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diy'er on LI 04-08-2012 09:24 PM

question about exterior vapor barrier integrity....

we had a shower disaster... mold on the plywood of an exterior wall. Because it's a second story shower, I hired a guy to replace the exterior wall plywood and shingles. (I don't do scaffolding or roof walking!) He ripped out a few sheets of plywood, replaced with clean/new ones, added a green colored vapor barrier and then shingled over it. Since then, no shower work has taken place. (still gutted to studs)

Well, on the first sunny day, to my horror, I saw light shining through that exterior wall! The new plywood had a few mm gap with the old plywood, there were no shingles in that tiny area, and light was shining directly through the vapor barrier. I could swear there's also a pinhole in the vapor barrier.

My gut instinct says I should be concerned and should do something to remedy this problem. I suspect I should be sealing-up the 2 pieces of plywood somehow (exterior grade wood putty?). But should I be worried about the vapor barrier having a pinhole in it? If need be, I would climb up there... I just feel unsteady on the roof :(

Obviously, priority #1 is avoiding another water damage nightmare.

Thanks for your feedback!!!

cortell 04-08-2012 10:30 PM

Let's hope he didn't put a vapor barrier over the plywood! Hopefully (and more likely, if he's a good contractor), he put a moisture barrier. Huge difference. A moisture barrier will prevent liquid water that makes its way behind the siding/brick/whatever to reach the plywood, yet it will let water vapor to escape. This is important because you're going to have (I hope) a vapor barrier on the warm side of the wall (behind the sheetrock). If there was a vapor barrier on both sides of the wall, you'd basically have a lot of wood trapped in a ziplock bag. Any moisture that ever gets in there is going to stay in there for a long time, and that will breed mold like crazy. So, vapor barrier on one side, moisture barrier on the other. If any wood gets wet in between, it has the chance to dry out--through the moisture barrier.

Now, regarding sheathing. Unless you live in a constant temperature environment (I lived in Queens, so I know you don't ;-), sheathing panels should be installed with 1/8" gap to allow for expansion. So, the light you're seeing...that's a good thing.

Now, as for the pin hole in the moisture barrier...if it's just a pin hole, I wouldn't worry about it. Nothing is ever going to be 100% perfect. In fact, how do you attach a moisture barrier to the sheathing? With fasteners that pierce the barrier! Each piercing is a potential failure point. So, I wouldn't loose sleep over one pinhole.

diy'er on LI 04-08-2012 10:52 PM

are you implying that lovely long island doesn't have as temperate and predictable climate as san diego? Yeah, you could say there are minor fluxes in temp here :) 0-100F.... that's not that large of temp range, right?

Thanks so much for replying... you really put my mind at ease. It makes perfect sense that a gap should be there, now that you mention it. Logically speaking, I know the MOISTURE barrier (see, I'm learning!) already has nail perforations... but to be able to see light shining through just put me on edge.

(and yeah, I'm putting up a vapor barrier... in the form of paper-backed insulation... then putting up the cement board and slathering the whole thing with laticrete. Sorry for the miscommunication. I sometimes have a brain fart with building supply jargon!)

KennMacMoragh 04-08-2012 11:18 PM

The plywood sheathing should have gaps, but not the shingles. The siding is installed wrong, did you call him and tell him the problem?

ratherbefishing 04-08-2012 11:22 PM

If you're covering the inside of the cement board with a membrane (Red Guard, Hydroban, Kerdi, etc), then no vapor barrier behind. I removed the paper from the FG batts before hanging Permabase and Kerdi. you don't want a moisture sandwich.

diy'er on LI 04-08-2012 11:28 PM

another good point. can you tell I'm still in the research stage of my project :whistling2: good grief. another crisis averted. thank you!

yeah, moisture sandwich is not a good idea. Easy enough to find insulation w/o the paper backing.

diy'er on LI 04-08-2012 11:31 PM


Originally Posted by KennMacMoragh (Post 894855)
The plywood sheathing should have gaps, but not the shingles. The siding is installed wrong, did you call him and tell him the problem?

well, this is the thing... I don't want to call him until I'm certain there IS a problem. I'll double check it tomorrow (need daylight). I believe it's just where 2 cedar shingles butt-up to one another. I'll have to get back to you on that......

Gary in WA 04-09-2012 12:21 AM

As Kenn said, you should NOT be able to see daylight from inside with shingles. A green color WRB may be specially for shingles for the required air gap- if he is good, Photo #3:

Again, notice the shingle lapping the course below, (same wall, different view) Photo #4:

Your paper facing on the insulation is a Class 2 vapor retarder, not a vapor barrier (Poly sheeting). It has a variable permeability- more open, the wetter it gets), from 1-10 perms- Ginger vs Mary-Ann:

Without a vapor barrier (poly) on the exterior, you won't have any problems with the facing on the inside. As it can dry to the exterior, it's not a v.b. sandwich.
Your location Code requires a Class 2 (paper-facing) on the warm side of the exterior wall:

"R601.3 Vapor retarders. Class I or II vapor retarders are required on the interior side of frame walls in Zones 5, 6, 7, 8 and Marine 4."

R601.3.2 Material vapor retarder class. The vapor retarder class shall be based on the manufacturer's certified testing or a tested assembly.

The following shall be deemed to meet the class specified:

Class I: Sheet polyethylene, unperforated aluminum foil.

Class II: Kraft-faced fiberglass batts.

Class III: Latex or enamel paint."



cortell 04-09-2012 07:42 AM


Originally Posted by KennMacMoragh (Post 894855)
The plywood sheathing should have gaps, but not the shingles.

I agree, and I may have misread the original post. The comment "there were no shingles in that tiny area" led me to believe the siding job was in progress and that the area he was seeing light through had yet to be done.

If the siding is up, you definitely should not be seeing light. Regarding butt joints in siding, there's a good amount of debate about how to best weather proof them. Regardless, if you see light through the siding, it's all but guaranteed rain will get in, and that's not good.

cortell 04-09-2012 07:47 AM


Originally Posted by diy'er on LI (Post 894867)
another good point. can you tell I'm still in the research stage of my project :whistling2: good grief. another crisis averted. thank you!

yeah, moisture sandwich is not a good idea. Easy enough to find insulation w/o the paper backing.

Make sure you understood the point here correctly. Certain bathroom wall products provide a vapor barrier. E.g., Kerdi. If, and only if, you are using such a product, you do not want a second vapor barrier at the interior side of the studs. If you're, e.g., using plain ol HardiBacker, you need an explicit vapor barrier behind it.

cortell 04-09-2012 07:59 AM


Originally Posted by GBR in WA (Post 894890)
Your paper facing on the insulation is a Class 2 vapor retarder, not a vapor barrier (Poly sheeting). Gary

Maybe you know something I don't, but I don't think there is such a thing as a "vapor barrier". The term is a misnomer. Anytime someone says "vapor barrier", they're really referring to "vapor retarder". Different products have different degrees of effectiveness, but I don't think any of them qualify as as technically being a "barrier".

House wrap, Grade D Paper and Asphalt Felt--those are, in fact, barriers. They're moisture barriers, though.

Gary in WA 04-09-2012 11:03 PM

Barriers stop all moisture/water from passing through them, retarders let some through;

A Class 1 vapor retarder (poly sheeting, vinyl wallpaper, foil-faced insulation board) is a true vapor barrier:

H.W., felt, and building papers are Code-accepted WRB's (water-resistive barriers), letting moisture (water vapor) pass through either direction. They are NOT moisture barriers as they let moisture through in varying degrees: felt/papers = 5-50 perms, housewraps = 11-58 perms;


diy'er on LI 04-10-2012 12:11 AM

ok, I checked again this morning. (the wall is east facing, so it's easiest to see when the sun is still rising).

DEFINITELY gaps between the shingles that sunlight penetrates. On the order of 1/8 to 1/4". In the tiny gap of the plywood, I see 2 of these gaps. So difficult to see up there from the outside with the way the roof lines are situated. ugh.

This means I'm basically missing some courses of shingles because they're not properly overlapping, correct? That probably would cause a lot of damage from unnecessary exposure to the elements, I presume.

I have to try to squint and examine the shingle job and then contact the guy back. Thanks for clarifying that!

as for the "vapor barrier" discussion, sorry once again for the incorrect terminology. You guys are entering a techinical conversation which I'm beginning to not be able to follow.

for my shower waterproofing, I'm going to use cement board and then lacitcrete hydro ban. I'm presuming this should be unfaced insulation, because that's a pretty decent moisture barrier of sorts.

thanks guys, once again! :)

cortell 04-10-2012 07:59 AM

GBR, I yield to your obvious expertise on this matter, and appreciate the reference material. Just when I thought I had a handle on a confusing enough situation, I find out it's even more confusing than I thought. Unfortunately, this does not bode well for the industry, as the likelihood that these subtle differences in terms and material properties are well understood and applied by contractors is pretty low. And the fallout of such misunderstandings are often serious mold issues, affecting peoples' health and costing a lot of money in remediation and remodeling.

I'll be studying the documents you've referenced to be better informed. Thanks again.

diy'er on LI 04-10-2012 05:28 PM

Cortell, I'm glad I'm not the only one confused by that subtle terminology.

Honestly, being a DIY'er, I never go by product name, rather what its recommended uses are.... If I went by name, I'd be tyveking my shower and putting lacitcrete on the exterior wall :huh:

I totally agree... it is a trainwreck waiting to happen though. the industry really needs to clarify the products and their purposes.

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