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Old 06-17-2008, 06:59 AM   #1
maynards
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closed cell foam insulation behind shower

A few months ago there was a discussion about a "moisture sandwich" within a wall insulated with closed cell foam. I was about to start my shower when I realized I would be creating this in my own home. Two of the future shower walls are exterior walls insulated with closed cell spray-in insulation (it really is closed cell foam - not open cell). The problem stated was that this insulation is a vapor retarder and by putting kerdi on the front of the wall, one would create a vapor lock within the wall. We are at the drywall stage of building this house. When I say "we" I really mean my husband and I because we are the general contractors, builders, everything! I have researched this problem for hours, gone over your discussion three times, I have the building science site bookmarked as it is one of our references for building this house, but I still dont know what to do. Any advice would be appreciated. I live in a mixed climate - mostly heating - South Dakota. Thank you in advance.
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Old 06-17-2008, 09:01 PM   #2
matman
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Maynard
You posted early this morning and no takers. That says something by its ownself. Okay I'll sucker and step up. There seems to be indecision on the matter. I started that thread because I had just such an install I was starting at that time, and I was looking for some input.

The job's done now and what I ended up doing was waterproofing the stall at the surface as you are thinking of doing. In that particular spot in the house, the shower stall, the heaviest amount of moisture will be coming from the inside, easily, and that needs to be stopped before it enters the wall, right behind the tile. If there were no barrier the frame would get wet time and again. It would have a better chance of drying with no moisture barrier to lock in the moisture, but with continual use everyday, would it ever really dry out anyway? We've all seen the damage and mold present when the frame area goes unprotected.

Do a bang up job on the Kerdi and it'll never get wet back there, right?
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Old 06-18-2008, 05:59 AM   #3
maynards
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Thanks for your reply, Matt. I know there is conflicting information about this subject, and I suppose time will give us the difinitive answer. But your reply just sounds like good common sense. I've read the kerdi shower book and spent a lot of time reading through this site, but I have to ask this: In this situation do you think the cement board would be a better choice than regular drywall? THe bathroom ceiling is 9' high, and we were thinking of just leaving it that way in the shower area rather than framing down as in a canopy. How far up do you think we should run the kerdi in this situation? thanks.
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Old 06-18-2008, 09:28 AM   #4
bbcamp
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Moisture sandwich notwithstanding, cement board offers no food for mold, so I think that would be the better choice behind Kerdi and in front of the insulation.

Another option would be to build a ventilated chase between the shower wall and the outside wall. Moisture that could get in there would be allowed to evaporate to the vented attic.

Kerdi can stop at the shower head, but you are free to go higher if you want.
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Old 06-18-2008, 10:16 AM   #5
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I'm shooting blind on this, but I think venting the moisture into the attic would be worse than having it in the walls. At least in our part of the country (SD, I'm in WI). I think you'd have better luck trying to provide a moisture path sideways, so it could dissipate through the drywall in adjoining walls.

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Old 06-18-2008, 01:00 PM   #6
matman
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As Bob said.
I did the install with cbu for the no mold advantage and if it were to get damp at all back there,(hopefully not), cbu easily outperforms in wet situations. I waterproofed the insulated wall all the way up and the other walls just above the shower head.

I think it's probably the best you can do, short of that ventilation system Bob's talking about. Might be room for a new product there.
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Old 06-18-2008, 02:55 PM   #7
maynards
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Thanks for your opinions, everyone. Theyre using this type of insulation a lot around here and it is great for the extreme wind and cold we experience in the winter, but I presume these walls are supposed dry to the inside (if any moisture were to accumulate inside them) and thats really the potential problem with covering the fronts of them with another moisture barrier. The ventilation thing is good idea, but we sure wouldnt want air that's house-temperatured to enter the attic, especially in winter. Which brings me to another conclusion. Maybe I'm over thinking this because our biggest problem with moisture is condensation caused by air (vapor) leakage. If the insulation is as good as claimed, there shouldnt be air (vapor) passing through the wall - right? I welcome any other opinions and information anyone else has to offer, and thanks for discussing this with me.
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Old 06-18-2008, 02:59 PM   #8
maynards
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I forgot to ask Matt what you meant by "room for a new product". I'm open to anything short of tearing out the plumbing and moving the shower to a new location.
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Old 06-18-2008, 05:09 PM   #9
jadnashua
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I think you''ll be okay using Kerdi over that exterior wall. It should prevent moisture from the shower migrating into the wall. Dealing with a surface waterproofing membrane verses a 'conventional shower, the surface membrane should win every time when installed properly.
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Old 06-18-2008, 06:07 PM   #10
matman
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Quote:
Originally Posted by Maynard
I forgot to ask Matt what you meant by "room for a new product".
Don't start ripping out anything yet Maynard. When I remarked ''room for a new product'', I was suggesting that a yet to be designed/invented wall ventilation system/product, might be brought to market by a creative mind. I know of no such system for sale.

Engineer Bob mentioned a chase wall creating a ventilated space in the wall cavity, and my ''new product opportunity'' comment was in reference to that.
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