Attic space behind shower wall

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Old 05-29-19, 06:29 PM
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Exclamation Attic space behind shower wall

Hello everyone, I am doing a bathroom remodel because of a spongy floor and found mold. I took out the fiberglass shower and the walls were in bad shape so I (started) to take out the drywall as well.

Behind the fiberglass shower, where the mixer is (was) - about 4 feet up from the floor there was unfaced insulation behind a clear plastic vapor barrier in the stud bay. I couldn't understand why it was installed 4 feet up and not all the way to the floor.. until I was about to take it out (there is mold growth on the vapor barrier and insulation.) When I went to remove it I was surprised to see that it opens up into attic space. The bottom four feet has drywall on the backside of the frame, but above that there is no backing - the frame is just open into the attic space, it is like this behind where the vanity is (was) as well.

From what I understand, you are not supposed to have a vapor barrier behind a vapor barrier because of condensation buildup. Would the fiberglass shower be 1 vapor barrier and the plastic sheeting over the insulation count as a second vapor barrier? New code in my area requires cement-board behind shower surrounds even if its fiberglass or acrylic, so I am going to need to put in cement board.

My main question is, since this opens up into the attic I am assuming I will indeed need to put in insulation in the stud bay. Would paper faced insulation be appropriate for this application? I live in the inland pacific northwest, and since its attic behind the frame,I plan on getting paper faced R-30 rated insulation.


My apologies for the rambling, I hope this is the correct forum for the question.
 
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Old 05-29-19, 06:53 PM
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Where I am located, you always use unfaced insulation behind a shower surround. Faced on exterior walls elsewhere. Since the insulation will be open on the back side, the back side needs an air barrier in order for the insulation to be effective. Either Tyvek, osb, or mold resistant drywall.
 
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Old 05-29-19, 07:15 PM
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Just wanted to make one more comment about air sealing... it is very important that there be no air exchange between a cold attic wall and the warm moist bathroom. So your drywall or cement board needs to all be taped and sealed at all corners and seams, and it might not hurt to caulk / seal against the bottom plate as you install it. All penetrations from plumbing, venting and electrical should likewise be sealed.
 
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Old 05-30-19, 06:43 PM
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WA State Code:
502.1.6 Moisture Control:

502.1.6.1 Vapor Retarders: Vapor retarders shall be installed on the warm side (in winter) of insulation as specified in the following cases.

EXCEPTION: Vapor retarder installed with not more than 1/3 of the nominal R-value between it and the conditioned space.

502.1.6.6 Walls: Walls separating conditioned space from unconditioned space shall have a vapor retarder installed. Faced batt insulation shall be face stapled.


I am in zone 5, but if someone could give me a hint on the appropriate R rating, or perhaps some other solution that would be appreciated.
 
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Old 05-30-19, 08:57 PM
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https://codes.iccsafe.org/content/ie...rgy-efficiency

Your attic wall is like any other exterior wall. The difference is that other exterior walls have plywood on the outside to provide an air barrier for the insulation. Air blowing through fiberglass reduces it's effectiveness. (It needs a dead air space to be the most effective.)

You don't need to insulate an attic wall to the same degree that you insulate the attic floor, as the chart in the link above shows.
 
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Old 05-30-19, 09:24 PM
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The latex paint you put on your walls is also considered a class III vapor retarder, and in most areas that is all that is required. Notice that the words "vapor barrier" aren't found in your code. They aren't the same thing... and kraft facing is not always required unless your inspector is old school and demands it.
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