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mnations 03-21-2010 03:10 PM

Vapor barrier stumper, where does the water go?
I have been chasing this rabbit down for a couple of weeks now, and I just can't find an answer.

I'm installing a standard tub and building a tub surrounding with tile with hardibacker as the cement board. Most places recommend bringing a 6mil plastic vapor barrier down past the flange of the tub so that it drapes over.

I completely understand why the vapor barrier is necessary and understand the theory behind bring any moisture back into the tub. This seems to be an accepted method across many different resources. But where does the water go?

If water does collect on the VB and slides down it, the water will settle in the space behind the tile on the surface of the tub that has been sealed with caulk. The same caulk designed to keep water out will also keep all this water in, which will have no where to go. Following through the process it will simply keep collecting until it reaches the hardibacker, at which point it will be wicked up and held.

The picture below is an example of what I'm referring to. At the bottom are blown ups of both situations, whether you install the CB on top of the flange or alongside it. It seems like if water vapor does collect it would be better pulled down below the tub where at least it has more change of evaporating then in the water tight space behind the tile.

Thanks for any input.

hereslookingatU 03-21-2010 03:53 PM


The problem is 'vapour' not moisture, not water. The purpose of the 'vapour barrier' is to keep the warm moisture laden air on the warm side of the construction, otherwise moisture in the vapour phase will permeate through the wall and as the air cools down it will become saturated and condensation will be deposited inside the structural element and could easily lead to problems with dry rot, and I can assure you, that you would not want to be finding out that you have a problem with dry rot!

Vapour = UK for vapor

mnations 03-21-2010 04:39 PM

Thanks for the response. I can definitely see what a problem can result if moisture continually creeps back into the walls.

The problem I see is that after a while moisture will accumulate on the vapor barrier and then run down the membrane, like a rain gutter. Then you have a pool of water trapped underneath the cement board and behind the tile.

Seems like if you want to protect the inside of the walls then covering the whole alcove with plastic sheeting would be enough to protect it. Then you may get some moisture creep through and hit that VB, but any accumulation would drop beneath the tub where it could evaporate.

Basically before I installed the tub I put 6mil plastic sheeting over the walls in the entire alcove to protect them. This was my understanding of what should be in place in the entire bathroom anyway to protect water vapor from escaping into the walls behind it, not just the tub part. I don't see how placing a sheet just behind the CB itself provides better protection than that.

troubleseeker 03-22-2010 09:50 AM

As lookingatU says, the operative word here is VAPOR. The minute amout of vapor that will get through correctly installed tile is minimal and will migrate back out the same way it got in. If conditions allow for the existence of enough liquid water to run down and pool as you hypothetically describe , there is a problem with the tile/grout installation, or water leaking around the plumbing penetrations.

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