# Basement shower wall insulation



## javan (Jun 9, 2008)

How will you deal with the weeping from the block wall? Not so sure you want to put the wall tight to the insulated block wall, might want an air space.


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## chronojosh (Feb 11, 2011)

I have put on two coats of dry loc on this section of wall that the bathroom will be on. The section that I am insulating with the foam board is only ~15' of a ~40' wall. The shower is only going to be 40"x40", so there will be plenty of un-insulated/uncovered walls left. I just want to try to make this area comfortable and usable while I renovate my main bathroom upstairs. So, any thoughts on the roxul between the kerdi and the foam?


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## Windows on Wash (Aug 30, 2011)

Dryloc does nothing to keep water out in the long term and will only create other issues. 

The rationale to not insulated that space would be to allow the pipes and wall to stay warm as long as the 2" XPS is well sealed and properly installed. 

You will have a bit of a vapor sandwich there if water gets into that space as it is not likely to be able to dry in either direction at that point.


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## chronojosh (Feb 11, 2011)

I understand that without fixing the outside drainage first, I am creating issues down the road. I will be addressing the outside issues this spring, since I created them by digging up my backyard last year to tear up an old patio there. But once I have it properly graded and the new patio is in, I am sure that the issue will go away. It wasn't there before I messed around back there...wooops! Anyway, will the fact that I will only have R-10 in that part of the wall be a problem? or should I just cut more foamboard for those stud bays and spray foam it in place?


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## spaceman spif (Jul 28, 2009)

Once you have your stud wall up you can add unfaced batt insulation between the studs, on top of the XPS.


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## chronojosh (Feb 11, 2011)

So the unfaced fiberglass won't have any issues being sandwiched between two waterproof materials?


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## spaceman spif (Jul 28, 2009)

There is no organic matter on unfaced batt, so there is no problem. And it will be between the XPS and your drywall, both of which allow a very small amount of moisture to pass in and out, which is actually what you want. What you don't want is a true moisture barrier, like a sheet of plastic, that will trap any condensation and hold it against organic matter and allow mold and mildew to grow.


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## chronojosh (Feb 11, 2011)

That's my concern, because it will be between the xps and the shower which will have a waterproof membrane, either kerdi or hydroban


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## spaceman spif (Jul 28, 2009)

chronojosh said:


> That's my concern, because it will be between the xps and the shower which will have a waterproof membrane, either kerdi or hydroban


Hmmm...that is a good question. I just got done remodeling a bedroom down in my basement, and part of that was insulating the outside facing walls. Gary in WA was a HUGE help in figuring out how to insulate it. So I was coming from the mindset of insulating a standard basement wall.

But that is a good question - if you have a shower in your basement that is against an outside facing wall, how do you prevent shower water from getting through while at the same time not creating a vapor barrier that will trap moisture coming in from the outside walls?

My first though is to put up XPS, then stud walls, then put up some type of cement board. Maybe leave an air gap? Curious to see what the pros say!


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## Windows on Wash (Aug 30, 2011)

Critical thing in my opinion is keeping the interstitial as dry as possible. Good waterproofing on the exterior and full schluter or other waterproofing system on the tile. Leaving the space between the XPS and the back of the cement board without fiber insulation should keep the foam surface warmer and help stave off condensation between the wall.


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## chronojosh (Feb 11, 2011)

Just out of curiosity, would cutting more rigid foam to fit between the studs and using great stuff to keep them in place be a problem in that area as well?


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## Windows on Wash (Aug 30, 2011)

Not at all. Outer edge of the stud still becomes colder then though.


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## chronojosh (Feb 11, 2011)

Is there an issue with condensation forming due to that?


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## chronojosh (Feb 11, 2011)

Also, should I frame the walls directly against the rigid foam, or leave an inch or so gap?


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## Windows on Wash (Aug 30, 2011)

You can frame the walls right to the foam and if you use an airtight drywall approach, you shouldn't have any moisture issues inside the wall as a result of condensation. 

http://buildingscience.com/documents/information-sheets/air-barriers-airtight-drywall-approach


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## chronojosh (Feb 11, 2011)

Cool. Thanks for that. Just because of the size of the shower I am framing, is it OK to have a gap between the foam board and the framing?


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## Windows on Wash (Aug 30, 2011)

Yes. 

Use shims to pinch the foam board so that it is compressed to the wall surface to help it stay tight. 

If you have a bit more space in there and are concerned about condensation, you could put another inch of foam on those walls and turn the studs sideways to hang your backer from. That would give you more R-Value in that cavity without the fibrous insulation.


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## chronojosh (Feb 11, 2011)

Thanks. I'll look into more foam. I just wasn't sure if the air gap between the roxul and the foam was a problem like it would be be between the foam and the cinder block walls. The gap between the 2x4 wall and the foam isn't even due to me framing my wall square based off of something else in the basement.


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## chronojosh (Feb 11, 2011)

Another thought I had would be to shim the foam board against the back of the framed wall like was suggested. Next I would fill the stud bays with unfaced fiberglass, and push it to the back of the stud bay. Would having the air gap between the fiberglass and the back of the drywall be better than having it between the fiberglass and the foam board?


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## Windows on Wash (Aug 30, 2011)

Don't leave an air gap.

One of Gary's favorites. 

http://www.joneakes.com/jons-fixit-...ULATION-IN-THE-BASEMENT-TO-AVOID-CONDENSATION


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## chronojosh (Feb 11, 2011)

Sorry if it feels like I'm beating a dead horse, but I'm just trying to make sure I do this right. So the rigid foam is up. Now, I want to have the stud wall be square (so I can use the kerdi shower pan I have), the wall would have to be built off the foam, but the distance isn't consistent. It ranges from < 1/4" to 1/2". To make up this distance, could I instead frame the wall out of square tight against the foam and somehow make up the distance on the drywall/cement board side to square up the wall for the shower? Or maybe I should just go ahead and leave it square at fit the various sizes of foam in behind the wall.


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## Windows on Wash (Aug 30, 2011)

No.

Frame the wall square. If you are worried about it, move the wall 1" further out and put 1" more of foam.


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## chronojosh (Feb 11, 2011)

Thanks. I figured framing the wall straight was the right answer. I was just looking to see if there was a way to save on buying more foam. Well, off to HD again. Thanks for all the help!


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## Gary in WA (Mar 11, 2009)

With an interior vapor barrier (Kerdi), any moisture that gets in the cavity will cause problems if using organic (wood) framing, IMO- use steel studs. 

2" of XPS is not nearly enough to stop vapor diffusion from the moist dirt/concrete wall. The concrete/foam interface is wet most of the year, until the warmed concrete dries it out or the interior (vapor open) wall assembly allows the HVAC system to remove excess moisture in the stud cavity. You are limited to exterior drying only, even though the wood stud holds 1000 times more moisture than air and lets it dry out slowly, it could build-up over time and cause mold, due to the very limited drying you now have (only in summertime). XPS at 1" is 1.5 perms, vapor semi-permeable. 4"= 0.375 perms a little less than asphalt coated paper on fiberglass insulation (0.4perms)--- vapor semi-impermeable. I would use enough to have a real Class1 vapor barrier= 0.1 perm or less (vapor impermeable. OR coat the wall behind the foam board (at shower only, air seal between other stud bays) install to be sure water/moisture cannot diffuse there.http://buildingscience.com/document...rol-for-new-residential-buildings?full_view=1

With 2"XPS, using Roxul in front of= for your climate; https://www.currentresults.com/Weather/Connecticut/Places/hartford-temperatures-by-month-average.php with 3 coldest months average temps= 29.33*F you could have condensation at 44% Relative Humidity at 68*F cavity temp. If the bathroom is 10*F cooler than the basement (seen that before as door is usually shut and no room vent supply) then at 30% RH and above moisture (ext. sourced as the interface is wet most of the winter, into spring) would condense in the stud cavity, wetting the wood framing. If any mold spores are present on the wood= mold. 

In any case, use non-paper faced drywall- with ADA because that only stops interior based moisture, not through the foam- ground based capillarity.

Gary


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