Then you can install your Kerdi waterproofing membrane
on the shower pan and up the walls.

I'm curious why you'd recommend that. This forum
is the first I've heard of plastic backer films. I've
used concrete board for many years. Companies like
Durock have never recommended moisture barrier.

Before Kerdi was available, the traditional way to make a waterproof
shower pan was to have a custom copper pan manufactured, or to use a
vinyl pan liner embedded in the mortar bed. In those cases, the tile
backer board usually overlapped the shower pan, so any water that found
it's way behind the tile "should" eventually work it's way down to where
it would drip inside the pan liner. (I believe you're supposed to keep
the bottom of the backerboard up off the pan so it doesn't wick moisture
up into the wall) Of course, it's always better to keep the water out of
the wall in the first place.

Kerdi is applied to the mortar bed AFTER the pan is built-up. It is
installed directly under the tile. In this case, the kerdi waterproofer
laps up the walls, and the manufacturer recommends it extend up above the
water source (shower head or tub spout, whichever is highest). It makes
sense as you wouldn't want any water getting behind the kerdi membrane
and into the structure. Basically the entire shower area becomes a
unified waterproof barrier.

By putting a sheet of plastic between tile and board,
or between tile and sandmix base, you're breaking the
continuity of the mortar, for no apparent reason that
I can see. If the tile were not, in itself, waterproof there
would be little point in going to the trouble of using it.

Tile is waterproof. Grout is not.

Also, Kerdi isn't just a sheet of plastic. It's some kind of fabric
material and you use regular thinset to install it.

Are there some kind of studies somewhere -- not paid
for by the plastic membrane companies -- showing
that the polyethylene layer is somehow an improvement?

I am not aware of any studies, but I seem to recall several topics on the
John Bridge forum talking about moisture problems in traditional vinyl
lined mud beds. It has been about 10 years since I really researched tile
work, but I believe the moisture soaks through the grout and into the
mortar above the liner. Mold, mildew, and probably other problems. You
might search the tile forum if you need details.

I used Kerdi on our entire master bathroom, even the flat areas of the
floor that didn't need to be waterproofed. Zero problems in the last 10
years. Compared to all the other costs of building a bathroom, the cost
of Kerdi was insignificant. Certainly cheaper than a copper pan.

Anthony Watson
www.watsondiy.com
www.mountainsoftware.com