Wall vapor barrier


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Old 01-09-06, 06:29 PM
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Question Wall vapor barrier

I'm preparing for the reconstruction phase of my completely gutted bathroom. I believe it would be wise to put up a vapor barrier behind the greenboard (and cement board in the to-be-tiled shower). I've got this roll of 30 # building paper I bought for the new window flashing. My plumber says this will work just fine, layered low to high, overlapped 2-3". Questions:

1. Is this material a good choice, or would another (e.g. plastic) be better?

2. Do I try to keep the paper as continuous (uncut) as possible around the room, or cut it between walls? The latter seems easier to fit and hang, but I don't know how important it is to keep this layer as seamless as possible.

3. If and where I do cut it, do I need to seal the vertical seams, or is overlapping the paper enough?

4. Does this layer overlap the nailing flange on the shower receptor, as the cement board will, or attach to the studs right down to the floor (I believe the latter)?

5. Any seal needed where it meets the floor?

6. Is there a recommended stapling technique (like covering the line of staples with the next layer)?

Any help with the specific details is much appreciated. I just installed the window, and the need for meticulous exterior sealing is on my mind...it may be that this level of concern is unnecessary here.

Thanks,

Mike
Ithaca, NY
 
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Old 01-10-06, 06:09 AM
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Hi Mike,

I am from Upstate NY as well. Schenectady actually.

1:They say you can use building paper. Personally I like 6 mil. poly the best but if you use building paper you may want to use a waterproof membrane over the cement board such as Redguard (Home Depot sells this for about $35/gal.)

2: I don't think it matters if it is uncut or not just as long as you overlap seams by 2"-3".

3: It is not necessary to tape the seams......building paper is not a vapor barrier, it is a vapor retarder so it will let moisture/condensation thru the paper but at a lesser rate so taping won't do too much.

4: Either way....if the paper doesn't get sealed to the tub flange, make sure it goes all they way to the floor.

5: No

6: No just staple, the less the better just make sure the paper isn't going to fall off.
 
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Old 01-10-06, 06:49 AM
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Ditto on everything except the tar paper on..er, to the floor.

I think the whole point is to give the water someplace to go
should it get behind the tile and cementboard.
If you run the paper to the floor, the floor has a chance to get wet,
which can lead to everything we are trying to avoid.
Run the paper or plastic to the tub and back seal with some caulk.

MHO.

Good luck and post some pics when you're done.
 
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Old 01-10-06, 08:21 AM
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Ty

Thank you both for your help. I'm inclined to use the 6-mil poly; the idea of a layer that is completely impermeable to water appeals to me, and I'm sure it's cheap. Leaves me with this huge roll of building paper; I'll probably find some use for it someday!

Thanks again for taking the time...

Mike
 
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Old 01-11-06, 05:55 AM
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On the topic of vapor barriers: in a shower/bath application Hardi board recommends a vapor barrier behind its product, while cement board does not. In a shower application, would it not make sense to have one regardless of the product or is this overkill?
Comments please.
 
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Old 01-11-06, 06:29 AM
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joeperi,

Check this out. I called USG and this is the response to use of W/R and Cement board with the use of vapor barriers.

http://dougaphs.smugmug.com/gallery/251530

Hope this helps!
 
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Old 01-12-06, 05:10 AM
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Doug, thanks. This was a good resource.
 
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Old 01-12-06, 03:16 PM
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New York State-No Vapor Barrier. Code Violation

It is a code violation in New York State and Pennsylvania and anywhere that the IRC 2003 or 2006 codes are in effect to install a vapor barrier behind moisture resistant drywall in a shower or tub enclosure.

Section R702.4.2 of both the IRC 2003 and IRC 2006 states:

"Water resistant gypsum board shall not be installed over a vapor retarder in a shower or tub compartment."

Section 702.4.3 further states:

"Water resistant gypsum backing board shall not be used in the following locations:

--1. Over a vapor retarder in a shower or bathtub compartment...."

The IRC 2003 Code Commentary on the above sections states:

"Although there are many gypsum board sheet products that are manufactured and approved for use in wet areas or areas exposed to moisture or humidity, there are still some extreme conditions where even water-resistant gypsum board will not provide the level of moisture protection necessary.

1. Gypsum board installed on the walls and ceilings at shower and bathtub areas must be finished to prevent moisture from penetrating the wall or ceiling finish and contacting the gypsum board. The finish applied to the exposed face of the gypsum board must create a water-resistant barrier that not only stops water from getting to the gypsum board but also prevents the release of moisture from within the wall or gypsum board itself. For this reason, gypsum board MUST NOT BE INSTALLED OVER THE OUTBOARD SIDE OF ANY VAPOR BARRIER OR RETARDER (sic: in a bath or shower compartment). This will create a waterproof membrane on both sides of the gypsum board causing moisture to be trapped in the gypsum board that will ultimately cause it to decompose and fail."

It is not permitted, then to place a vapor barrier or vapor retarder under moisture resistant drywall and over the face of the studs in any shower or tub area.

Doing so can actually create moisture issues and cause the MR board to fail.
 
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Old 01-12-06, 04:33 PM
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Manhattan42, thanks for the response. But in the details lies the devil. The IRC specifies "water resistant gypsum back board". I would interpret that as "green Board". You will note that the code singles out water-resitant gypsum : "shall not be installed over a vapor retarder in a shower or tub compartment". The reasons are clear. Green board can absorb moisture. If you have a vapor barrier behind it, voila either rot and failure and/or mold. However, I can find nothing in the IRC relative to masonry backer. Cement backer board is billed a "impervious" as is hardibacker. So If I've missed it please let me know.
Again, thanks for the comments.
 
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Old 01-13-06, 08:10 AM
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Yes, I'd like to clarify this and be sure as well. I'm about to put up this shower wall: plastic sheeting, Durock, thinset and tile. I believe this is OK, right?

On the details of this subject, a couple questions:

I. Is it a good idea to use plastic behind the greenboard in the rest of the bathroom (i.e. not in the shower)? I know we're talking about water in vapor form, and not the direct soak of a shower, but if moisture gets there, the same trapping and decay would occur. I guess that we just trust the greenboard to adequately repel vapor, right?

How about the greenboard directly above the cement board in the shower? If not, what is best?

II. The junction between cement board and greenboard: Home Depot guy says I need to carefully use thinset on the cement board side and regular joint compound on the greenboard. Does this sound right? Then, fiberglass tape across the joint, and cover with what?

III. Previous threads have discussed the junction of the cement board with the shower receptacle, giving 2 acceptable options:
(a) 1/4" furring strips on the studs to bring the cement board out;
(b) caulking the plastic sheet over the nailing flange, then bringing the cement board to 1/8" from the flange.

Option (b) would be easier for me at this point; questions are:

1. Is this considered just acceptable, or is it just as good a choice as (a)? I want to do it the best way.

2. What happens next? I imagine the tile then overlaps the 1/8" gap and gets a caulk seal to the shower flange. (I don't think the seal can be to the horizontal surface of the rim of the shower base, since that is probably close to an inch for the tile to span without backer board). Will this leave a nice finished look? In terms of visibility, I do realize it's a few inches from the floor!


Can't say thanks enough; the collective wisdom of the experts in this forum has been invaluable!

Mike
Ithaca, NY
 
  #11  
Old 01-13-06, 01:46 PM
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Concrete Backer OK

Joeperi and DoggieDoc:

You both have it correct. The restriction applies only to greenboard (aka MR drywall) just as Doug Aleshire's gypsum links affirm.

I wasn't sure whether DoggieDoc was planning to use MR board "and" cement board in his shower compartment because it was a bit confusing when he stated:

" I believe it would be wise to put up a vapor barrier behind the greenboard (and cement board in the to-be-tiled shower). "

Sounded like he was set on using greenboard and cement board in the shower enclosure.

Anyhow, I'd though I'd throw out the code and code commentary on greenboard and vapor barriers just to help clarify what was already stated.

___________________________________________

Comments on DoggieDoc's questions:

I. It is not a good idea to use plastic sheeting as a vapor barrier in ANY location in a house.

The reason is that plastic sheeting is TOO restrictive to water vapor flow.

You actually want gaseous water to pass through the walls but in a controlled fashion. You don't want it completely stopped.

Plastic is too restrictive.

Kraft facing on insulation is the only vapor retarder you need.

Short of that the latex paint on the walls itself will act as the vapor retarder.

Only place you want plstic sheeting for a true vapor 'barrier' is under concrete floor slabs and perhaps over a masonry foundation wall in a basment. No where else.

II. Make your joints between the cement board and greenboard occur an inch or so under the tile. This will allow you to use only thinset and not have to worry about joint compound at all.

Where joint compound absolutely must be used, used a setting type calcined joint compound. Setting compound contain plaster and are better equipped to resist water if they get wet.

III. I'm not much of a tile man and these question would best be addressed by tile professionals.
 

Last edited by manhattan42; 01-13-06 at 01:58 PM. Reason: a
  #12  
Old 01-19-06, 09:07 AM
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Question

Thanks again for all the help. Sorry I was a bit ambiguous (ironically, when I was trying hard to be completely specific and clear! Part of being a novice, I guess ) There is only to be cement board in the shower, not cement board over greenboard.

One more related question:

I believe I remember reading that there should be some barrier behind the green board on an exterior wall (the wall is of course insulated). If this is true, what is the reason? Should it be building paper, since it is somewhat permeable, and won't trap moisture behind the greenboard?

Thanks again,

Mike
Ithaca, NY
 
  #13  
Old 01-19-06, 06:38 PM
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Good Question.

Funny you should mention that.

I just posed that very question to 2 Master Code Professionals at a training conference yesterday.

In discussing the IRC requirement to not use a vapor retarder under MR drywall under ceramic tile in a shower compartment, I raised the question about what to do with the code requirement to have a vapor retarder on every wall that is part of the building's thermal envelope (ie. an exterior wall.)

They were like a deer caught in the headlights.

But after banging heads with several other code officials I think we all concluded that no vapor barrier should be installed for thermal barrier reasons under the MR board because the tile and paint or other surface treatments would be all the vapor barrier needed to comply with the intent of the Code.

The Code recognizes surface treatments including paints as being compliant vapor barriers.

So despite the Code sections that require a vapor retarder on the warm-in-winter side of the thermal envelope, you already have the required barrier in the form of the surface mounted tile, mastics, paints and so forth.

So NO vapor retarder under MR board where tile will be used in a shower compartment.

Hope that helps.
 
 

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