[chronojosh]

So I have read a ton of stuff on here about basement insulation, framing, and showers. All of it has been very informative, but I can't seem to find out whether my plan is going to work or not. Here's the situation: I am building a bathroom in my basement, 1950's ranch in CT with concrete block foundation, minor weepage at the concrete floor/wall intersection when it rains. I plan on insulating the concrete walls with 2" xps, then framing a 2x4 wall directly against that. I plan on having one of the shower walls be directly against part of this 2x4 outer wall, and it will be waterproofed with either kerdi or hydroban...not sure which yet. Anyways, I want to insulate the stud bays with roxul, but I am not sure if I should do that for the shower wall. Will these create a moisture problem in that section of wall? In case it helps, I will be putting foam board under the PT bottom plate of my walls to also keep any moisture from wicking up. The weepage should be solved once I re grade the outside yard to install a paver patio. Thanks in advance for any advice!

 

[javan]

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.

 

[chronojosh]

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?

 

[Windows on Wash]

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.

 

[chronojosh]

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?

 

[spaceman spif]

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

 

[chronojosh]

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

 

[spaceman spif]

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.

 

[chronojosh]

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

 

[spaceman spif]

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!

 

[Windows on Wash]

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.

 

[chronojosh]

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?

 

[Windows on Wash]

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

 

[chronojosh]

Is there an issue with condensation forming due to that?

 

[chronojosh]

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

 

[Windows on Wash]

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

 

[chronojosh]

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?

 

[Windows on Wash]

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.

 

[chronojosh]

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.

 

[chronojosh]

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?