vapor barrier behind cement board?

[minnesotahusker]

I'm starting to finish the lower level bath and I'm going to insulate the whole thing for noise purposes but I'm wondering if I have to install a vapor barrier behind the cement board or not?

[311Hemi]

I assume you are referring to a shower/bath installation?

I have been researching the proper way to build a shower and everything I have read says yes, a vapor/moisture barriar should be installed. This can either be tar paper or 6 ml poly behind the cement board, or a roll on membrane like Red Gard which goes on the front of the cement board. If you go the route of poly it should drape over the edge of the shower pan liner. Also, you do not want to install Red Gard if you already have poly behind the cement board.....which I have read they call a moisture sandwhich. Do a search for johnbridge and do some reading there!

The Red Gard would probably be my choice, and it can be purchased at Home Depot. I will be putting it on a shower in my basement tonight over Durock.

[NoWiser]

I agree with 311......if you are talking about a shower, use the red guard after you get the cement board installed. I just did a shower and did two coats of the redguard. 3 coats on the shelf and niche. You don't want a leaky shower!

Or, are you talking about an actual vapor barrier like you would find on an exterior stud wall??

[minnesotahusker]

I'm wondering if I have to put the 6 mil poly behind it just as you would with an exterior wall or if I can put something similar to the RedGard that was suggested, maybe the orange stuff that comes in rolls(I think it's Ditra). I don't want to have to rip it out because I have a leak later on.

[311Hemi]

One or the other, not both.

NoWiser, how did the coverage work out for you with the redgard? I picked up 1 gallon but I think I may need to buy the bigger 3.5 gallon to do 80 sq/ft of shower wall.

Any tips for putting it on? I was going to try the roller application.

[NoWiser]

I had about 80-90 S.F. of wall. It took me a gallon to get one coat on. A buddy of mine had some blue stuff that was similar that he gave me to use. I put a coat on and was not happy with it at all. It was too thin. So, I went and got a gallon of the red stuff and feel very good about it now. I think it will take you about a gallon per coat. I'd use two just to be safe.

I used a cheap paintbrush and roller to put it on so that I could just toss them when I finished. It worked very well.

[frazwood]

I have placed Ditra onto cement board. It works well and its easy enough to use. Just be sure to read the directions as far as what type of thin-set mortar to use (you cannot use modified thin set).

[311Hemi]

For Ditra you should use modified between the subfloor and the Ditra mat, and un-modified thinset between the Ditra and tiles.

Although, I don't know that you would actually use Ditra on walls.....is that what you are planning on doing? Maybe you are thinking Kerdi?

[LightningBG]

I've used Red Gard with good results.

One thing I did learn though....

The area I was doing was a back splash / surround around a whirlpool. The walls were already painted, so I taped off the area I didn't want the product on. I used a brush to put it on, two coats, then let it dry. The problem was, it formed a sheet that included over the tape. When I went to remove the tape, it peeled the redgard off in areas. Lesson: take tape off when wet, or score a line with a blade at the edge of the tape before pulling it off.

[Stratosman]

I think the real lesson there is not to apply it over a painted surface, as it won't bond...

I would always recommed Redgard instead of poly. If you like Redgard you will love Hydroban, made by Laticrete. Mapei has a good product as well. The key is to apply it to the proper mill thickness (measured wet) to assure you get the performance you want...

[LightningBG]

I think the real lesson there is not to apply it over a painted surface, as it won't bond...

I would always recommed Redgard instead of poly. If you like Redgard you will love Hydroban, made by Laticrete. Mapei has a good product as well. The key is to apply it to the proper mill thickness (measured wet) to assure you get the performance you want...

Well next time i will unpaint it first. It was scored and scuffed up a bit to help adhesion. Been there for years now with no problems yet. Probably wouldnt have been needed at all, given the wall just gets the occasional splash.

[minnesotahusker]

Ok, thanks for all the input. My problem is that the back of the tile wall is an exterior wall that has the vapor barrier up on it as it should and if I want to put Kerdi on it, that would make the "moisture sandwich" on the back wall. I don't really care to put a vapor barrier on the studs behind the duroc, I'd rather just put some type of membrane over the Duroc to prevent the water from getting behind the lip of the tub and running down the studs if it ever got that far.

Am I find just painting the Duroc with a Redgard or a similar product as mentioned even though there is a vapor barrier behind the exterior wall already?

[frazwood]

For Ditra you should use modified between the subfloor and the Ditra mat, and un-modified thinset between the Ditra and tiles.

Although, I don't know that you would actually use Ditra on walls.....is that what you are planning on doing? Maybe you are thinking Kerdi?

Yep, I meant Kerdi, not Ditra. Oops. Sorry about that.