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Old 03-11-2012, 07:12 PM   #1
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Insulation behind shower walls


I'm about to install PVC liner and Durock in my alcove shower. The alcove is 3'd x 4'w. Two of the walls are outer facing east and south. Wall is 2x4 wood stud, black board, brick. Climate is deep south, hot and humid 6-8 mos. of the year.

Given that the PVC liner will act as a vapor barrier, I think I should probably insulate with unfaced R-13 FG. Is my thinking correct?

Thanks!

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Old 03-12-2012, 01:16 PM   #2
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Insulation behind shower walls


If you are going to insulate, Roxul is better than fiberglass. Hot/humid may be tricky w/ a shower on the exterior wall. Some climates are better suited to a vapor barrier on the outside. I'd suggest you do some reading on buildingscience.com, check w/ a good, local builder, and/or your local gov't building department.

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Old 03-12-2012, 05:01 PM   #3
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Insulation behind shower walls


+1

Roxul is better but a high density batt and well sealed exterior (i.e. no air movement) will work pretty well too.

You are correct, no moisture barriers so it can dry to outside but make sure the exterior sealed up tight.
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Old 03-12-2012, 09:35 PM   #4
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Insulation behind shower walls


Change of plans. Due to some timely advice, I'm ditching the liner and using just durock and RedGard (then tile), though this still would constitute a vapor barrier on the living space. Googling Roxul has me agreeing that this would be a good solution.

Windows o Wash:
"...well sealed exterior (i.e. no air movement) will work pretty well too."

"...make sure the exterior sealed up tight."

Does this mean just make sure there are no holes or seams in the black board?

jklingel: In fact, I've read much of the data from buildingscience.com and have downloaded many of their pdfs, including many regarding vapor barriers. The problem is that there is so much data and there are so many different opinions to be found that it becomes confusing.

But does this make sense?: since there will be an impermeable surface on the living space side, placing a vapor barrier on the exterior side would prevent any moisture that does occur from escaping?

Thanks for your comments.
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Old 03-12-2012, 09:44 PM   #5
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Insulation behind shower walls


Quote:
Originally Posted by rightit View Post

jklingel: The problem is that there is so much data and there are so many different opinions to be found that it becomes confusing. It does take some digging!

But does this make sense?: since there will be an impermeable surface on the living space side, placing a vapor barrier on the exterior side would prevent any moisture that does occur from escaping? Yes. A vapor barrier sandwich is always a bad idea. Walls WILL get a bit of moisture in them, and need to dry somewhere. That is why air sealing is so important. If you minimize the air movement, you minimize the water vapor movement. Vapor diffusion through materials is generally minimal.
see after the bullets.
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Old 03-13-2012, 11:33 AM   #6
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Insulation behind shower walls


Quote:
Originally Posted by rightit View Post
Windows o Wash:
"...well sealed exterior (i.e. no air movement) will work pretty well too."

"...make sure the exterior sealed up tight."

Does this mean just make sure there are no holes or seams in the black board?
Yes.

If the blackboard is full of holes, you can use a semi-permeable cover over the black board on the inside of the wall.

EPS foam is a good option as would be something like a Tyvek.

Get the exterior sealed up tight.
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Old 03-13-2012, 12:54 PM   #7
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Insulation behind shower walls


Thanks again!

Back to the Roxul...given its mold/water/fire resistant/air permeable properties and that it comes in R-Values equivalent to FG (with less thickness), I'm considering using it (or another brand rockwool) for the rest of the room, including the exterior walls. Is there any reason not to use it instead of FG?

Last edited by rightit; 03-13-2012 at 12:59 PM.
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Old 03-13-2012, 02:30 PM   #8
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Insulation behind shower walls


FG batts are the worst batt insulation on the planet. You can read about them on greenbuildingadvisor.com, etc. There is no reason to ever use them unless you just don't want to spend a little extra money right now, but spend it as time goes on via more heat loss. It may have the stated R value of other insulations, but in real world applications it will likely not perform up to that metric. It is just not that good of a product, but has/had great marketing. I have 13" of it in my house, and it is working OK, but I now know that dense packed cellulose would have been far better, and Roxul would have been as well. In 1980 I did not know better.
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Old 03-13-2012, 03:32 PM   #9
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Insulation behind shower walls


That's all I needed to hear. Thanks!
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Old 03-13-2012, 04:16 PM   #10
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Insulation behind shower walls


While the wall is open, air seal the boards/Celotex to the studs, as said. This will limit the amount of warm, humid exterior air entering your colder framing cavity to condense water there. Air seal with canned foam, the wring/plumbing holes in the top and bottom plates to limit the “stack effect” from mechanical/natural ventilation: http://www.wag-aic.org/1999/WAG_99_baker.pdf
Seal the crawl/basement if one. Then go into the attic and air seal all holes/chases. If still not tired, install foam board on the face of the bathroom studs, behind the shower, and/or in the cavities next to the drywall to raise the condensing temperature of the cavity framing above the dew point. Fig.1 and 2. Wait a minute….. vapor retarder inside??? With the cavity warmer from the colder room air, diffusion air and exterior air won’t as likely condense because the temperature drive is diminished, and air movement is about none (from good air sealing). This is brought on by rain, solar-warmed brick, and exterior sprinklers/sunshine moisture trying to go from warm to cold (room temp.). http://www.buildingscience.com/docum...e-humid-south/

ADA the drywall: http://www.buildingscience.com/docum...wall-approach/
Air seal the rim joists, if not on a slab: http://www.buildingscience.com/docum...l_seal_rev.pdf

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Old 03-14-2012, 11:47 AM   #11
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Insulation behind shower walls


Thanks Gary. Good info. I also found this, which has some good info specific to climate zones:

http://www.naima.org/insulation-know...retarders.html

I'm in zone 2, BTW. And IRC 2009 states no VB required.

Also, I think what everyone is saying is to use the EPS behind the shower, but just air seal the other walls.

For the room walls, the west wall should be no problem, given that it's ply board (and has Tyvek on the outside). But the south wall, with its aged blackboard (a bit 'crumbly' in a few places) could be a bit of a problem.

Is there some sort of spray foam solution that will adhere to seal not only the BB to the studs, but also fill small holes and seal exposed seams? Or would stapling tyvek in the stud cavities to cover the BB altogether be the better solution?

The foam, obviously would be the easier task, but the functionally best solution is what I'm after.

And, come to think of it, will the celing joists need some sort of extra attention as well, or will the Roxul w/no vapor barrier be OK there? It's 2x8 joists with 1/2" ply board decking.

Thanks!
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Old 03-14-2012, 11:56 AM   #12
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Insulation behind shower walls


Forgot to mention. The space will be air conditioned with its own system. Fresh outside air will be added to the space via the return air of the A/C, so the space will have a positive pressure with conditioned fresh air.

Also: The south facing wall (20' long) is (from outside in): Brick, blackboard (or so I say. See image. TO the left is bathrm...board is black. Main wall is black outside but brown inside?), wood studs. This wall was pre-existing and short of tearing the brick down, there is no way to add Tyvek to the outside of the black board. There also is no vent slot in the soffit for this wall. I'm working on how to ventilate it, but have not decided in a solution as of yet.

Insulation behind shower walls-southwall_-img_9141.jpg


Last edited by rightit; 03-14-2012 at 01:17 PM.
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