Plastic Vapor Barrier in Bathroom?


  #1  
Old 09-10-10, 05:31 PM
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Plastic Vapor Barrier in Bathroom?

I'm in the process of remodeling my master bathroom and will be tearing out all the drywall on the exterior wall and replacing the insulation. Currently, there is R13 batt insulation in the wall, and I planned on replacing it with R19. My question is, after installing the batts, should I cover the entire wall with a plastic vapor barrier/sheet or will the vapor barrier on the insulation suffice? Reason I ask is because I've seen TV shows where there was a plastic vapor barrier, but this may have been because the batts didn't have a barrier. I've also saw an episode of Holmes on Homes where they renovated a bathroom and tore all the plastic out because there was mold.

I've been reading up on insulating bathroom exterior walls and have read that an air barrier is a good thing to have between the drywall and vapor barrier to inhibit capillary action. Has anyone drywalled and exterior wall with an air barrier? The general idea is that any moisture that passes through the wall will be able to dry back to the interior instead of soaking into the wall studs, etc.
 

Last edited by mossman; 09-10-10 at 06:04 PM. Reason: Added comments
  #2  
Old 09-10-10, 06:12 PM
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The first objective is to prevent water/moisture from getting into your walls. Leaks should not occur and moisture laden air should not seep into cold places. If the roof leaked a bit and the wall cavity got wet, it would need to be able to dry in one direction or the other. Once you install a plastic vapor barrier, the only practical direction left is to the outside. Wood sheathing will absorb the moisture and pass it to the outside. The housewrap is designed to allow moisture vapors to escape. So as long as your siding will let some air flow you should be all set. Exception would be floods or major leaks.

The other source of moisture is condensation due to moist air leaking into the wall cavities and reaching a cold surface. Or the reverse, cold air being forced into the wall cavity and cooling that plastic to below the dew point. The solution here is good air sealing and a dense insulation. The mineral wool option is dense and resistant to mold. You'll see Mike using it a lot.

Bottom line is to seal all air leaks, fill with a dense insulation, and cover it with a good vapor barrier/air barrier. You mention R-13 and R-19, those are usually associated with 3.5" or 5.5" walls. Which do you have?

Bud
 
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Old 09-10-10, 07:02 PM
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Walls are 3.5". This is what I'm thinking for the exterior wall:
- Tear out R-13 batts and install R-19 (without vapor barrier)
- Install 6mm poly over the entire exterior wall
- Install furring strips along full length of exterior wall studs to create air gap
- Install Hardybacker cement board in areas around tub that will be tiled
- Install regular gypsum board on the remainder of the exterior wall.

Does this sound good?
 
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Old 09-11-10, 04:58 AM
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If the walls are 3.5", installing R-19 will do no good and may compact the insulation enough to actually reduce its R-per-inch value. R-19 only gets its full R-value when installed in a 6" cavity. Fiberglass insulation is approximately R-3.5 per inch. Over-stuffing fiberglass is bad.

6 mil poly is a vapor barrier, but does not act as an air barrier until it is in direct contact with a rigid surface like the drywall. Otherwise it will billow in and out with air pressure which will pump air through the inevitable holes and actually tear where attached to the studs.

The strapping and air gap sound like an attempt to create a drying plane. Moisture or moist air cannot be allowed to enter this space. It will not dry to the inside and your VB will not allow it to dry to the outside. If your cement board is done correctly it will be impervious to leaks and you can leave out the 6 mil poly. But any seams on the backer board need to be properly sealed.

For proper disclosure, I am not a tile pro, I deal with moisture in the context of energy management. Moisture always needs a way to escape, thus the vapor barrier and the backer board are a problem as moisture that gets in between cannot get out.

Bud
 
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Old 09-11-10, 09:03 AM
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I'm a bit confused. I've read that an air gap is just that--a small gap between the drywall and the vapor barrier that's behind it, which allows moisture to dry back into the room as opposed to continuing through the wall. The instructions that came with the bathtub also say to do this (install furring strips on studs to create air gap). This is website I've been referencing: Welcome To Home Energy Magazine Online
 
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Old 09-11-10, 03:31 PM
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It all goes back to moisture getting through the backer board. If it is water impervious or you install it over a impervious membrane then, theoretically no moisture will get in. Requiring an air gap can either be viewed as they expect moisture to enter of they are being cautious. You must decide what your materials are capable of. IMO, it is difficult to expect water/moisture to exit through a material that wasn't supposed to allow it enter in the first place. In the real world, water will seep in through a small hole through which it can't be expected to escape. Thus I recommended no vapor barrier behind the backer board. If there is no vapor barrier, then there is no need for the strapping to create an air gap. From their pictures, you might want the strapping to hold the backer board out over the lip of the shower stall, but that concern is based upon the installation instructions.

Again, I'm not a pro on bathrooms and tile, but I would rather have no VB than to have one that will trap moisture in a place where its exit is not fully understood.

Also, note they are referring to vapor retarders and not vapor barriers. A little bit of diffusion goes a long ways and 6 mil has virtually none.

Let's see if some of the pros will kick in here.

Bud
 
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Old 09-11-10, 06:55 PM
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Just going to leave the existing R-13 insulation with vapor retarder on the exterior wall (adjacent to right end of tub) and furr the studs per the installation instructions to allow the backerboard to hang over the lip of the tub. This will create a small air gap behind the backerboard, which is unavoidable, but not a bad thing. If moisture happens to saturate the backerboard, which it likely won't since this is a bathtub only, I would think that it would dry to the outside as the surface of the tile and grout drys out. Can anyone else provide some insight?
 
 

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