Home>Discussions>BATHROOMS>Felt paper vapor barrier behind cement board?
3 posts / 0 new
Last post
Felt paper vapor barrier behind cement board?

I am remodelling my bathroom. My tub is in a 3 wall alcove with 2 of the walls being exterior walls. The plumbing is on one of the exterior walls. I have read different opinions about using felt paper for a vapor barrier between the studs and cement board. If I use felt paper should I use faced or unfaced insulation on the exterior walls? Also, what needs to be done to the places where the plumbing fixtures come through the felt? Won't this leave holes for water to get into the walls? I can't find any instructions or videos on this. None of the videos show this area which I would think would be the trickiest and most important area. Am I wrong? Do I just cut holes in the felt for the plumbing and not worry about it? I sweat the details, especially the ones that get covered up and are hard to go back and fix if they are wrong. Any ideas, instructions, or links to good sources?

Re: Felt paper vapor barrier behind cement board?

The felt paper is not a vapor barrier, its a water barrier. You still need a vapor barrier for the insulation. Your plumbing should be inside the insulated envelope and inside the vapor barrier. If you have plumbing in a 2x4 exterior wall, and you live in cold country, you are going to have problems.

You should have an insulted exterior wall, then a wet wall inside of that for your plumbing, but putting plumbing in any exterior wall will eventually cause problems. How do you plan to get to the backside of the plumbing fixtures when they need replacement?

The best book on tiling is written by a guy named Burnes. I can't find my copy right now but you should be able to find his book at one of the big box home stores, that's where I got mine, but I don't remember which one.

Re: Felt paper vapor barrier behind cement board?

Let me see if I'm understanding correctly. The plumbing should be between the insulation and the felt? This is how I've understood other instructions. The pipe for the tub spout and the valve and the shower pipe need to come through the felt leaving holes for moisture to get through. Don't these need to be sealed some how? My question is how to seal them if they need sealed?

I don't have much choice where the plumbing goes. The plumbing was in the exterior wall when the house was built in 1960. Would be way too much trouble and expense to do otherwise. I have room to put about 8 inches of insulation between the plumbing and exterior wall. I did not have problems before. I hope the only thing I will ever need to replace is the cartridge in the valve body. This can be done without tearing up the wall.

Thanks for your time and info. I will look into the book, I have seen reference to that name elsewhere.

Sponsored Stories

TV Listings

Find TV listings for This Old House and Ask This Old House in your area.