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Hey guys so gutted my small bathroom and I’m a little confused with this whole vapor barrier thing. So I live in NY Long Island so our winters are up and down but they are still mostly on the cold side some warmer than others. I have 2 walls of this bathroom that are on the exterior of this house built in 1958 when I took down the old insulation it had kraft paper on both sides of the insulation which kinda threw me off im assuming they wanted vapor barrier on both sides and insulation in between, some of the insulation looked fine and some of it was black.Moving forward with my reno my wife wants the bottom half of the bathroom tiled and the top portion sheetrock, my plan is to air seal all the gaps,cracks and crevices including all the stud bays top and bottom plates with caulk and foam and then put in roxoul insulation,the other 2 walls face the interior of the house do air seal in between rooms also?From what I read if I put redgard on top of my durock cement board (underneath tiles) that would be my vapor barrier but what about the upper portion under the sheet rock should i staple a 6mil plastic on that portion of the wall for the vapor barrier? Or Should I just put 4 inches Of blue foam board in the stud bays (Cost is not a factor in such a small bathroom) and just cement board and sheetrock over that without using any plastic vapor barrier just the redgard on the cement board.
 

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Red Seal Electrician
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Sounds similar to my master suite bath, and climate (but maybe not quite as cold in winter)

I did roxul between the studs, and 6mil poly VB on the inside/warm side. Sheetrock walls... I don't like installing tile.

Good use of the exhaust fan, and modest heating pushes any moisture out.
 

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Discussion Starter #3
Sounds similar to my master suite bath, and climate (but maybe not quite as cold in winter)

I did roxul between the studs, and 6mil poly VB on the inside/warm side. Sheetrock walls... I don't like installing tile.

Good use of the exhaust fan, and modest heating pushes any moisture out.
Did you use cement board at all? Or u dont have tiles in the bathroom???? I’m also wondering about putting the vapor barrier under the cement board and creating an area for mold growth
 

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Red Seal Electrician
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Did you use cement board at all? Or u dont have tiles in the bathroom???? I’m also wondering about putting the vapor barrier under the cement board and creating an area for mold growth



No. No tiles. One-piece moulded shower.


I used this (or similar product) on cement board 15 years ago.

https://www.custombuildingproducts.com/products/surface-preparation/waterproofing-membranes-underlayments/redgard.aspx#


(Can't say how it held up... I moved out ~3 years later.)
 

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As long as a wall assembly can dry in at least one direction it should be good and your description sounds like that direction is to the outside.

As long as you do not have a vapor barrier on the exterior all is good.

As for felt paper it is not classified as a vapor barrier, just a vapor diffusion retarder, it does allow some drying. Modern house wraps are designed to allow moisture to pass while blocking water under the siding.

Air sealing and a good exhaust fan as mentioned are important.

Bud
 

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Discussion Starter #6
As long as a wall assembly can dry in at least one direction it should be good and your description sounds like that direction is to the outside.

As long as you do not have a vapor barrier on the exterior all is good.

As for felt paper it is not classified as a vapor barrier, just a vapor diffusion retarder, it does allow some drying. Modern house wraps are designed to allow moisture to pass while blocking water under the siding.

Air sealing and a good exhaust fan as mentioned are important.

Bud
Thank you Bud appreciate it
 

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For those who read this thread later on. You should never use 6 mil plastic as a vapor barrier, with or without kraft faced insulation batts. The use of plastic vapor barriers was popular in the 90s and is no longer a recommended building practice. The reason is that when its warm outside and cool inside, moisture will build up on the exterior side of the plastic vapor barrier. Eventually, it will rot the stud bays and promote mold growth. I have seen this happen on several properties, including my own house. Simply use Kraft faced batts on exterior walls without any additional vapor barrier.
 

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Simply use Kraft faced batts on exterior walls without any additional vapor barrier.

What about when using Roxul? It is unfaced. I have a similar situation redoing a bathroom with 1 exterior wall. I put in rock wool and was planning to add 6mil plastic vapor barrier.
 

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I should have mentioned in my prior post that the use of vapor barriers depends on the climate zone in which you live (ref 1). I live in Zone 5. Maryland is Zone 4 (ref 3). The recommendation for Zone 4 is a class III vapor retarder. It is supposed to be semi-permeable. 6 mil plastic is impermeable and is the wrong vapor barrier for Zone 4. In fact, 6 mil plastic is the wrong vapor barrier for all locations except southern Florida (reference 3). So to be specific, the references suggest you can add a class III vapor retarder though it appears to be a recommendation and not a requirement.

Sources:
1. https://www.buildingscience.com/documents/digests/bsd-106-understanding-vapor-barriers

2. https://www.certainteed.com/insulation/vapor-retarders-and-moisture-management/

3. https://www.certainteed.com/insulation/resources/do-i-need-vapor-barrier/
 
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