On Fri, 23 Nov 2007 18:55:21 -0800 (PST), wrote:

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?

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.