# Outside wall tub surround vapor barrier question



## shotsy (Mar 29, 2010)

I am getting ready to install a new tub/ shower in freshly renovated bathroom (gutted, reinsulated, vapor barriered (outside walls) and partially mr dry walled) I plan on using cement backer board for a tile backer in surround. My question is should I or can I add another vapor barrier over existing full wall vapor barrier to go behind cement backer and end at tub flange? Is there a reason that I should not do this? Being an outside wall I am thinking that there should still be a full vapor barrier on said outside wall. Any input would be greatly appreciated.


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## Bob Mariani (Dec 1, 2008)

you need a waterproofing over the cement board, so a vapor barrier behind it makes no sense. However some inspectors will still require it. Normally we then install it, get the inspection and then remove the poly. Never have a double vapor barrier. So protect the work I would not use a poly barrier.


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## shotsy (Mar 29, 2010)

OK just to make sure I am understanding you I should do the following:

-remove vapor barrier
-install cement backer right over studs and bare insulation
-apply a waterproof membrane such as kerdi

Have I understood you?

I should have maybe given a little more info in my first post sorry for that. My plan was to put up the cement board and cover with redguard; is this going to be sufficient for water proofing? Is a waterproof membrane a better way to do it? If I have a leak won't all my insulation be wrecked? I thought the idea of putting the vapor barrier behind the cement board was so if any water ever say seeped through a grout line it would make it's way back to the tub. Sorry for all the questions I just want a tub shower done right the first time that doesn't leak and doesn't give me a headache in a few years. I live in Alberta Canada and it's common practice to put vapor barrier on outside walls over fiberglass insulation.


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## Bob Mariani (Dec 1, 2008)

your inspector will require the vapor barrier in that climate.

Use Kerdi... far better than the redguard which is latex based and if water gets in it re-emulsifies.


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