| The coating is over the board, board tape, and thinset applied at those
Yes, I understand that, which is why I said
you're losing the mortar integrity. You're
gluing your tile to a plastic coating rather
than getting a thinset -> concrete bond.
| The redgard keeps any moisture that gets through the grout from
| getting through the concrete board. Then it either has to find its way
| through the grout, or find a path, hopefully back into the tub area,
| caulk at the bottom of the tile.
That sounds like Redgard marketing hocus pocus
to me. Tiling on mortar has been done for thousands
of years. Why, all of a sudden, is grout considered
to be a non-waterproof material? Even if tiny bits
of moisture can go through some grout, the concrete
board is designed to let it migrate out. The top of
the concrete board is not plastic wrapped.
I've been doing the same for years and never had
a problem. I also built a steam room about 12-15
years ago. It's been heavily used with no sign of
problems. Just as nearly every other steam room
ever built has been some version of tile on mortar
and has worked just fine. If moisture were getting
through then even steam rooms built with concrete
wall on metal lath would eventually break down.
The Shluter system is, in theory, similar to the
Redgard approach, except that it would provide
full waterproofing where Redgard probably won't.
But it's also similar in that it's a plastic sheet glued
to the wall. So the tile is only as stong as the bond
between the thinset and that plastic sheet. The
Shluter people even show their sheet being used
over drywall! In that case the bond depends on
tile -> plastic and also plastic -> paper. That
seems idiotic to me. And why do they claim it
makes sense? Just to keep the moisture barrier on
the front side of the wall, with the theory that
otherwise the concrete board will somehow get wet
due to water leaking through the grout. The reasoning
just doesn't hold water, so to speak. :)
| The plastic you suggest would likely create a problem with condensation
| wall/floor space below the plastic, or the same potential water issue at
| caulking at the bottom of the tile.
No, because the plastic is sealing it out. The
plastic comes down behind the concrete board
and the gap gets caulked before tiling. It's
always possible that water can get through --
at the top of a porrly caulked tub or around
a poorly sealed mixing valve, for instance. But
in general a plastic sheet is going to provide
a good seal.