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There is discussion of vapor barrier behind a shower at this question, and many recommend not to have a vapor barrier behind the backer board if you are using a membrane on top of the backer, as it will create a "moisture sandwich" which cannot dry out.

HOWEVER: I am putting the shower against an exterior wall, I am reluctant to follow the above advice as it will not maintain a continuous vapor barrier for the exterior wall; that is, I cannot join the shower membrane to the existing exterior wall's barrier.

What should I do? I am considering keeping the existing vapor barrier in place and leaving a gap in the membrane at the top and bottom, so the backer board can breathe to the inside of the house. Will that prevent a moisture sandwich? Will it cause other problems?

Part II of my question: I am using a pre-fab pan and just tiling the walls, not pouring or tiling the pan. The instructions with the pre-fab show using a silicone bead to seal the lip of the pan to the wall lining (vapor barrier I guess) BEHIND the backer board, and no membrane is shown in front of the backer. So another reason I like leaving the bottom of the membrane open is that I don't see how to connect it to the shower pan lip in this configuration. But if I change my mind, how is the membrane supposed to connect to the shower pan?

  • Where within the wall assembly is the existing vapor barrier, and where on this planet are you? – ThreePhaseEel Nov 6 '18 at 0:20
  • The existing barrier is inside the studs and inside the insulation, which is typical for our cold climate in eastern canada. – user93139 Nov 15 '18 at 3:54
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I think you’re confusing MOISTURE barrier with VAPOR barrier. They are two different things and go in two different places.

Moisture barrier is placed on the OUTSIDE of a wall and keeps rain, etc. from coming into the building.

Vapor barrier is placed on the INSIDE of a wall and keeps vapor from entering the wall, condensing, turning to moisture and then creating dryrot because it cannot escape or be vented out of the wall.

It use to be that we installed a vapor barrier in an attempt to keep all vapor out of the wall. Now, we know that it’s nearly impossible to keep 100% of all vapor out of the wall so we provide for the vapor to escape from the wall. (Attics are different...we vent attics.)

Vapor travels from a warm environment to a cooler environment. It will pass through building materials based on its “perm rating”. You want to use a vapor barrier with a good enough perm rating to keep most of the vapor out, but allow a small amount of vapor to pass when the weather changes. If you use plastic sheeting it has a perm rating that does not allow vapor to pass. However, vapor will find a way into the wall cavity around cracks, outlets, etc. Then, when the weather changes to WARM outside air and cooler indoor temperature (air conditioning) then the vapor in the wall will reverse and move to the interior. If there is plastic sheeting, the vapor is trapped in the wall. However, using a mild vapor barrier, like paint, it will prevent most vapor from penetrating the wall cavity in the winter and allow trapped vapor in the wall to escape in the summer.

So, to answer your question, do not connect the interior vapor barrier to the exterior moisture barrier.

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