Hi all,
I am re doing a small upstairs bath. I plan on using Durock cbu in the shower and greenboard for the rest of the room. I plan on only going 6 feet high(in the shower area) with the cbu and the top 2 feet will be greenboard. As far a a vapor barrier, I was told to use 6mil poly on the studs behing the cbu. My question is, what do I use behind the greenboard? I was told if I used poly behind the greenboard and then paint it, I would be creating 2 barriers which will cause a problem. If I dont use poly beind the greenboard, will I end up with wet insulation on the outside walls? There is currently Kraft faced insulation on these walls. I was told to replace with unfaced or cut slits in the paper to defeat that barrier. Opinions would be greatly appreciated!!!

For the exterior wall I would replace the kraft faced bats with regular insulation and install a 6 mil vapor barrier floor to ceiling.
The other walls where there is CBU should have a minimum 4 mil poly.
If you have a good exhaust fan there's actually no need for greenboard above the CBU or anywhere else in the bathroom. Regular drywall is fine.

Any drywall product that has a moisture resistant face or core material should not be installed over a poly vapor barrier.

Thanks for the reply.
How about waterproofing the cbu? If I put redgard on the cbu does it change the way the vapor barrier is used?
And, how about the walls that are not outside walls? Do I need a vapor barrier between the bathroom and adjoining bedroom? Will it hurt if I put the 6 mil poly on all walls?

Sorry to ask what probably seems like basic questions for the pro's, but after reading several different forums on several different sites, it's unbelievable how much conflicting information is out there!! The guy (kid actually) I spoke with at Lowes wasn't sure what a vapor barrier was!!) I spoke with two different contractors, and they had different opinions on how things whould be done.
I live in Michigan and have to deal with heating and cooling seasons if that matters for the answer.

Topical waterproofing will require the removal of the poly vapor barrier or kraft paper faced bats.

Is this a tub/shower or a shower only remodel?
Each requires a different approach when it comes to choices for materials and installation.

There are a lot of variables when it comes to the proper placement and use of vapor barriers,vapor retarders or air barriers for home construction.
Building codes don't really address the true technical issues required to control vapor transmission thru wall cavities. Unfortunately they tend to take a more generic approach to moisture control in wall structures which can lead to water damage and mold.

You need to decide on which type of construction and materials you plan on using so that the proper advice for that particular method can be given.

Thanks again for the reply.
Currently I am down to the studs on all 4 walls. Exterior walls still have the old kraft faced insulation for now.
I guess I'm looking for the best way to proceed.
I know I'll be using the durock in the shower area..which is actually tub/shower. I have put in the new subfloor, and 5/16 durock cbu will go on that(using mortar and screws with alkali resistant tape per manufacturer). I've read about using Kerdi, but with no experience using it, I honestly don't feel confidant(skillwise)attempting it.
So i guess I'm looking for the best way using the combination of the CBU and drywall(or recommended substitute). I plan on tiling the shower tub area, and painting the rest of the room.
A good quality exhaust fan will be used.

Thanks again!

If you're uncomfortable with Kerdi, a paint-on waterproofing might be best. This would negate the need for the vapor barrier behind the CBU, and keeps the CBU from getting damp to begin with. That's what I use on tub/showers. (Redgard, to be specific, but any quality brand will work)

Here's my suggestion on how to proceed with what is existing.

Leave the kraft paper bats in place and do not puncture or tear the facing where contact is made by the CBU. Usually this product is semi-permeable to water vapor so no other protection should be necessary.
Make sure all other exposed studs are covered with a minimum 4 mil poly where contact is made by the CBU panels only.

If you already have greenboard for the top section then the kraft paper face should be removed or slit where contact is made by the greenboard on the exterior wall. No poly should be behind the greenboard anywhere else.

Make sure to leave a 1/8"to 1/4" space between the top of the tub and the CBU panels. This area will be caulked with pure silicone before tile tile goes on.

Thanks Kman, If I was to Redgard the cbu in the show/bath area, I would still use a vapor barrier behind the drywall in the rest of the room , correct?
Also, does the Redgard affect how the tile is put on the wall?

Is it better to waterproof the face of the cbu or use the poly barrier behind it, or is either method acceptable?.

Thanks

If a topical waterproofing is used, then no poly should be used.

The same modified thinset is used for either assembly.

Waterproofing the face will give better protection for the tile underlayment and wood studs.

Either method will work if done properly.

Thank you RC, I think I like the idea of having the 4 mil poly on all walls, so here is my new plan:
I will use unfaced batts, covered by 4 mil poly on all walls.

Shower/bath wall area will be 1/2" Durock 6 ft. high, then ceramic tiled. Balance being standard drywall/paint.

Rest of the room will also be standard drywall and painted.

The floor is covered with the 5/16 durock already, and I think I may apply redgard to the floor for waterproofing.

If something doesn't sound right , PLEASE let me know. I've got to get this job done and I'm running out of excuses to stall!!

Thank you very much!