Air sealing VS Poly vapor barrier.


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Old 01-13-17, 05:58 PM
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Air sealing VS Poly vapor barrier.

I am so confused. I was going to put up a 6 mil poly vapor barrier around my new bathroom and put airtight boxes around electrical outlets and seal them as well. Some say not to use vapor barrier because your drywall is your vapor barrier, some say it causes problems in summer with moisture in the walls.

If the drywall is airtight and sealed with a moisture barrier paint, im not sure how the drywall would allow the wall to breath, I am confused.

If i do not use poly in the wall, how do I go about air sealing the wall? I know that the best way is a nice tight housewrapped exterior, but that's not going to happen any time soon unfortunately.

If i do use poly I was going to use accoustical sealant where I could not tape seams, but it is impossible to find it here. The nearest menards that has it is 250 miles from me.

I did get my Roxul Insulation in today and its nice looking stuff.

Thanks
 
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Old 01-13-17, 06:17 PM
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The latex paint you will put on your drywall is a class III vapor retarder (1 to 10 perms)... it is not a vapor "barrier". Poly is a vapor barrier, (impermiable) and allows no moisture to pass. Current thinking is that the class III barrier is sufficient in our climate and helps avoid creating mold problems associated with trapping it when you use poly, which is a class i, (< .1 perm) impermiable barrier. Drywall is an air barrier (not a vapor barrier)... meaning when it is taped, no wind or air should come through it. Of course you need to use air tight fixtures, insulate behind outlets and such in order to make them as efficient as possible. You would also air seal any penetrations (fire caulk or foam) that pass through your top plate or bottom plate (pipes, wires, etc) before you insulate so that air can't just freely travel up or down as it pleases.
 
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Old 01-13-17, 08:04 PM
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X covered it very well. You can caulk the framing to the exterior sheathing in those cavities along with other penetrations before the insulation goes in. Caulk the bottom plate to the subfloor as well. If your local codes wants to see the poly it will not create a double vapor barrier as X explained the paint is slightly permeable.

You said "I was going to put up a 6 mil poly vapor barrier around my new bathroom". If used it should only be necessary on the exterior walls. Perhaps that is what you intended.

Bud
 
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Old 01-14-17, 06:44 AM
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You said "I was going to put up a 6 mil poly vapor barrier around my new bathroom". If used it should only be necessary on the exterior walls. Perhaps that is what you intended.
Yes, that is what I meant. I may put some behind the bath walls, one of those is exterior anyway.
 
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Old 01-14-17, 11:30 AM
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Does anyone have any thoughts about using Tyvek instead of 6 mil poly? Tyvek is intended as an air and moisture barrier, but not a vapor barrier. So you can then have the vapor barrier on the interior side as I believe most would recommend for Indiana. I know it's a wrap, but you could staple it in the inside of the studs just like I'm assuming he's planning for the poly. On the interior you can use Kraft faced batts or there are now "smart vapor barriers". One's called CertainTeed MemBrain, that's more forgiving than Poly. MemBrain is intended to be used on the interior side of the wall and if, during the summertime it becomes a moisture trap, then (unlike Poly) it's vapor barrier opens up at a certain humidity point allowing it to dry inward. I'm not nearly as familiar with this stuff as some of you, but it would be my thought to do it this way.
 
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Old 01-14-17, 07:38 PM
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I was thinking the same thing about the Tyvek, it sounds like it would work in theory. I am really considering the Smart membrane, I looked and Menards has it but its listed as "Discontinued" however a few stores have it.

It seems with the smart membrane you cant go wrong.

Towsonite

Does anyone have any thoughts about using Tyvek instead of 6 mil poly? Tyvek is intended as an air and moisture barrier, but not a vapor barrier. So you can then have the vapor barrier on the interior side as I believe most would recommend for Indiana. I know it's a wrap, but you could staple it in the inside of the studs just like I'm assuming he's planning for the poly. On the interior you can use Kraft faced batts or there are now "smart vapor barriers". One's called CertainTeed MemBrain, that's more forgiving than Poly. MemBrain is intended to be used on the interior side of the wall and if, during the summertime it becomes a moisture trap, then (unlike Poly) it's vapor barrier opens up at a certain humidity point allowing it to dry inward. I'm not nearly as familiar with this stuff as some of you, but it would be my thought to do it this way.
 
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Old 01-14-17, 10:25 PM
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The orange store has it for about $59 for an approximately 9' by 50' roll, which I don't believe is unreasonable, likely comparable to poly in price, and something you may eventually use on other projects. Our local store doesn't have it, but they ship for free to stores I believe and I never pass up an excuse to go there. Tyvek on the other hand is not cheap since they only sell huge rolls and it may or may not be something you'd use again. If you are interested in Tyvek, then hopefully you have a contractor friend who can give you some leftover scraps or can find someone selling remnants on Craigslist. Either way, I'm not an expert on this by any means and know people use poly, but personally it scares me. I've just read a lot horror stories and they always seem to involve poly.
 
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Old 01-15-17, 06:09 AM
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Tyvek is applied to the exterior side of the sheathing, period.
 
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Old 01-16-17, 12:03 PM
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I see where the blue store sells Tyvek in 3ft by 100ft rolls for about $44. That size would likely work for these purposes.
 
 

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