Hey guys,

I am at a crossroads. Here is my set up. Retangular prefab (Maax) shower base. Enclosed by three sides. One side is and exterior wall that is insulated and vapour barriered. The other two walls are interior. One wall being within the bathroom (wetwall) and the other sharing the wall with a bedroom.

The problem is that I did not put any vapor barrier on the two interior walls. I used fiberock Aqua tough tile backerboard. My question is do I really need to have the vapor barrier there? I know people say that the condensation can form and start the rot the wood studs.

I am aware that I can use the Kerdi system which acts like a vapor barrier and waterproofing. But I am trying to stay within by budget and would like to get away without having this expense. Of course if it is absolutely necessary then I will do I.

Please share with me what you think on this subject.

John, the location of vapor barriers varies depending upon climate. The only case of tile installations calling for a true "vapor barrier" is in steam shower/sauna rooms. Other than that, you follow guidelines for your climate regarding vapor barriers. What is called for in shower and tub surrounds is a moisture barrier. 15 lb roofing felt or 4 mil plastic sheeting are both allow vapor emission, but effectively protect the stud wall cavities from moisture in the cbu backing the tile. I don't have any scrap left over of fiberock, but if you do, just do the test I was going to do... silicone a piece of pvc to the fiberock. Allow it to cure. FIll the pvc pipe with water and let it sit. Does it drip through the board? If so, use a moisture barrier behind the fiberock or a membrane like Laticrete's Hydroban or Custom's Redgard. You could also buy some Kerdi if you want, but with the one wall being exterior with the vapor barrier already in place, you'd want to take down that (those) sheet(s) and slice up the vapor barrier on that wall or you could leave the vapor barrier in place and just use roofing felt around everything.

Agree with Bljack's response. Moisture barrier material will cost you few dollars. Not a big expense issue. As he said: Get roll of 15 lbs asphalt paper, gal. roofing nails and nail it with 6" overlap on the walls - that should do.

Agreed but the problem is that my fibrerock is already installed and taped. Therefore I would like to not have to take it down. Fibrerock claims that there Aquatough backerboard is water resistant.

Unfortunately I do not have any left to do any trials. My question is being a shower wall that will only have intermittent water contact, is there really that much of a chance that water will penetrate down to the studding?

As for the Kerdi you are saying that since one wall is already vapor barried that I can not use it on that wall? Why is that? Does that mean I can put the kerdi on only the two other walls and not that one?

Thanks again for the help guys

In this case, I would not worry too much about it. I have seen it many times installed w/o moisture barrier with no negative effect on framing. Just make sure your tile is properly grouted and sealed. Re-apply sealer once a year afterward.

Thanks that is great to hear.