vapor barrier does not allow any water vapor to pass, creating mold factories.

There is a link to a good paper on the subject here: http://buildingscience.com/documents/reports/rr-0912-spray-polyurethane-foam-need-for-vapor-retarders-in-above-grade-walls/view.

Anyone still using poly on the walls is at least ten years behind the times, closer to twenty. Building science has come a long way since then. Unfortunately when people misunderstand things like vapor barriers vs. vapor retarders it makes it hard for those just learning to figure out what's going on. Archinect is a good forum but for building science, greenbuildingadvisor.com is a great place to learn the basics and to ask questions.

I would encourage you to play around with WUFI as Gruen suggests, so you better understand what is happening inside the walls. (free version here: https://wufi.de/en/service/free-wufi-versions/) But please do not rely on anything it tells you, as even the smartest building scientists in the country get conflicting answers or answers that don't match field measurements.

Jan 15, 16 10:25 am  · 
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PandasAreSexy

Have you read: Builder's Guide: Cold Climates ?

I agreed with Wood Guy about the buildingscience.com papers and greenbuildingadvisor.com as a good source.

Aug 2, 18 1:32 am  · 
1  · 
whistler

Just a head up but.... my understanding is that open cell foam is not a competent vapour barrier. Closed cell foam is a vapour barrier, one becomes hard the other always remains semi soft.

Mar 18, 21 6:39 pm  · 
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natematt

That's one of the selling points. Like if you're doing a double layer of sealant you use open cell foam backer rods on the rear layer so that the sealant can dry out properly from the back.

Mar 19, 21 12:51 am  · 
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Wood Guy

Open cell foam is vapor permeable but when installed at least a few inches thick it's a good air barrier. It's important to understand the difference.

Closed cell foam is not a complete vapor barrier, the way poly sheeting or foil are. When installed at 1-2" thick it's a class 2 vapor retarder, allowing some water vapor to pass through, which can be good or bad. Installed at thicker depths it's a class 1 vapor retarder but still not completely impervious to water vapor.

Mar 20, 21 10:27 am  · 
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