Do I need a vapor barrier behind cement boards in shower?

According to the tile bible by M. Bryne I do but what I don't understand is this: If water gets through the tiles/grount and then penetrates the cement board then wouldn't it be better to let it essentially evaporate into the air void between the walls rather than trap it? Especially if you have the vapor barrier sealed off and the bottom of the tiles is caulked etc. where is the water going to go except get trapped. Could someone explain this please?
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When I redid my bathroom, the building inspector told me to use a permeable membrane behind the cement board, like 15 lb felt.
Cheers, Wayne
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Just to follow up, the logic behind using a permeable membrane is this. Water will eventually make its way through the membrane, e.g. through the holes from the staples holding it up. So given that there will be some moisture in the wall cavity, the permeable membrane will allow some water vapor to escape back into the shower.
Cheers, Wayne
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Taking logic a step further, using a non-permeable membrane and the appropriate adhesive (avoiding staples), there will be no moisture transmission problems. Paul F, is spot on in his recommendations in this regard. The building inspector who regarded roofing felt as permeable is puzzling. If that were true, then Tyvek and other brands used as house wrap today could be replaced by cheaper felt.
Joe
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I don't think it's that easy to ensure that no moisture will get into the walls. The staples are just one example, there are also corners, lap joints, etc.

Indeed, I believe that synthetic house wrap has little or no advantage over traditional felt. For example, see: <http://www.umass.edu/bmatwt/publications/articles/housewraps_feltpaper_weather_penetration_barriers.html
Cheers, Wayne
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Just my opinion but isn't tyvek touted more as being a better air barrier than felt in that the seams are sealed better...always assuming of course that it is applied correctly. I don't see that there is much difference in moisture barrier over felt. If there is any it would be on the side of the tyvek.
Harry K
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On Fri, 23 Nov 2007 18:55:21 -0800 (PST), poison snipped-for-privacy@yahoo.com wrote:

The whole point is to try very hard to keep the water *out* of the wood behind the cement board. Since tile/grout and backer board are not completely waterproof, you need a waterproofing membrane behind them. Properly installed, any water that gets through the backerboard will be stopped by the membrane and will either flow down the membrane where it will eventually make it's way back into the shower pan (the membrane should lap over the shower pan or mortar bed liner), or will evaporate back out the way it came in, slowly over time.
Think of it as a tank with a tile lining. You want all water to stay inside the tank (or go out the drain) and not get out any other way. The problem with letting water get into the wall assembly behind the backer board is that is will take a very long time to dry because there is no where for it to go except to diffuse through the wall assembly. When houses had lots of air leaks and little insulation this wasn't as much of a problem. Now wall are sealed better and once water gets in, it can stick around long enough to foster mold and rot. The materials inside the "tank" can tolerate being wet or damp for long periods. the materials outside the "tank" can't.
Building felt makes a pretty good membrane, and there are self-adhesive rubber based products that are even better. Just make sure to install it from the bottom up, lapping the seams so any water that gets to it will not be able to go anywhere but back into the shower area. If you're using a pan, the liner should lap over the walls of the pan. If you're laying a mortar bed floor, the pan line should extend up the walls 6-8 inches and the wall liner should lap over it.
HTH,
Paul F.
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wrote:

Ahhh, but that's the part that is troubling me, if I understand the "tile bible" correctly then I don't see how the water can make it's way back into the shower pan (looking at the pictures in the book.) If all along the bottom most row of tiles is caulked (between the tiles and pan or tub) how is any water going to make its way back? Obviously I am missing something. Unless it is supposed to make it's way back into the pan "through the tiles and grout" - and as you said - very slowly.
I want to fully understand this before I retile this shower. btw this shower enclosure has a "one piece" 3'x3' or so floor. Not a traditional pan with tiles etc. I'm not sure what type of laping it has although I would venture a guess of "none."
Thanks!
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I'm sure you have already proceeded on your project, but here is a very different direction to take on your shower: http://www.johnbridge.com /
I think this is some of the best tile advice available, period. Now click on "tile and marble", scroll down to the Kerdi shower link. Schluter is a very respected name in the tile industry. Here is the shortcut: http://www.johnbridge.com/kerdi_shower.htm
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