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-   -   Exterior Wall Insulation require plastic barrier? (http://www.diychatroom.com/f19/exterior-wall-insulation-require-plastic-barrier-32466/)

BillyDIY 11-23-2008 02:15 PM

Exterior Wall Insulation require plastic barrier?
 
While tearing down ceramic tile and rotted drywall in a bathroom shower repair I noticed that the pink fiberglass insulation had some slight black mold growing on it, so I am opting to replace those columns.

My house was built in the late 1970s and I noted that all of my exterior walls including this bath one had a aprx 2-3 mil thick piece of clear plastic over top of the insulation. This bathroom insulation appears not to be "faced" in that it is pink on both sides (no paper on one side).

I have 2x4 outside wall construction so I purchased R-13 PinkPro or ProPink insulation. Because this is the "good stuff" and is "faced", do I need to put a 2-3 mil thick sheet of clear plastic over top of it?

It is my assumption that because this is "faced" that if I staple the faced part interior so I can see it that is all I should require.

If it matters I will be putting cement board over top of the insulation when I redo the shower stall.

AtlanticWBConst. 11-23-2008 04:11 PM

Quote:

Originally Posted by BillyDIY (Post 189060)
....do I need to put a 2-3 mil thick sheet of clear plastic over top of it?....

No, since it is already "faced", you have a vapor barrier over it (kraft paper face).
Installing an additional vapor barrier over another one, is actually against building code, since such an arrangement will create an "envelope" to trap moisture.

BillyDIY 11-23-2008 09:22 PM

Thanks Atlantic. Turns out on another forum I got the exact opposite advice so I did a little more searching and you are spot on.

http://saveenergy.owenscorningblog.c...raftfaced.html

RDS 11-24-2008 02:57 PM

On a similar subject: Is it true that one should not put greenboard over kraft-faced fiberglass insulation, for a similar reason? (i.e. to avoid creating a double vapor-barrier) I've read conflicting and/or unclear advice on this.

(Note I'm not talking about greenboard around a tub or shower or under tile -- just a regular exterior wall and ceiling, under attic, in a bathroom.)

Perhaps I should ask the more basic question: Is it overkill to use greenboard on a bathroom ceiling and/or on a bathroom wall that isn't directly part of a bath/shower enclosure?

Basically, since I've already put up the kraft-faced insulation and bought the greenboard but not yet installed it, I'm wondering whether I should (a) switch to unfaced insulation, (b) switch to regular drywall, or (c) neither.

Grateful for any guidance.

Termite 11-24-2008 04:02 PM

RDS, don't confuse greenboard with a moisture barrier. It isn't. It is only different than regular sheetrock in that it is a little bit mold resistant. Greenboard is just fine to use in bathrooms, provided it isn't used in the wet areas (shower/tub walls).

RDS 11-24-2008 04:35 PM

Thanks termite.

If 'greenboard as vapor barrier' is a misconception, it seems to be a common one. For instance, it turned up in this thread recently:
http://www.diychatroom.com/f15/green-board-over-vapor-barrier-32055/

and I first encountered it in Myron Ferguson's book Drywall, which seems to be a well-regarded source on the subject. On page 8 he says: 'no greenboard over a vapor retarder IF the board will be finished with tile, vinyl, or oil paint'; but later on the same page he says 'on ceilings, no greenboard over faced insulation, period'. For a remodeling newbie the advice out there is confusingly contradictory to say the least.

But when in doubt I'll stick with what the termite says. :)

AtlanticWBConst. 11-24-2008 04:48 PM

I'll back up KCT, Green board IS NOT, in any way, shape, or form..... a moisture barrier.

bjbatlanta 11-24-2008 06:29 PM

With proper ventilation and appropriate paint, there's no NEED to use greenboard at all in my opinion. I only use it if a customer insists. Since you have it, use it unless you WANT to exchange it. Regular rock is a little cheaper, but not enough to justify loading up the green and taking it back. Your faced insulation is fine.

Tracymc 11-25-2008 01:24 AM

I would give using cement board another thought. Look into DensShield or DensGuard they are a far superior product and unlike Durock or WonderBoard they with not draw moisture in to them, CBU will wick moisture like nothin else if it can. Take a peek @ USG's website regarding the steps u have to take as far as vapor barriers behind when using the CBU products. Plus, the Dens products are very user friendly and more water resistant. No VB behind Dens a must have VB behind CBU per all manufacturer install specs.

Tracy.

AtlanticWBConst. 11-25-2008 07:46 AM

Quote:

Originally Posted by Tracymc (Post 189842)
I would give using cement board another thought. Look into DensShield or DensGuard they are a far superior product and unlike Durock or WonderBoard they with not draw moisture in to them, CBU will wick moisture like nothin else if it can. Take a peek @ USG's website regarding the steps u have to take as far as vapor barriers behind when using the CBU products. Plus, the Dens products are very user friendly and more water resistant. No VB behind Dens a must have VB behind CBU per all manufacturer install specs.

Tracy.

Superiority of one product over another, is subject to interpretation, preference, and relative comparisons to particular characteristics of each material.

There is often confusion between "Vapor Barriers" (VB) and "Moisture Barriers" (MB).
FWIW - Both Denshield, and Cement Board, "are" neither.

Denshield "has" a moisture barrier coating applied to it's surface. Installed, it requires additional work, to "become" an actual vapor/moisture barrier system.

1.) Denshield loses any concept of a sealed surface, if you break the surface materials with a nail, or screw. Moisture will also be able to enter at any other broken/penetrated locations, as well as at any seams.

2.) With Denshield, you would still have to seal all seams and corners with a moisture proof material, once you install it, in order for it to be a "completely sealed barrier" (MB)...

Example; Denshield Installation Instructions - to seal seams and water penetration points:
"... 2” 10 x 10 glass fiber mesh tape–embed in a skim coat of material used to set tiles on all joints and corners that are to receive tile. Caulk or seal penetrations and abutments to dissimilar materials..."

3.) If you are getting water into any substrate material, you have issues already. The point is, that "if" water were to migrate behind the tile, you want mold and rot resistant material there.

4.) Your primary layer is to be installed water-tight. That encompasses, the proper installation of the tile, grout, and any caulking used. That is your primary water-tight surface. The substrate, is your secondary surface (for adhering the tile to).
With CBU -The VB is placed underneath the cement board, over the wood framing, to protect it, should the primary surface material (Tile/grout/caulk) fail for any reason.

IMHO - Whether that tile substrate material is your secondary VB/MB, or the VB/MB is behind a rot-resistant cementitious material (CBU), does not designate, nor does it classify, one material, as being superior to the other.

joelbuckley 11-26-2008 08:41 AM

Would an additional vapor barrier behind densshield be a good thing or a problem?

AtlanticWBConst. 11-26-2008 09:48 AM

Quote:

Originally Posted by joelbuckley (Post 190371)
Would an additional vapor barrier behind densshield be a good thing or a problem?

Problem, don't install one if you are going to use DS

Maintenance 6 11-26-2008 11:39 AM

As a little bit of back up to Atlantic's info:

(From USG's spec)
When tested in conformance with ASTM E 96, DensShield panels achieved a perm rating of less than 1.5 alone, 1.0 with dry set mortars and .5 when applied with a Type 1 tile mastic.

For comparison, the kraftpaper face on fiberglass batts have a permeability rating or .4 while 6 mil poly is 0.06.

Anything with a perm rating of 1.0 or less is considered a vapor retarder.

rdavis84 03-05-2009 08:12 AM

Unfortunately I made the mistake of installing 6 mil poly over kraft-faced insulation in a renovation a few years ago. What would my best coarse of action be? There are no signs of mold but I would want to prevent any problems from occuring down the road. Is it safe to keep it or should all the plastic be removed?

jogr 03-05-2009 10:19 AM

Quote:

Originally Posted by rdavis84 (Post 240470)
Unfortunately I made the mistake of installing 6 mil poly over kraft-faced insulation in a renovation a few years ago. What would my best coarse of action be? There are no signs of mold but I would want to prevent any problems from occuring down the road. Is it safe to keep it or should all the plastic be removed?

Best course of action is to not worry about it. No inside vapor is getting past the poly. If outside moisture vapor works it's way all the way through your wall the kraft paper will stop it. Yes in theory if you got moisture between these two layers it would take a long time to dry out so you don't want to intentionally do it this way. But in reality the chances of trapped moisture between these two layers that are essentially in contact with each other is remote. Certainly not worth tearing out drywall.


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