Building envelope


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

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

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
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

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

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

Great, thanks again!

Jan 14, 16 5:55 pm
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:

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, 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: 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

I would like to recommend you the <a href="">Water Vapour Permeability Tester</a>, to determine the water vapour permeability of various textile and garment fabric, coating fabric, composite, sportswear and industrial fabrics.

Aug 1, 18 3:41 am

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

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

Aug 2, 18 1:32 am

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