RedGard before tiling?

[DocBlake]

I'm tiling a shower. I put 6mil vapor barrier behind the cement board but weant to be sure the application is entirely waterproof. I'd like to cover the cement board with RedGard prior to tiling. I've read a few posts on the internet saying this is a bad idea because it will "trap" water in the cement board! I realize that the Red Gard may be overkill, but would it be problem to use it over cement bpard that has a vapor barrier behind it?

[geniescience]

over the internet, it will be hard to say whether your vapor barrier was really that well done that it has made a continuous wrap.

Since it's sheet material, it has seams. Humidity will not get trapped much. How much is "much" ?? No-one can say.

I vote in favor of overkill since a vapor barrier is not a real barrier like RedGard, and since you like overkill.

David

[jadnashua]

One or the other...

Neither tile, grout, nor cbu is waterproof and some moisture will get behind the tile. Now, not much and if you have the vapor barrier properly installed, it won't get into the walls. You don't want to trap what does get there inside.

Mildew takes three things to grow: the presence of the mold spore (almost universal), food (neither tile, grout, nor cbu is mold food), and moisture. If you keep the shower clean (soap scum, body oils, etc.) and it can dry out inbetween (good ventillation and not holding moisture), mold won't grow. It helps to seal the grout - it slows, but does not stop moisture absorbtion, and helps prevent stains from penetrating.

If you are really paranoid about it, you could tear it out and install Kerdi from www.schluter.com (http://www.schluter.com). I trust it much more than Redgard. You'd need to redo the pan, but could leave the walls up. Check out www.johnbridge.com (http://www.johnbridge.com).

[geniescience]

slope your shower floor and RedGard the slope. Then moisture and soap will have nowhere to go but down the drain.

If you Redgard a flat floor it will give you the same problems as any other membrane installed over a flat floor.

David

[frenchie]

If your plastic was done well, I vote against redguarding the cbu as unecessary, and promoting the odds of a mold problem by trapping moisture.

just my 2c.