# Vapor barrier back to the tub?



## albertpu (Mar 20, 2009)

Hello.

I am about to install my new bathtub. I have 4mil plastic over the studs and under the cement board. My question is that I'm not sure how to connect the plastic back to the tub.

I'm not sure what the typical flange/lip of a tub looks like, but mine is a drop-in tub and its flange is small bump. It is not a nail in flange, just a hump at the ridge that returns downward. 

After the tub is in, the cement board will start 1/4" above the lip of the tub. So, do I just cut the plastic so a small bit overhangs the lip? What would I use to glue it? Would the final silicone cover up that material?

Any help would be great - thanks.


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## glennjanie (Mar 21, 2009)

Welcome Albertpu:
I would not cut the plastic, but make sure you have a full and complete job of caulking with silicone upon completion.
To make the silicone job fill the crack; cut the nozzle on the caulking tube to the size of the crack being filled, point the nozzle in the direction of travel at a 45* angle, begin to squeeze the trigger and move the gun only as the silicone builds up around the tip (ever so slightly). This is called 'pressure caulking' and is the only way to be assured that the crack is filled to the bottom. As soon as the calulk is complete you will need to 'tool' it with a finger or piece of clear plastic tube. Don't worry about the overflow; after it has set for 30 minutes or so the thin overflow can be rubbed off, while the thick overflow will need to be cut off with a knife.
Glenn


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## GBR (Mar 21, 2009)

Hi. I'm not an expert in this field. If it was mine, I wouldn't use a plain cement backer board product, but one that has a waterproof surface towards the tile, like DensGuard or DensShield. It's 30% lighter than cbu, and it scores with a knife. It also doesn't encourage mold growth, just Google either word. It just makes sense to me to stop the water (leaking through the grout) at the thinset instead of a 1/2" later at the vapor barrier on the studs (which has 80 or so holes through it from the fasteners). In demo I have seen a lot of cbu with mold on the wood.
Could you  silicone caulk the plastic VB to the front of the raised portion? I'm sure a tile pro or experienced DIY will be along shortly. Perhaps search past threads? Be safe, GBR


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## inspectorD (Mar 21, 2009)

The vapor barrier should continue down past the tub to the bottom of the wall. The barrier is not for the tub to collect water, it is for any water vapor which moves through the wall and finds the next place to condensate. 
Build the wall as you normaly would , then install the tub against the wall. 
If you are installing tile, use a durarock or tile cement backer board, then seal it with redguard or another waterproof coating for tile board. Then start your tile job as normal.
Good luck.


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## Hube (Mar 21, 2009)

I assume this is a exterior wall since you are using a vapor barrier. As InspectorD suggested, just take the vb down to the floor. 
The tub will be then installed against the vb without any cutting.


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## GBR (Mar 21, 2009)

Is the vapor barrier, required by the cbu maker, there to stop vapor from outside getting in? Or is it to protect the wall assembly from water damage from the shower? Grout, thin set, and cbu are not waterproof.
If a house has ply, osb, or other wall sheathing(perm of .75 to .5), wouldn't that slow down moisture on the outside going in? To the point the vb on the inside would be more beneficial to stop shower moisture? Hence, onto the tub lip? 
I understand warm climates -- vb on outside of wall, or none, and cold climates- vb on inside of wall. Do you lose any maker warranty, by going to floor instead of in pan or tub lip?  I searched 5 tile forums with the same results- vb into tub. Does anyone have a TCA handbook for their answer? 
To cut or not to cut----- I think this is a very good question, one I'm going to ask my County Inspector on Monday. I enjoy this forum and hope it's in the right talk area (newbie).  Be safe, GBR


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## inspectorD (Mar 22, 2009)

Now you have me thinking. 
If you were to just do it with the lip into the tub, you could continue with another piece at the bottom below the tub.


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## GBR (Mar 22, 2009)

That is true, and yet wouldn't the tub act as a vapor barrier? They must have a pretty low permanence rating, SMILEY HERE. I've removed tubs and the only water damage seen was from a leak elsewhere. Usually there aren't even any spiders present, lack of water. When framing, we always installed un-faced batts behind the built-in tub platform as well as wood fire stopping. Is there anything in UBC or IRC on this exact situation? I only have the '76 and '88 of UBC.   Be safe, GBR


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## jdougn (Mar 22, 2009)

The vapor barrier is to protect the insulation and wall from from damage caused by  moisture vapor moving from inside the house into the wall cavity. The vb should go all the way to the floor without being cut to do this properly.  

The cement board should extend just over the lip of the tub. Any leakage will move down the cement board to the solid tub lip and weep back through. I have not seen any mfgr installation guide that dictated additional waterproofing for the cement board as long as it, and the tile are installed properly.
just my .o2, Doug


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## GBR (Mar 24, 2009)

Here is an answer from a TCA board member, the last 5  paragraphs.    Water and Waterproofing Facts and Fantasies - Cover Story - Floor Covering Installer         Be safe, GBAR


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